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A geological reconnaissance of Bowie Seamout Herzer, Richard Howard 1970-12-31

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A GEOLOGICAL RECONNAISSANCE OF BOWIE SEAMOUNT  BY  RICHARD HOWARD HERZER B.Sc.  Queen's U n i v e r s i t y , 1 9 6 6  A THESIS SUBMITTED I N PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In t h e Department of ' Geology  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e required standard  THE UNIVERSITY OF B R I T I S H COLUMBIA April, 1970  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  f u l f i l m e n t o f the requirements  an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree the L i b r a r y  s h a l l make i t f r e e l y  I f u r t h e r agree tha  available for  permission for extensive  r e f e r e n c e and copying o f t h i s  It  i s understood that copying o r  thesis  permission.  Depa rtment The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8 , Canada  Date  or  publication  o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my written  that  study.  f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department by h i s r e p r e s e n t a t i v e s .  for  F r o n t i s p i e c e - Photograph taken on the e a s t e r n f l a n k o f Bowie Seamount at a depth of 3 » C 0 0 f t .  ABSTRACT Bowie Seamount, a submerged v o l c a n o s i t u a t e d o f f c o a s t o f Canada a t 5 3 ° l 8  the west  ;  N, 1 3 5 ° 3 9 '  W,  has a r e l i e f  of 1 0 , 0 0 0  f t . and comes t o w i t h i n 1 0 0 f t .  of the ocean  surface.  I t i s made up o f a s e r i e s o f  i n t e r s e c t i n g . r i d g e s which t o g e t h e r g i v e the mountain o v e r a l l n o r t h e a s t - southwest e l o n g a t i o n .  an  I t appears  t o be a c o m b i n a t i o n c e n t r a l and f i s s u r e t y p e v o l c a n o w h i c h has been b u i l t up o v e r a s y s t e m o f f r a c t u r e s i n the oceanic c r u s t .  Two  intersecting  terraces  form  t h e f l a t t e n e d summit o f t h e v o l c a n o a t a p p r o x i m a t e d e p t h s o f 4 5 and. 1 3 0  fathoms.  These a r e t h o u g h t t o be  t h e r e m a i n s o f p l a t f o r m s p r o d u c e d by combined e r o s i o n and s h a l l o w - w a t e r v u l c a n i s m d u r i n g Qu^ernary today.  t i m e when s e a l e v e l was  The  l a s t phase  summit o c c u r r e d a f t e r  late  lower than i t i s  of volcanic a c t i v i t y the f o r m a t i o n of the  t e r r a c e no more t h a n 1 8 , 0 0 0 y e a r s ago. dredged  wave  on t h e upper  Samples  from the upper h a l f o f the v o l c a n o i n c l u d e :  p i l l o w fragments, fragments of n o n - p i l l o w e d flows, p i l l o w b r e c c i a s , bombs, t u f f s , a s h , and u n s o r t e d t e p h r a . The  rocks are mainly a l k a l i o l i v i n e  accompanied  basalts,  by r a r e a n d e s i t e s w h i c h , p r e s u m a b l y ,  were  d e r i v e d by d i f f e r e n t i a t i o n o f t h e b a s a l t i c magma. F e l d s p a t h i c and g a b b r o i c i n c l u s i o n s , many o f w h i c h  appear-  t o be c u m u l a t e s ,  a r e common i n t h e b a s a l t ; I c e -  r a f t e d r o c k s a r e r a r e on t h e summit o f Bowie Seampunt b u t a r e common on i t s n e a r e s t n e i g h b o u r Seamount.  - Hodgkins  A ferro-manganese d e p o s i t , a p p a r e n t l y over  1 m i l l i o n y e a r s o l d , t h a t e x i s t s on t h e summit o f H o d g k i n s Seamount, s u g g e s t s t h a t t h i s p e a k i s r e l a t i v e l y much o l d e r t h a n t h e . s u m m i t a r e a o f Bowie Seamount.  P a l a g o n i t e appears  t o form a s t h e i n i t i a l  phase o f w e a t h e r i n g o f g l a s s y b a s a l t s i n t h e a r e a o f s t u d y b u t t h e p r o d u c t s o f more a d v a n c e d w e a t h e r i n g a r e m o n t r a o r i l l o n i t e and z e o l i t e s .  Rock fragments  t h a t have  been r o u n d e d by c h e m i c a l w e a t h e r i n g a r e common.  TABLE OF CONTENTS I. II.  III.  IV.  1  -Introduction Field  Work  A.  1967 cruise  k  B.  1968 cruise  5  Geomorphology A.  Topography  13  B.  T h e o r e t i c a l Development o f f l a t - t o p p e d seamounts.  17  C.  G e o m o r p h i c D e v e l o p m e n t o f Bowie Seamount 2 3  Petrology A.  Lava flows  38  B.  Inclusions  51  C.  Tephra  66  D.  V e s i c l e L i n i n g s and amygdules  °?  E.  Post-volcanic  9^  F.  Chemical weathering  deposits  97  V.  Summary and C o n c l u s i o n s  102  VI.  References  107  VII.  Appendices  L I S T _ p ^ TABLES Table I  - Chemical c o m p o s i t i o n s o f r o c k samples  43  Table I I  - Normative  44  c o m p o s i t i o n s of r o c k s t a p l e s  Table I I I - Modal c o m p o s i t i o n s of a n a l y s e d r o c k samples L I S T OF  45  ILJJJ^ATIONo FIGURES  Figure 1  - I n d e x map  2  Figure 2  - Sample L o c a t i o n Map  6  Figure 3  - Ship's Track  8  Figure 4  - Topographic  d e t a i l o f summit  10  Figure 5  - Topographic  d e t a i l o f summit ( 1 9 6 9 )  11  Figure 6  - S h i p ' s t r a c k o v e r summit  12  Figure 7  - T o p o g r a p h y o f Bowie Seamount  14  Figure 8  - S i l h o u e t t e s o f Bowie Seamount  16  Figure 9  - Model f o r f o r m a t i o n o f f l a t - t o p p e d Seamounts I  20  F i g u r e 10 - Model f o r f o r m a t i o n o f f l a t - t o p p e d 21  Seamounts I I Figure 11  - Kolocene  Figure 12  - Suggested  sea l e v e l  2o  changes  s t r u c t u r e o f Bowie Seamount  F i g u r e 1 3 - O r i g i n o f d i f f e r e n t l a v a s from same magma chamber  J>k  the 65  PLATES. Plate 1  - Sonar  Plate 2  - Reworked l a p i l l i  profiles  o v e r the summit deposits  18 26  Plate 3  - Deposit of p i l l o w  Plate 4  - Deposit of small  Plate  5  - Types o f flow  Plate  6  - Textural  35  fragments pillow  cru.-;t f r a g -  ..ts 3 6 .39  rock  change f r o m t a c h y l y t e t o  sideromelane  •  40  Plate  7  - Microscopic  Plate  8  - Photomicrographs of the andesites  50  - Pyroxene  53  Plate 9  textures  o f the b a s a l t s  cumulate  Plate  10  - Andesine - o l i v i n e  Plate  11  - .Plagioclase  Plate  12  - Harrisitic  cumulate  - hornblende cumulate o v e r g r o w t h on  Plate  14  - Plagioclase  55 57  anorthosite  xenolith Plate 13  47  60 - magnetite x e n o l i t h  62  - Graphic a l t e r a t i o n texture of hornblende  64  Plate  15  - Bombs f o u n d on t h e seamount  68  Plate  16  - Bombs f o u n d on t h e seamount  69  Plate  17  - Bombs f o u n d on t h e seamount  71  Plate  18  - Bottom photographs o f t u f f o c c u r r e n c e  74  Plate  19  - Cementation of the t u f f s  75  Plate 20  - Cementation o f the t u f f s  76  P l a t e 2 1 • - Cementation o f the tuffs^  77  Plate 22 Plate 2 3  - Loose l a p i l l i d e p o s i t o f t h e 4 5 fathom terrace - R i p p l e marks i n s h e l l m a t e r i a l  )  79 80  P l a t e Zk  - Bedded a s h  82  Plate 25  - Pillow breccia  85  Plate 26  - Bottom p h o t o g r a p h s i n a r e a o f p i l l o w breccia  °°  Plate 27  - R u b b l y s u b s t r a t e on t h e  seamount's ^7  flank Plate 28  - V e s i c l e l i n i n g s and a m y g d u l e s  90  Plate 29 Plate 30  - V e s i c l e l i n i n g s and a m y g d u l e s - V e s i c l e l i n i n g s and amygdules  92 93  Plate 31  - Ferro-manganese n o d u l e s from Hodgkins Seamount  96  Plate 32  - P a l a g o n i t e i n hand s p e c i m e n  98  Plate 33  - Alteration  •  101  ACKNOWLEDGEMENTS The  author wishes to express h i s thanks to  Drs, K.C. who  McTaggart, R.E.  acted as t h e s i s a d v i s o r s .  co-operation  The  of the o f f i c e r s and  ENDEAVOUR and  J.W.  Murray  enthusiastic  crew of CNAV  of f e l l o w U.B.C. students  i s also appreciated. and  G a r r i s o n and  and  technicians  Thanks must go to Dr. W.H.  White  h i s t e c h n i c a l s t a f f at U.B.C. for t h e i r K - Ar age ^  determination,  to Dr. B.E.B. Cameron of the  Geological  Survey of Canada f o r h i s work on the microfauna, to Dr. J.G.  Souther of the G e o l o g i c a l Survey f o r h i s help  and  advice  on the c r u i s e and  and  f i n a l l y to Dr. J.A.  f o r p r o v i d i n g rock  analyses,  Scrimger of D.R.E.P. for h i s  w i l l i n g exchange of data on the seamount. F i n a n c i a l support provided  f o r t h i s p r o j e c t was  kindly  by the N a t i o n a l Research C o u n c i l of Canada,  the I n t e r n a t i o n a l N i c k e l Company of Canada and  the  Deans Research Fund of the U n i v e r s i t y of B r i t i s h Columbia.The  author i s indebted  Research Establishment  t o the personnel Pacific  help i n p r o v i d i n g the s h i p .  of Defence  f o r t h e i r w i l l i n g n e s s and  Without the support  of these  o r g a n i z a t i o n s t h i s p r o j e c t could not have been undertaken.  1  INTRODUCTION A.  PurjD ojb e_and Scojp e_of The  area of study  submarine volcano  the  Study  e n c o m p a s s e s Bowie Seamount, a  i n the n o r t h e a s t  Pacific  m i l e s w e s t o f t h e Queen C h a r l o t t e I s l a n d s , Columbia.  The  purpose of the p r e s e n t  to determine the processes t h e age and  form of the  is  investigation  of i t s f o r m a t i o n .  Geologic  geologic  I t s p r o x i m i t y to t h e surface  coast  (15  fathoms)  investigation.  seamount, ( 5 3 ° l S » N , 1 3 5 ° 3 9 ' W ) ( S c r i r n g e r the  continental rise  o f the P r a t t - W e l k e r  chain  A l a s k a Seamount P r o v i n c e Welker group i s a l i n e s t r i k e s southeast  on  the  1969)  southern  ( F i g u r e I) i n the Gulf (Menard, 1 9 6 4 ) .  o f s e a m o u n t s and  The  o f Canada.  Pratt-  guyots which  f r o m K o d i a k Seamount i n t h e A l e u t i a n  East  o f B o w i e Seamount, a l o n g  c o n t i n e n t a l margin, l i e s ( S t . Arnand, 1 9 5 7  The  a major  part  of  T r e n c h t o w a r d s t h e Queen C h a r l o t t e I s l a n d s o f f t h e  be  and  Setting  s i t u a t e d on  coast  was  t h a t have c o n t r i b u t e d t o i t s d e v e l o p m e n t ,  t h e n e a r n e s s o f i t s summit t o the  The  British  seamount, t h e  make i t w e l l s u i t e d f o r g e o l o g i c 3.  100  Ocean  and  the  t h e Queen C h a r l o t t e F a u l t  Sutherland  Brown, 1 9 6 8 )  s t r u c t u r a l element of the East  r e l a t i o n s h i p s of the P r a t t - W e l k e r  Seamount t o t h e E a s t  west  Pacific  which  Pacific  c h a i n and  may Rise.  Bowie  R i s e , to the c o n t i n e n t a l  150°  INSET  145°  MAP BASED  ON U.S. N A V A L OCEANOGRAPHIC  140°  CONTOUR SHEETS:  135°  410 AND 411.  |35°  130"  125°  |3Q°  125°  3  margin and t o t h e r e c e n t l y d e l i n e a t e d m a g n e t i c anomaly bands i n t h e r e g i o n a r e s t i l l  not clear.  P r e v i o u s Work on the'Seamount O n l y one p r e v i o u s g e o l o g i c - e x p e d i t i o n h a s been made t o Bowie Seamount, published i n abstract  t h e r e s u l t s o f w h i c h were  form  ( H u r l e y a n d Nayudu,  1961).  These a u t h o r s f o u n d an e x t e n s i v e t e r r a c e n e a r t h e summit b e t w e e n 1 2 0 a n d 1 ^ 0 f a t h o m s , above w h i c h , p i n n a c l e s r i s e an a d d i t i o n a l 1 0 0 f a t h o m s . recovered palagonitic material fresh basalt  small  They  from t h e t e r r a c e a n d  from t h e peaks and c o n c l u d e d t h a t t h e In 1 9 6 2 ,  p e a k s were formed by p o s t - g l a c i a l v u l c a n i s m .  Nayudu p r o p o s e d t h a t t h e t e r r a c e was n o t e r o s i o n a l b u t a product of primary vulcanism. i n c l u d e d an a l k a l i  basalt  E n g e l and E n g e l ( 1 9 6 3 ) (  f r o m Bowie Seamount i n t h e i r  analyses of northeast P a c i f i c  basalts.  O t h e r work on  t h e seamount h a s been l i m i t e d t o i n f r e q u e n t  crossings  d u r i n g g e n e r a l b a t h y m e t r i c s u r v e y s c a r r i e d o u t by t h e U.S. Navy and t h e U.S. C o a s t and G e o d e t i c S u r v e y  around  t h e t i m e o f t h e Second W o r l d War. D.  Scope of_ t h e F i e l d j f t o j r k The f i e l d w o r k was c a r r i e d o u t o v e r a t o t a l o f  about  two weeks d u r i n g two s h o r t v i s i t s t o Bowie  Seamount i n t h e summers o f 1 9 6 7 a n d 1 9 6 8 .  The v e s s e l  u s e d f o r b o t h c r u i s e s was t h e C a n a d i a n N a v a l  Auxiliary  V e s s e l ENDEAVOUR, Establishment  f u r n i s h e d , by D e f e n c e R e s e a r c h  Pacific  i n Esquimalt,  British  Columbia.  J u s t o v e r 600 m i l e s o f echo s o u n d i n g l i n e s were r u n i n a p a t t e r n o v e r t h e seamount a n d b o t t o m s a m p l e s were obtained  f r o m s e v e n t e e n l o c a l i t i e s and u n d e r w a t e r  p h o t o g r a p h s from  four,  II. A.  F I E L D WORK  1967 Cruise The  first  phase o f t h e f i e l d w o r k on Bowie  Seamount was c o m p l e t e d d u r i n g t h r e e d a y s o f a two-week c r u i s e i n 1 9 6 7 devoted p r i m a r i l y to a continuous profiling  survey  west c o a s t .  seismic  o f t h e c o n t i n e n t a l s h e l f o f f Canada's  At t h a t time,  a p r e l i m i n a r y survey  was  made by echo s o u n d e r f o l l o w e d by d r e d g e a n d g r a b sampling  s t a t i o n s i n t h e s h a l l o w w a t e r s o f t h e summit.  Since  sufficiently  accurate  f i x i n g o f p o s i t i o n s by  LORAN o r by c e l e s t i a l n a v i g a t i o n v/as i m p o s s i b l e , n a v i g a t i o n was t o have been e f f e c t e d by f r e q u e n t  radar  f i x e s on an aluminum r e f l e c t o r mounted on a t a l l  spar  buoy w h i c h was moored on t h e summit o f t h e seamount. Unfortunately, shortly after  l o s s of the r e f l e c t o r t h e buoy was s e t o u t ,  i n rough weather necessitated  r e s o r t i n g t o n a v i g a t i o n by dead r e c k o n i n g  with  o c c a s i o n a l LORAN f i x e s a n d s i g h t i n g s o f t h e c r i p p l e d buoy,  5  The elongate  Maltese  circles. survey  p r o p o s e d s h i p ' s t r a c k was  T h i s p a t t e r n was  u s i n g the  covered  c r o s s w i t h two  and  a p p r o x i m a t e the a c t u a l course.  vicinity  adjacent  The  the  sonar  distance  Gyro compass t o Visual contacts with  one-mile square  of  s run m  one  sampled.  continuous  sounding  lines,  obtained  a f t e r c o r r e c t i o n s were made f o r c l o s u r e on  b a s i s of  five visual  f i x e s on  the  s h i p had  and  ocean c u r r e n t s d u r i n g the  the  seamount was Six  taken 3 •  on  t h e buoy, r e v e a l e d  been s e t so f a r t o t h e  southeast  survey  by  the  that wind  t h a t only h a l f of  a c t u a l l y mapped.  d r e d g e h a u l s and  the  the  to o b t a i n d e t a i l e d  c o n t r o l i n t h e a r e a t o be c o m p i l a t i o n of the  an  concentric  . A further close pattern  o f t h e buoy i n o r d e r  topographic  superimposed  s h i p ' s l o g to estimate  t h e buoy were r e c o r d e d .  form of  r e t a i n e d d u r i n g the  t h e h e a d i n g s on t h e  four mutually  i n the  s e v e n g r a b s a m p l e s were  c e n t r a l p e a k s and  the  terraces (Figure  2).  1 9 6 , 8 _ C r u i se  During  the 1 9 6 8  c r u i s e , t h e e n t i r e seamount  resurveyed.  More a c c u r a t e  c o v e r a g e was  obtained  out onto the  flanks (Figure 2 ) .  and  and  detailed  topographic  sample c o n t r o l was  run i n c o n j u n c t i o n w i t h the  was  extended  A magnetic survey  sounding survey  by  the  was  7  Department o f Geophysics a t t h i s U n i v e r s i t y ( M i c h k o f s k y , • 1969).  The m a g n e t i c  survey r e q u i r e d very close spacing  of l i n e s and c o n s e q u e n t l y a t o t a l o f 4 7 5 m i l e s o f simultaneous magnetic  and s o u n d i n g l i n e s were o b t a i n e d .  E m p l o y i n g t h e same b a s i c n a v i g a t i o n t e c h n i q u e a s t h a t o f t h e p r e v i o u s summer, t h a t i s , r a d a r f i x e s on a single in  s t a t i o n a r y t a r g e t , t h e s h i p ' s t r a c k was p l a n n e d  t h e f o r m o f an e x p a n d i n g r e c t a n g l e s u p e r i m p o s e d on a  series of sixteen r a d i a t i n g l i n e s ,  t h e whole r e s e m b l i n g  a r e c t a n g u l a r s p i d e r web ( F i g u r e 3 ) .  The r a d a r t a r g e t ,  a n c h o r e d on t h e summit o f t h e seamount,  naturally  formed t h e c e n t r e o f the p a t t e r n . S i n c e the s u c c e s s o f t h e e n t i r e v e n t u r e depended on a c c u r a t e and r e l i a b l e  n a v i g a t i o n a more e l a b o r a t e  n a v i g a t i o n s y s t e m t h a n t h a t e m p l o y e d i n 1 9 6 7 was u s e d . A r a d a r t r a n s p o n d e r ( A p p e n d i x A) was mounted on t o p o f a 2 0 f t . l o n g s t e e l s p a r buoy, 8 f t o f w h i c h above t h e w a t e r .  The u n i t ,  projected  t r i g g e r e d by t h e beam f r o m  the i n t e r r o g a t i n g r a d a r aboard the s h i p ,  replied  a u t o m a t i c a l l y w i t h a s t r o n g s i g n a l which produced a d i s t i n c t i v e d o u b l e b l i p on t h e s h i p ' s r a d a r s c r e e n . T h i s n a v i g a t i o n s y s t e m , w h i c h p r o v e d t o be h i g h l y reliable,  f u r n i s h e d a r a n g e a n d b e a r i n g t o t h e buoy a t any  t i m e w h i c h were a c c u r a t e - t o w i t h i n 0 . 2 m i l e s and 0 . 5 d e g r e e s r e s p e c t i v e l y a t d i s t a n c e s up t o 11+ m i l e s .  r' 3  I JO-  I5J*  FIGURE 3- . BOWIE  SEAMOUNT  SHIP'S TRACK CONTOUR INTERVAL - 100 fmt SCALE '  NAUTICAL WILES KILOMETRES  0  /J&OO'  135'50  I35'*0  I3S'30':.  "  I . 2  S «  5  .i • T  I/O'-..  9  I n t e r m i t t e n t weak s i g n a l s were p i c k e d up when t h e s h i p was  once l 6 m i l e s  f r o m t h e buoy.  During the survey,  f i x e s were t a k e n e v e r y h a l f m i l e o r a t even intervals,  t h e p o s i t i o n was p l o t t e d , a n d c o r r e c t i o n s  were made i n t h e s h i p ' s h e a d i n g ing set.  closer  to counteract the vary-  3y t h i s method, t h e s h i p was a b l e t o f o l l o w  c l o s e l y the planned the e n t i r e  ship's track, with the r e s u l t  that  s u r v e y a r e a was u n i f o r m l y c o v e r e d w i t h  accurate continuous sounding  profiles  (Figure 3 ) .  A d e t a i l e d s u r v e y o f t h e summit a b o v e t h e 2 0 0 fathom  l e v e l was made a t t h e end o f t h e 1 9 6 8 c r u i s e  ( F i g u r e 4 ) , a n d i n M a r c h o f 1 9 6 9 t h e summit was }  r e s u r v e y e d by D e f e n c e R e s e a r c h  Establishment  u s i n g CSS PAR1ZEAU ( F i g u r e 5 ) . t h e s e two i n d e p e n d e n t  not exceed system  the i n t r i n s i c  used  The maps r e s u l t i n g  s u r v e y s agree  n a u t i c a l miles (Scrimger, 1 9 6 9 ) ,  Pacific from  to within 0 . 2  a f i g u r e which  does  error of the radar n a v i g a t i o n  on t h e s u r v e y .  The U.B.C. s u r v e y  a s e r i e s o f p a r a l l e l t r a c k s spaced  employed  a quarter of a mile  apart w i t h n a v i g a t i o n f i x e s taken every quarter mile. To  t h i s was added d a t a f r o m  the sounding  lines  which  c r o s s e d t h e summit d u r i n g t h e l a r g e s c a l e s u r v e y o f t h e seamount ( F i g u r e 6 ) .  The D.R.E.P. s u r v e y u s e d  p a r a l l e l l i n e s spaced  one q u a r t e r o f a m i l e a p a r t w i t h  navigational  similar  f i x e s t a k e n a t t h e b e g i n n i n g a n d e n d o f eac  FIGURE k  BOWIE SEAMOUNT TOPOGRAPHIC DETAIL OF SUMMIT AREA ABOVE 200 FATHOMS CONTOUR INTERVAL -  20 FATHOMS  CONTOURS CONTROLLED BY SOUNDING LINES  < 1/2 MILE APART  CONTOURS CONTROLLED BY SOUNDING LINES 1 - 3 MILES APART SCALE  .  NAUTICAL MILES i i i  1/2  i  i  r  0  1  i  i  1/2  I  I 1/2  i  2  KILOMETRES II  Ml  I  V I  I  13 5° 39 200'W  FIGURE  5  -  Topogrophic  map of the summit of Bowie Seamount ( after Scrimger, 1 9 6 9 ) .  F  /  'J  LOCATIONS  OF PROFILES  APPEARING  Lines  run in 1968  Line  run in 1967. (location  IN  PLATE  approximate)  13  line. D r e d g i n g a t f o u r s t a t i o n s and u n d e r w a t e r  photo-  g r a p h y a t f o u r o t h e r s t a t i o n s were s u c c e s s f u l l y  under-  t a k e n d u r i n g the 1 9 6 8 c r u i s e but a t t e m p t s at s a m p l i n g i n deep w a t e r w i t h g r a b , s n a p p e r , d r e d g e with l i t t l e  -  met  success. III.  A  and c o r e r  Topography  GE OMOR PI-IOLO G Y  ,  C o m p i l a t i o n o f the s o u n d i n g r e c o r d s r e v e a l e d a c l o s e agreement i n d e p t h s a t s o u n d i n g l i n e usually  to w i t h i n 10 fathoms.  intersections,  Only a t 7 out o f a  o f 6 7 i n t e r s e c t i o n s d i d d e p t h s d i s a g r e e by 5 0 .or more.  The  o c c a s i o n a l discrepancy of t h i s  total  fathoms magnitude  i s t o be e x p e c t e d i n v i e ? ; o f t h e p o t e n t i a l s o u r c e s o f error.  The.maximum s l o p e m e a s u r e d on t h e seamount i s  3 0 ° b u t most s l o p e s r a n g e b e t w e e n 1 0 ° and 2 0 ° . • water depths range  from 1 5  t o 1,800  fathoms.  The  It i s  e s t i m a t e d t h a t the s t e e p s l o p e s , the g r e a t d e p t h s  ;  the  n a v i g a t i o n e r r o r o f 0 . 2 m i l e s , - and t h e 3 0 ° s p r e a d o f t h e s o n a r beam c o u l d e a s i l y 1 0 0 f a t h o m s o r more. in  combine t o p r o d u c e an e r r o r  A l l the major  of  discrepancies did,  f a c t , o c c u r i n deep w a t e r o r where s t e e p s l o p e s and'  rapidly The  changing bottom map  (Figure 7)  topography was  prevailed.  c o n t o u r e d by hand.  discrepancy i n depths at a - p r o f i l e i n t e r s e c t i o n  Where t h e was  15  l a r g e enough t o p r o d u c e a s i g n i f i c a n t contour l i n e s ,  s h i f t i n the  the l i n e w i t h the l e a s t  b a t h y m e t r i e r e c o r d was  discarded.  survey, m a l f u n c t i o n s i n the sonar  reliable  During p a r t s of  equipment r e s u l t e d i n  d i s c o n t i n u o u s r e c o r d s , hence t h e l e s s r e l i a b l e r e f e r r e d t o above. where r e l i a b l e soundings satisfy  bathymetry  I f a major d i s c r e p a n c y o c c u r r e d  l i n e s c r o s s e d each o t h e r , the  were d i s c a r d e d and  conflicting  t h e c o n t o u r s were drawn t o  s u r r o u n d i n g p o i n t s w h i c h were i n a g r e e m e n t .  A l t h o u g h the survey a r e a d i d not eastward map  extend  f a r enough  t o i n c l u d e a l l o f Bowie Seamount, t h e  resulting  shows t h a t t h e seamount g e n e r a l l y has t h e f o r m o f a  n o r t h e a s t - s t r i k i n g m o u n t a i n 1 3 m i l e s w i d e on t n e and a t l e a s t 3 0 m i l e s l o n g ( F i g u r e 7 ) abruptly The  the  which  from the g e n t l y s l o p i n g ocean f l o o r  r e g i o n a l s u r v e y by t h e U.S.  average  rises (Figure 8).  Navy ( F i g u r e 1 ,  inset)  i n d i c a t e s t h a t t h e n o r t h e a s t t r e n d o f Bowie Seamount meets the n o r t h w e s t  t r e n d of Hodgkins,  Davidson  and  Denson Seamounts a p p r o x i m a t e l y a t r i g h t a n g l e s on i t s northwest  s i d e , and  t h a t s e v e r a l o t h e r seamounts on  this  p a r t of the P r a t t - W e l k e r c h a i n are . s i m i l a r l y e l o n g a t e i n these  two  The  directions..  h i g h e s t peak, which  ocean f l o o r ,  stands 1 0 , 0 0 0  dominates the southwestern  seamount and i s e l o n g a t e d i n a  f t . above t h e  h a l f of  the  northeast-southwest  VERTICAL  HORIZONTAL  SCALE  0  NAUTICAL  0  500  1000  1000  2000  1500  0  SCALE MILES  1  2  3  KILOMETRES D  3000  I  2  3  4  5  6  7  2000  NW  SE  FIGURE 8. SILHOUETTES  O F  BOWIE  SEAMOUNT  17  direction.  To  the n o r t h e a s t , a lower t  ridge  continues  a l o n g t h e same t r e n d b u t a t a s l i g h t n o r t h e r l y o f f s e t , and  comprises  mountain.  most o f t h e r e m a i n i n g  Together,  these  two  o v e r a l l northeast-southwest Seamount.  half of  the  features control  the  e l o n g a t i o n o f Bowie  A s i m i l a r r i d g e s t r i k e s eastward  from  the  m a i n mass o r summit r i d g e , f o r m i n g a m a j o r s p u r like  the n o r t h e a s t r i d g e , extends  limit  o f t h e map  area.  To  beyond the  which,  eastern  the south of t h i s r i d g e , a  s m a l l l o b e p r o j e c t s beyond the  f o o t o f the  mountain  i n t o the s u r r o u n d i n g p l a i n of the ocean f l o o r .  The  e a s t e r n f l a n k o f t h e seamount i s f u r t h e r c o m p l i c a t e d a s e r i e s o f s m a l l p e a k s and The be,  flattened  by  valleys.  summit i s s e e n i n F i g u r e s k and  i n e f f e c t , a d i s c o n t i n u o u s t e r r a c e at a depth  5 to of  130  f a t h o m s w h i c h i s s u r m o u n t e d by a s e r i e s o f p e a k s , most of which are a l i g n e d a l o n g the n o r t h e a s t characteristic and  summit h i l l s  c u t by a t e r r a c e b e t w e e n 3 5 and B.  F i g u r e s 4 and  o f t h e seamount i t s e l f .  P l a t e 1 r e v e a l that these  trend  55  thought  f l a t - t o p p e d searnounts  o f e i t h e r a s t h e t r u n c a t e d and  of a n c i e n t v o l c a n i c i s l a n d s ,  are, i n turn,  fathoms.  T h e o r e t i c a l Development o f F l a t - T o p p e d Traditionally,  8,  Seamounts  have b e e n  subsided  foundations  o r a s a t o l l s t h a t have  PLATE 1 - S o n a r p r o f i l e s o f t h e s u r a a & t o f Bowie Seamount. C l e a r l y v i s i b l e a r e : the l o w e r t e r r a c e between 1 2 0 and I ' - L O f a t h o m s ( p r o f i l e s E-F, G ~ H and J - K ) , t h e summit h i l l s w h i c h r i s e a b o v e i t ( p r o f i l e s E - F and J - K ) , t h e u p p e r t e r r a c e b e t w e e n 3 5 and 5 5 f a t h o m s w h i c h f o r m s t h e f l a t t e n e d t o p s o f t h e summit h i l l s ( p r o f i l e s A-B, C-D, E-F and J - K ) , and t h e s m a l l , r u g g e d p i n n a c l e s w h i c h p r o j e c t , i n turn", above t h e l a t t e r t e r r a c e ( p r o f i l e s A-B, C-D and J - K ) . The l o c a t i o n ox t h e p r o f i l e s i s shown i n F i g u r e {$ ( o v e r l a y ) . 1  18  19  submerged due t o a t o o r a p i d r e l a t i v e r i s e Nayudu  proposed that the l a r g e r  (1962)  become more o r l e s s f l a t - t o p p e d a large  that  (Figure  following evidence:  slopes,  seamounts  due t o t h e g r o w t h o f  dome o f magma w i t h i n t h e v o l c a n o b e n e a t h a  p y r o c l a s t i c mantle the  of sea l e v e l .  9).  1.  H i s argument i s b a s e d on  G u y a t s have g e n e r a l l y  steep  i n the o r d e r o f 1 8 ° t o 2 5 ° which would i n d i c a t e  t h e y were composed o f f r a g m e n t a l m a t e r i a l  t h a n l a v a flows..  He c i t e s t h e o c c u r r e n c e o f  rather palagonite  t u f f s on t h e t e r r a c e s  o f Bowie and Cobb Seamounts a s  supporting  2 . S e i s m i c r e f r a c t i o n w o r k on  evidence.  Bikini Atoll high  ( R a i t t , 1 9 5 4 ) h a s shown t h e e x i s t e n c e  v e l o c i t y , probably b a s a l t i c core,  of a  c a p p e d by a  l o w e r v e l o c i t y , p o s s i b l y p y r o c l a s t i c l a y e r , upon w h i c h the  reef  limestone i s b u i l t .  Jones ( 1 9 6 6 )  and K j a r t a n s s o n  (1966)  proposed  that  flat-topped  seamounts a r e submarine a n a l o g u e s o f t h e  Pleistocene  i n t r a g l a c i a l v o l c a n o e s o f I c e l a n d and  British  Columbia (Figure  10).  These  flat-topped  v o l c a n o e s a r e t h o u g h t t o have grown w i t h i n l a k e s t h r o u g h a n i c e c a p by v o l c a n i c h e a t and  Mathews,  usually  1947).  composed  melted  (Kjartansson, 1 9 4 3  The b a s e Of s u c h a v o l c a n o i s  o f p i l l o w l a v a s , p i l l o w b r e c c i a s and  h y a l o c l a s t i t e s w h i c h f o r m e d by q u i e t  effusion of  20  -  '  sea level  I.  .  sea level  2.  Figure 9 Method of f o r m a t i o n suggested by Nayudu ( 1 9 6 2 ) .  of f l a t - t o p p e d  seamounts  Stage I : Eruption on sea floor produces a mound of c l a s t i c debris derived from the aqueous brecciation of lava flows. Stage 2 : Growth of a dome beneath the clastic mound. The clastic mantle continues to grow, fed by lava reaching the surface. Stage 3 : Continued growth as in stage 2 with the development of more complex structure.  product of successive eruptive phases during  (d)'Terraced' and subsided.  (b) Emerged.  (a)Submerged. FIGURE 1 0 suggested  by  Method of formation of Jones  (1966).  flat-topped  seamounts  22  basaltic  l a v a i n deep w a t e r  where p e r h a p s  p r e s s u r e was g r e a t enough t o p r e v e n t volcanic and  the c o n f i n i n g  p h r e a t i c and  e x p l o s i o n s (McBirney, 1 9 6 3 ; K j a r t a n s s o n , 1 9 6 6 ;  Jones,  1967  in-Moore  and F i s k e , 1 9 6 7 ) .  The u p p e r  r e a c h e s o f t h e v o l c a n o a r e dominated, by c l a s t i c d e b r i s which  commonly o c c u r s i n s t e e p ,  b e d s , a n d above t h i s ,  volcanic  outward-dipping  t h e f l a t mountain t o p i s formed  by a s h i e l d o f s u b a e r i a l b a s a l t  flows.  The c l a s t i c  m a t e r i a l i s apparently the product p a r t l y o f p h r e a t i c e x p l o s i o n s , which  c h a r a c t e r i s t i c a l l y accompany  e r u p t i o n s i n s h a l l o w v a t e r , and p a r t l y subaerial lake  of brecciation of  f l o w s where t h e y e n t e r e d t h e w a t e r s o f t h e  ( F u l l e r , 1 9 3 1 ) ••..If the l a v a s h i e l d i s absent, i t  i s presumed e i t h e r  t h a t i t was removed by p r o l o n g e d wave  erosion, exposing the c l a s t i c  p l a t f o r m , or t h a t i t never  e x i s t e d and t h a t t h e v o l c a n o succeeded  only i n b u i l d i n g  a b r o a d , wave-washed p l a t f o r m o f t e p h r a . (1966)  Kjartansson's  w o r k on S u r t s e y s u g g e s t s t h a t t h i s y o u n g v o l c a n o  may be a r e c e n t m a r i n e mountains.  analogue  of the Icelandic  Moore a n d F i s k e ( 1 9 6 9 )  observed  s u b s t r u c t u r e o f K i l a u e a V o l c a n o , H a w a i i , may fit  basaltic  t h i s model.  table-  that the similarly  R a i t t ' s s e i s m i c d a t a on B i k i n i  Atoll  a p p a r e n t l y s u p p o r t s t h i s model a s w e l l a s i t d o e s Nayudu's.  23  C.  G e o m o r p h i c D e v e l o p m e n t o f Bowie Seanio_unt_ Because r o c k s  dredged from the  exclusively basalt vitrophyres volcanic textures presumed t o be opinion, affect has  of v o l c a n i c o r i g i n .  seamount i s  In the  author's  on a s c a l e l a r g e enough t o  g r o s s v o l c a n i c m o r p h o l o g y o f the  i s known o f t h e  Bowie Seamount b u t  seamount  the  volcanic substructure  of  p r o m i n e n t r i d g e s t h a t make  v o l c a n o s u g g e s t t h a t i t grew f r o m e r u p t i o n s  a system of along  f i s s u r e s ( F i g u r e 7,  overlay).  a main n o r t h e a s t - s t r i k i n g set of  formed the feeders  of  occurred.  Little  the  w i t h a wide v a r i e t y  s t r u c t u r e s , the  submarine e r o s i o n  the  not  and  seamount care a l m o s t  long  summit and  northeast  p r o b a b l y u n d e r l i e the  southeastern  flank.  along  Extrusions  f i s s u r e s probably  ridges.  s p u r s on  up  the  Subordinate  volcano's  A s e c o n d m a j o r f r a c t u r e i s assumed  t o u n d e r l i e H o d g k i n s and  D a v i d s o n Seamounts, w h i c h e x t e n d  northwestwards i n a l i n e  f r o m Bowie Seamount ( F i g u r e s  and  7).  The  i n t e r s e c t i o n of the  f r a c t u r e b e n e a t h Bowie w i t h t h e f r a c t u r e of the afforded  northeast-trending northwest-trending  Hodgkins - Davidson system ray  t h e magma r e l a t i v e l y  have  easy a c c e s s to the  and  t h u s have a c c o u n t e d f o r t h e  the  summit r i d g e .  1  huge v o l c a n i c p i l e  Bowie Seamount, t h e r e f o r e ,  surface of  appears  24  t o bo a c o m b i n a t i o n c e n t r a l a n d f i s s u r e The  network of i n t e r n a l  conduits  type  volcano.  i n a volcano of t h i s  s i z e a n d shape must be f a r f r o m s i m p l e  a n d many o f t h e  s m a l l e r p e a k s on t h e s e a m o u n t ' s f l a n k s may be p a r a s i t e cones. The are  n a t u r a l 1 0 ° t o 2 0 ° s l o p e s o f t h e summit r i d g e  t r u n c a t e d .around 1 4 0 f a t h o m s , f r o m w h i c h d e p t h a  broad t e r r a c e slopes  g e n t l y up t o a d e p t h o f 1 2 0 f a t h o m s  ( F i g u r e 8 and P l a t e 1 ) .  Above t h i s p l a t f o r m r i s e  hills  300 t o 7 0 0 f t . high which b a r e l y a l t e r the f l a t - t o p p e d s i l h o u e t t e of the 1 0 , 0 0 0 highest  f t . high volcano.  o f t h e s e summit h i l l s  are, i n turn,  The f o u r terraced  between 3 5 and 5 5 fathoms ( F i g u r e 4 and P l a t e 1 ) .  Small,  r u g g e d p e a k s p r o j e c t an a d d i t i o n a l 1 0 0 t o 2 0 0 f t . above t h i s upper t e r r a c e l e v e l , 100  the t a l l e s t  f t . , o f t h e ocean s u r f a c e The  hills  surface  coming t o w i t h i n  (Scrimger,  1969)•  m a t e r i a l on t h e t e r r a c e s and summit  i s made up m a i n l y  of tephra  The  freshness  the  terraces to sea l e v e l  and b r o k e n  o f t h e v o l c a n i c e j e c t a and t h e n e a r n e s s o f s u g g e s t t h a t t h e t e r r a c e s were  p r o d u c e d by c o m b i n e d wave e r o s i o n a n d s h a l l o w vulcanism  flow-rock.  during  water  l a t e Q u a t e r n a r y s e a l e v e l l o w s , and  t h a t t h e summit h i l l s mark t h e c e n t r e s A d d i t i o n a l evidence supporting  of eruption.  wave e r o s i o n i s s e e n i n  25  the d i s t i n c t l y abraded  and winnowed n a t u r e o f some o f  the l a p i l l i  Two s u c h d e p o s i t s wore d i s c o v e r e d :  one  deposits.  on o r j u s t b e l o w t h e 4 5 f a t h o m  67 - 7, A p p e n d i x  w e l l rounded and  (sample 6 7 - 2 ,  3) and t h e o t h e r j u s t o f f t h e 1 3 0 f a t h o m  Appendix (sample  terrace  shell  lapilli  B).  These a r e b o t h composed o f  and c o n t a i n a b r a d e d  fragments;  such s h e l l  phenocrysts  fragments being very  c o n s p i c u o u s i n sample 67 - 2 ( P l a t e 2 ) . s o r t i n g o f t h e g r a i n s cannot  terrace  Although the  be s a i d t o be good i n  e i t h e r d e p o s i t , i t i s much b e t t e r i n sample 67.-- 7 t h a n in  any o t h e r l a p i l l i The  d e p o s i t f o u n d on t h e seamount.  present depth o f the t e r r a c e s suggests t h a t  must have b e e n formed either level  they  a t some t i m e i n t h e p a s t when  t h e seamount s t o o d h i g h e r above t h e g e o i d o r s e a stood lower.  An e s t i m a t e d maximum s e a l e v e l  l o w e r i n g o f 5 4 0 f t . i s g i v e n by S h e p a r d Pleistocene accounted  (1963)-  A low  s t a n d , t h e r e f o r e , c o u l d c o n c e i v a b l y have  f o r t h e upper  t e r r a c e , whose d e p t h  varies  from 2 1 0 t o 3 3 0 f t . but a p p a r e n t l y not f o r t h e lower t e r r a c e , which has a depth o f 7 2 0 t o 8 4 0 f t . r e a s o n a b l e t o suppose,  however, t h a t  It i s  t h e seamount  itself  may have s u b s i d e d s i n c e t h e f o r m a t i o n o f t h e l o w e r Subsidence are  terrace.  o f o c e a n i c v o l c a n o e s i s commonplace and g u y o t s  common on t h e P r a t t - V.'elker c h a i n .  PLATE 2 - Reworked L a p i l l i  Deposits  (a.) W e a k l y c e m e n t e d v o l c a n i c conglomeratef o u n d i n t h e v i c i n i t y o f t h e 4 5 fathom terrace. The r o c k i s composed o f w e l l rounded l a p i l l i , f r o s t e d p h e n o c r y s t s , and a b r a d e d s h e l l f r a g m e n t s ( w h i t e i n t h e p h o t o g r a p h ) ( S t a t i o n 67 - 2 ) . (b) L o o s e d e p o s i t o f w e l l - r o u n d e d l a p i l l i •and f r o s t e d p h e n o c r y s t s , f o u n d n e a r t h e edge o f t h e 1 3 0 f a t h o m t e r r a c e . The s o r t i n g o f t h i s d e p o s i t i s b e t t e r t h a n t h a t o f any o t h e r on the- seamount. The t h u m b - t a c k i n t h e l o w e r r i g h t hand c o r n e r p r o v i d e s t h e s c a l e ( S t a t i o n 67 -  7)  26  27  Ignoring for  s u b s i d e n c e , one can  t h e upper t e r r a c e  curves (Figure  11)  Hopkins, 1 9 6 7 ) . unstable, certain 330  loose  that  estimate  from p u b l i s h e d  a maximum age  Quaternary sea l e v e l  (Kenney, 1 9 6 4 , C u r r a y , 1965> and  The u p p e r t e r r a c e i s composed o f o r w e a k l y cemented m a t e r i a l a n d i t seems  the t e r r a c e , which has a depth o f 2 1 0 t o  f t . , w o u l d n o t s u r v i v e i f s e a l e v e l were t o d r o p f o r to the l e v e l  This  t e r r a c e t h e n c o u l d n o t have been i n e x i s t e n c e  to the low stand  of i t s l a t e Wisconsin  low o f 4 0 0 f t .  long  prior  o f 1 8 , 0 0 0 t o 2 2 , 0 0 0 y e a r s 3 . P . a n d must,  t h e r e f o r e , have f o r m e d d u r i n g According  1  to the sea l e v e l  the Holocene  curves,  transgression.  Holocene sea l e v e l  was  a t t h e p r e s e n t u p p e r t e r r a c e l e v e l b e t w e e n 1 3 , 0 0 0 and 18,000  since  y e a r s ago. that  time,  I f the volcano has, i n f a c t ,  subsided  t h e t e r r a c e must be even y o u n g e r ,  The  p r o p o s e d mjoximum age o f t h e u p p e r t e r r a c e i s , t h e r e f o r e , 18,000  years.  No s a t i s f a c t o r y  age c o u l d be e s t a b l i s h e d  lower t e r r a c e but there  i s a high  f o r the  probability  that i t  t o o was f o r m e d d u r i n g  the Pleistocene.  material i s virtually  a b s e n t on b o t h t e r r a c e s , b u t  o c c u r s i n abundance on t h e n o r t h o f n e a r b y H o d g k i n s Seamount  Ice-rafted  f l a n k and on t h e summit  (Appendix B ) .  The summit o f  B o w i e Seamount may, t h e r e f o r e , have stood' h i g h  enough  28  METRES  BELOW  PRESENT  SEA  LEVEL  during  low  Pleistocene  debris-laden  icebergs which, during  have c a l v e d i n g r e a t sea. it  s t a n d s to p r e v e n t the  would n e v e r t h e l e s s  tephra  terraces contain only recent f a u n a s (Cameron, 1 9 6 9  s u r v i v a l of a  itself  f e a t u r e was  on  pre-Pleistocene In view of  the  fathom t e r r a c e , i t i s  also  formed d u r i n g  of low P l e i s t o c e n e  evidently instrumental  i n the  However, e v i d e n c e o f  v o l c a n i c a c t i v i t y i n d i c a t e s t h a t the due  terrace  late  time.  these t e r r a c e s .  i s not  might  c o n t i n u e d • u n t i l so r e c e n t l y  Wave e r o s i o n d u r i n g p e r i o d s l e v e l s was  Post-  pre-Pleistocene  to obscure the  e x c e l l e n t p r e s e r v a t i o n of the 130  Pleistocene  micro-  communication).  t e r r a c e w o u l d seem h i g h l y u n l i k e l y .  probable that t h i s  both  accumulated g l a c i a l m a t e r i a l .  i n fact,  summit t h a t t h e  debris.  the l o w e r t e r r a c e , and  However, i t w o u l d a l s o t e n d  the  Pleistocene  m i g h t have o b s c u r e d a  a l s o have b u r i e d any  V u l c a n i s m has,  and  personal  f a u n a , i f s u c h e x i s t e d .on  to  q u a n t i t y of i c e - r a f t e d t u f f s of  the feature  ample o p p o r t u n i t y  d e p o s i t s and  of  must  were a p r e - P l e i s t o c e n e  have had  accumulate a considerable  terrace vulcanism  such p e r i o d s ,  numbers f r o m g l a c i e r s r e a c h i n g  I f the l o w e r . t e r r a c e  Furthermore the  passage  t o wave e r o s i o n  alone.  formation  sea of  post-glacial summit m o r p h o l o g y  30  The  summit p e a k s a r e s t r i k i n g i n t h e i r  parallel  alignment  t o t h e l o n g a x i s o f t h e seamount ( F i g u r e 5 ) ,  an a l i g n m e n t w h i c h was p r o b a b l y s t r u c t u r e o f the volcano.  c o n t r o l l e d by t h e  I f these  are erosional  r e m n a n t s o f a once g r e a t e r p e a k t h a t h a s been d i s s e c t e d , they  w o u l d have t o be made o f r e l a t i v e l y  r e s i s t a n t rock t o preserve i n which case, dykes.  deeply  their  structural  t h e y m i g h t be a l i n e  more  alignment,  of volcanic plugs or  Work on p a r t o f t h e c e n t r a l L - s h a p e d s y s t e m o f  peaks (Figure 2 ) has i n d i c a t e d that the b u l k o f the surface tephra  cover' i s composed o f l o o s e o r w e a k l y and l e s s e r amounts o f f r a g m e n t a l  shallowest be  flow rock.  p i n n a c l e was f o u n d by d r e d g i n g  The  incoherence  are not simply  volcanic  tuffs.  plugs.  o f t h e m a t e r i a l m a k i n g up t h e c e n t r a l  summit p e a k s p r e c l u d e s  the p o s s i b i l i t y  of their  survival  f o r l o n g a s an i s l a n d d u r i n g t h e l o w s e a l e v e l w h i c h formed t h e lower that a tephra  The  and d i v i n g t o  composed o f s t r o n g l y cemented f e r r u g i n o u s  Some p e a k s , t h e n ,  cemented  sea c l i f f  terrace.  Richards  stand  (1965)  w i t h an a v e r a g e h e i g h t  reported  of 2 0 0 f t . ,  f o r m e d by t h e 1 9 5 2 e r u p t i o n o f B a r c e n a on I s l a San Benedicto,  west o f M e x i c o , was c u t b a c k b y wave  a t an a v e r a g e r a t e o f 5 . 5 f t . p e r d a y .  erosion  An a s s o c i a t e d  a a l a v a f l o w was s i m i l a r l y e r o d e d a t an a v e r a g e  daily  31  r a t e o f 0J+ f t . p e r day.. At s u c h r a t e s , t h e summit p e a k s , i f t h e y were i s l a n d s , w o u l d have been w i t h i n a year.'  I t i s probable, therefore,  destroyed  that the  summit h i l l s were f o r m e d by v o l c a n i c e r u p t i o n s the  formation  and submergence o f a summit  w h i c h now e x i s t s i n r e l i c  after  plateau,  form as t h e 1 3 0 fathom t e r r a c e .  By t h e same t o k e n , t h e 4 5 f a t h o m t e r r a c e l e v e l may be a later planation The t e p h r a  surface  c a p p e d by l a t e t u f f  cones.  on t h e summit o f t h e seamount a r e , f o r t h e  most p a r t , e x t r e m e l y f r e s h ( C h a p t e r IV), a s w o u l d be expected the  from s u c h r e c e n t  vulcanism.  summit i m p l i e s , t h e r e f o r e ,  continued  a f t e r the formation  that  The e v i d e n c e on eruptive  and submergence o f t h e  u p p e r t e r r a c e l e s s t h a n 1 8 , 0 0 0 y e a r s ago. the  conclusion  130  This  r e a c h e d by H u r l e y and Nayudu  p o s t - g l a c i a l vulcanism  activity  confirms  (1961)  that  f o r m e d t h e summit p e a k s on t h e  fathom t e r r a c e . T h i s apparent c l o s e a s s o c i a t i o n i n time between t h e  latest  episodes of vulcanism  summit t e r r a c e s a t s e a l e v e l Seamount i s n o t s i m p l y old  and t h e f o r m a t i o n  s u g g e s t s t h a t Bowie  the truncated  v o l c a n i c i s l a n d t h a t was f i n a l l y  l o n g p e r i o d o f wave e r o s i o n . likely  of the  foundation  o f an  washed away a f t e r a  Instead,  i t a p p e a r s more  that the volcano reached sea l e v e l  during  the l a s t  32  stages  o f i t s a c t i v i t y and s t r u g g l e d  build itself  f o ra while to  up above t h e s e a s u r f a c e .  Any v o l c a n i c  i s l a n d s w h i c h may have t h u s been f o r m e d were repeatedly  planed  o f f by wave a c t i o n e v e r t h e c o u r s e o f  months o r p e r h a p s y e a r s , flat  platforms  giving rise,  i n the end, t o  composed o f p y r o c l a s t i c and e r o s i o n a l  d e b r i s , w h i c h now r e m a i n a s t e r r a c e s . islands did, i n fact, lava  probably  exist  I f ephemeral  the b r e c c i a t i o n of s u b a e r i a l  flows e n t e r i n g the sea ( F u l l e r ,  1931) say also  c o n t r i b u t e d s u b s t a n t i a l l y t o the growth o f these  platforms  by b u i l d i n g o u t f o r s e t b e d s o f f l o w - f o o t b r e c c i a s . b e h a v i o u r o f Bowie Seamount  during  i960)  and S u r t s e y  (Thorarinsson,  196k a n d  w h i c h have been o b s e r v e d ' e r u p t i n g a t s e a l e v e l .  The p o s t - P l e i s t o c e n e  r i s e i n sea. l e v e l ,  the subsidence  of t h e v o l c a n o and t h e t a p e r i n g o f f o f v o l c a n i c apparently and  to that  submarine v o l c a n o e s such as C a p e l i n h o s  (Zbyszewski, 1965)  The  l a t e P l e i s t o c e n e and  H o l o c e n e t i m e may, t h e r e f o r e , have been s i m i l a r o f modern  have  activity  b r o u g h t an end t o i t s b r i e f s u b a e r i a l h i s t o r y  i t has probably  remained i n a c t i v e ever  since.  The o r i g i n o f t h e t e r r a c e s a p p e a r s t o f i t t h e model s u g g e s t e d by J o n e s ( 1 9 6 6 )  and K j a r t a n s s o n  t h a n t h a t p r o p o s e d by Nayudu  (1962).  (1966)  The t e r r a c e s a r e  known t o be p r e s e n t l y c o v e r e d l a r g e l y by c l a s t i c and  t h e summit p e a k s a r e a p p a r e n t l y  rather  composed  debris  of t u f f s  33  b u t w h e t h e r a l a v a s h i e l d e v e r e x i s t e d on t h e summit o f t h e seamount i s n o t known. breccias  Pillow lavas  and p i l l o w  a r e a p p a r e n t l y more common on t h e s l o p e s o f  t h e seamount t h a n on t h e summit ( C h a p t e r I V ) . modified version  o f Jones's diagram  suggested present s t r u c t u r e .Seamount a p p e a r s i n F i g u r e Late flank eruptions  A  d e p i c t i n g the  o f t h e summit o f Bowie 12.  appear  t o have o c c u r r e d ,  p o s s i b l y a t t h e t i m e o f t h e f i n a l summit e r u p t i o n s , f o r deposits  of broken p i l l o w s of very f r e s h , b l a c k ,  basalt, virtually are  free of o r g a n i c growth or a l t e r a t i o n ,  found 8 0 0 fathoms  down t h e west f l a n k n e a r a  cone on t h e same l i n e a r t r e n d  boulder o f v e s i c u l a r basalt,  2).  f i s h e d from t h e upper  a t s t a t i o n 6 8 - 9 was d a t e d by t h e w h o l e - r o c k  p o t a s s i u m - a r g o n method and f o u n d t o be 7 5 > 0 0 0 100,000  small  a s t h e summit p e a k s  ( s t a t i o n 6 8 - 7 ) ( P l a t e s 3 and 4 , a n d F i g u r e  terrace  glassy  years o l d .  method f o r d a t i n g come u n d e r f i r e  The u s e o f t h e p o t a s s i u m ~ a r g o n  young, g l a s s y ,  recently.  half l i f e  1/8400  submarine  lavas has  P o t a s s i u m makes up l o s s  Z% o f most b a s a l t s and K ^ , c o n s t i t u t e s about  -  the r a d i o a c t i v e  isotope,  o f the t o t a l potassium.  o f 1'}''® i s 1 , 3 1 0 m i l l i o n y e a r s and  t h e amount o f r a d i o g e n i c  argon i n a b a s a l t  than  less  The therefore than  SE  NW  FIGURE 1 2 Suggested structure of Bowie Seamount according to the Jones hypothesis. The diagram represents a hypothetical northwest - southeast cross - section through the main body of the seamount.  \ \ \  Tuff,  ash,  Pillow lava,  and possible  flow-foot  pillow breccia  breccia.  and hyaloclastites.  PLATE 3 - P i l l o w f r a g m e n t s on t h e west f l a n k o f Bowie Seamount a t S t a t i o n 6 8 - ?. The d i a m e t e r o f t h e compass i s 3 i n c h e s and t h e l e n g t h o f t h e vane i s 1 0 i n c h e s .  35  PLATE 4 - L o o s e d e p o s i t o f s m a l l p i l l o w c r u s t f r a g m e n t s and l a r g e r p i l l o w f r a g m e n t s on t h e west f l a n k (Station 68 - 7 ) . In the lower photograph, t h e t h i n c o v e r o f f i n e v o l c a n i c and p e l a g i c s e d i m e n t h a s b e e n d i s t u r b e d by t h e c a m e r a frame and t h e compass. The d i a m e t e r o f t h e compass i s 3 i n c h e s and t h e l e n g t h o f t h e vane i s 1 0 i n c h e s .  36  1 m i l l i o n years o l d i s exceedingly small. volcanic  glass w i l l  l o s e argon w i t h the passage  r e s u l t i n g i n a younger Baadsgaard, 1 9 6 7 ) .  Furthermore  than a c t u a l date  Fluid inclusions,  ( H i l l s and  common i n  p h e n o c r y s t s , may c o n t a i n a r g o n , w h i c h w i l l  g i v e an  a n o m a l o u s l y o l d age ( a s much a s + 8 8 0 11.Y.) et  a l , 1966).  submarine  of time,  (Funkhausor  G l a s s p r o d u c e d by r a p i d q u e n c h i n g o f  l a v a s may c o n t a i n t r a p p e d a r g o n - 1+0 w h i c h d i d  n o t have a c h a n c e  t o e s c a p e by v e s i c u l a t i o n ,  or, i f the  h y d r o s t a t i c p r e s s u r e i s g r e a t enough, a r g o n w i l l  be  s i m i l a r l y r e t a i n e d due t o l a c k o f v e s i c u l a t i o n ,  both  s i t u a t i o n s y i e l d i n g o l d e r than a c t u a l and Moore, 1 9 6 8 ) .  (60  inclusions,  (Dalrymple  To a v o i d a s much e r r o r a s p o s s i b l e ,  a vesicular rock, free of c h i l l e d fluid  ages  s u r f a c e phenomena a n d  w h i c h was f o u n d i n s h a l l o w w a t e r  f a t h o m s ) was s e l e c t e d  f o r t h e age dating..  S i n c e Bowie Seamount was a p p a r e n t l y s t i l l  active  t o w a r d s t h e end o f t h e P l e i s t o c e n e i t i s o f g r e a t interest floor. (1969)  t o know when i t f i r s t  a p p e a r e d on t h e s e a  On t h e b a s i s o f m a g n e t i c  data, Michkofsky  b e l i e v e s t h a t t h e m a i n body o f t h e seamount was  formed d u r i n g a p e r i o d o f  geomagnetic  reversal,  p u t t i n g i t s age b a c k a t l e a s t 0 . 6 9 m i l l i o n y e a r s ( C o x , 1969).  I t s age beyond  this  c a n o n l y be g u e s s e d a t .  38  IV. A.  PETROLOGY•  Lava Flows 1.  T y p e s and d i s t r i b u t i o n o f f l o v ; r o c k •  R o c k s w h i c h a p p e a r t o have o r i g i n a t e d by q u i e t were r e c o v e r e d  from n i n e  flow  s t a t i o n s s c a t t e r e d over the  seamount ( A p p e n d i x B ) .  On t h e b a s i s o f t h e i r p h y s i c a l  appearance, these  f a l l i n t o three  rocks  pillow  fragments, tops  of non-pillowed  highly  j o i n t e d non-vesicular rock.  categories: l a v a f l o w s , and  Most o f t h e s e  occurred  as s u r f a c e r u b b l e i n t h e areas sampled.  one  ( s t a t i o n 67 - 4 ) were s a m p l e s t a k e n  case  a p p e a r e d t o be a c o h e r e n t l a v a (i)  Pillow  rocks In only  f r o m what  flow,  Fragments  About h a l f o f t h e samples o f f l o w r o c k a r c fragments o f p i l l o w s . istic  pyramidal  shape  Most o f t h e s e  have t h e c h a r a c t e r -  ( P l a t e 5a) and a q u e n c h e d  s u r f a c e marked by a t r a n s i t i o n  outer  from a m i c r o l i t c - r i c h ,  tachylyte vitrophyre i n the i n t e r i o r melane v i t r o p h y r e a t t h e s u r f a c e  t o a c l e a r sidero;-  (Plate 6).  Palagonite  r i m s a r e uncommon, b u t may be l o c a l l y w e l l d e v e l o p e d (Plate 3 2 ) . Sideromelane i s thought t o form a t t h e s u r f a c e o f a s u b a q u e o u s f l o w where b a s a l t i c  lava i s chilled  so q u i c k l y  t h a t i r o n i s r e t a i n e d i n s o l u t i o n , i m p a r t i n g an amber c o l o u r t o the t r a n s l u c e n t g l a s s , whereas t a c h y l y t e  forms  PLATE 5 - T y p e s o f f l o w - r o c k f o u n d on Bowie Seamount (a) P i l l o w f r a g m e n t s . The l o c a t i o n s o f t h e s a m p l e s a r e , from l e f t t o r i g h t : S t a t i o n 6 8 - 7 , Station 6 8 - 1 , and S t a t i o n 6 7 - 2 . I n t a c t p i l l o w s were f o u n d a t Station 6 8 - 1 . (b) F r a g m e n t from t h e s u r f a c e o f a f l a t , non-pillowed l a v a flow. ( S t a t i o n 68-1) ( c ) B l o c k l a v a o r t a l u s m a t e r i a l e r o d e d from an o u t c r o p p i n g dyke o r d i s s e c t e d f l o w . (Station 67-7)  PLATE 6 - The t e x t u r a l change f r o m t a c h y l y t e t o sideromelanc i n the c h i l l e d surface of a vesicular basaltic lava. The s u r f a c e o f the r o c k i s out o f v i e w , j u s t beyond the top of the photograph. (Plane l i g h t ) ( T h i n s e c t i o n 68 - 7 - B ) . ( x 2 7 )  j u s t b e l o w t h e s u r f a c e where t h e l a v a c o o l s  less  rapidly,  magnetite  allowing the p r e c i p i t a t i o n of fine  which renders  t h e g l a s s b l a c k a n d opaque ( P e a c o c k , 1 9 2 8 )  It i s d i f f i c u l t  to t e l l  fragments represent  whether these  the products  have b r o k e n up i m m e d i a t e l y  pillow  of p i l l o w s which  after  formation, or of  p i l l o w s w h i c h have d i s i n t e g r a t e d d u r i n g p o s t - v o l c a n i c erosion. cover  P i l l o w fragments formed 1 0 0 % o f t h e l o o s e  a t S t a t i o n 6 8 - 7 on t h e s t e e p l y s l o p i n g w e s t  f l a n k o f t h e seamount ( P l a t e s 3 a n d 4 ) a n d were a l s o f o u n d i n a b u n d a n c e a t S t a t i o n 6 8 - 1 on t h e e a s t  flank  ( P l a t e s 2 5 and 2 6 ) . (ii)  Tops o f n o n - p i l l o w e d  flows  Fragments o f l a v a flows without  pillow  s t r u c t u r e were r e t r i e v e d a t s e v e r a l l o c a t i o n s ( A p p e n d i x B).  The t o p s o f t h e s e  sideromelane  chill  flows e x h i b i t  the t a c h y l y t e - t o -  t e x t u r e d e s c r i b e d above a n d may have  a thin skin of palagonite.  Bread c r u s t f r a c t u r i n g i s  common on t h e u p p e r s u r f a c e  (Plate 5 b ) .  (iii)  Angular,  non-vesicular  Whether t h i s t y p e  rock  of rock represents a flow  o f " B l o c k Lava", a s d e s c r i b e d by W a s h i n g t o n ( 1 9 2 6 ) a n d Finch  (1933),  o r t a l u s m a t e r i a l from an exposed dyke o r  p l u g c a n n o t be a s c e r t a i n e d .  The o n l y two s p e c i m e n s a r e  42  h i g h l y but i r r e g u l a r l y  jointed, angular, non-vesicular  ( P l a t e 5 c ) , the only massive v o l c a n i c rock  blocks  obtained 2 •  from t h e seamount. £^ t r o g r a p h y o f _ t h c_ _La v as  Ba_salt_ The  r o c k s o f Bowie Seamount a r e m a i n l y  basaltic,  a l t h o u g h a n d e s i t i c v a r i e t i e s do o c c u r r a r e l y . b a s a l t s are of the a l k a l i 1962)  olivine  type  (Yoder  environments  volcanic  Their occurrence  structure i s compatible  E n g e l and Engel  (1964),  which  l a r g e r oceanic volcanoes. previous workers,  on a h i g h  with the hypothesis of  l i m i t s such r o c k s t o t h e  Their results,  suggest  that a l k a l i  r e p r e s e n t l a t e phases of v u l c a n i s m which formed p r i m a r i l y o f t h o l c i i t i c listed  similar  ( c f . E n g e l and E n g e l , 1 9 6 3 , 1 9 6 4 , 1 9 6 6 a n d  Engel et a l . , 1 9 6 5 ) -  of  and T i l l e y ,  ( T a b l e s 1 a n d 2 ) and a r e a p p a r e n t l y s i m i l a r i n  c o m p o s i t i o n t o o t h e r o c e a n i c b a s a l t s from  of  The  basalt.  and t h e r e s u l t s olivine basalts crown  volcanoes  The b a s a l t s  i n t h e t a b l e s were d r e d g e d from t h e summit r i d g e  Bowie Seamount i n d e p t h s  s h a l l o w e r than 5 0 0 fathoms.  I t i s p o s s i b l e t h a t t h e r o c k s on t h e l o w e r r e a c h e s o f t h e seamount a r e t h o l c i i t i c . The  alkali  o l i v i n e b a s a l t , which  i s t h e most  s p r e a d r o c k t y p e , i s a "dark g r e y , v e s i c u l a r ,  wide-  porphyritic  Oyide  ?7 - 7 .  53.5 Ti0  1.07  2  67.j:„.6  -„12.  45.4 2.60  oo ~,..4-„.  48 ;0  63„9  45.2 3-10  2.43  16.4  PV  50  45-33 3-53 15.50.  19.8  18.5  18.5  1.7  1.0  1.5  FeO  6.7  9.7  7.5  MnO  0.25  0.19  0.15  0.18  0.23  NgO  1.  5.9  5.0  6.9  6.77  CaO  3.6  8.5  8.1  8.50  ^a 0  5.1  8.5 -7 O  4.0  3-8  4.30  ICO  3.1  1.9  1.6  2.4  2.36  H0  1.1  0.3  0.6  0.6  0.21  0.43  0.70  0.07  0.71  0.73  ,i o 2  3  r  2  2  co  2  TABL'3 I -  <0.1  3.0  <0.1  <0.1  .  0.6  10.3  1.81 10.64  < 0.1  C h e m i c a l c o m p o s i t i o n s o f r o c k s a m p l e s f r o m Bowie Soamount The f i r s t two p a r t s o f a sample number i n d i c a t e t h e s t a t i o n a t w h i c h t h e sample was f o u n d ( F i g u r e 2 ) . Sample .67 - 7 i s an a n d e s i t o . The others arc a l k a l i o l i v i n e basalts. 67 - 6 - 12 i s a x e n o c r y s t - r i c h b a s a l t , from w h i c h t h e x e n o c r y s t s were removed p r i o r t o a n a l y s i s . PV 50 i s a b a s a l t f r o m Bowie Seamount, a n a l y z e d by E n g o l . a n d E n g e l (19b3). The o t h e r a n a l y s e s wore made by t h e L a b o r a t o r i e s o f t h e G e o l o g i c a l S u r v e y o f Canada, Ottawa.  ^  •lineral  67  i 7 . _ - . .7.  "'6 -  12  . -  .68  8  9.9.. '  Jr'V  50  Albito  44. 01  1 8 . 86  2 5 . J-5  15. . 2 0  13- . 6 2  Anorthito  1 3 . 62  2 8 . 08  2 8 . 36  21 .13  16 . 1 2  Orthoclase  1 8 . 90-  1 1 . 12  9 . 45  lb. . 4 6  13 . 9 0  7 . 38  5 - 11  Nepnoline  o  Diopsidc  o.  Hyper sthc-no  26  2 . 87  Magnetite  12 . 2 1  12 . 3 0  17 . 5 3  -  1 1 . 15  Olivine  1 2 . 12  9 • 37  1 7 . 59  1 2 . 70  10 . 1 6  15 • 41  2 . 55  1 . 62  09  0 .93  2 • 55  Ilmenito  2 . 13  5 . 02  2 . 89  b .04  6 .69  Apatite  1 . 01  r ~> 1 . oo  I . 60  1 .68  Corundum  3 . 16  -  :  -  TABLE 2 - N o r m a t i v e c o m p o s i t i o n s o f r o c k s a m p l e s f r o m Bowie Seamount The f i r s t two p a r t s o f a s a m p l e number i n d i c a t e the s t a t i o n a t w h i c h the sample was f o u n d ( F i g u r e 2 ) . Sample 67 - 7 i s an a n d e s i t e . The others are a l k a l i o l i v i n e b a s a l t s . 67 - 6 - 12 i s a x e n o c r y s t - r i c h b a s a l t from w h i c h t h e x e n o c r y s t s w.*ro removed - p r i o r t o a n a l y s i s . PV 50 i s a b a s a l t from Bowie Seamount a n a l y z e d bv E n g l c and G n g e l (1963).  67. - 6 - 1 2  Component  68.. r _ l . ~ . 3.  63....- 9.  0  10  py_.  1 0 . ,0  19  20  Pyroxene  1  J?  T X  J  ,0  Olivine  1  2  1  4  4.,1  Opaque  1  <1  <1  ^>  2..1  Plagioclase  Groundless Vesicles  (glass)  70  47  42  _  28  48  52  52..6 20. .0  TABLE 3 - M o d a l c o m p o s i t i o n s o f r o c k s a m p l e s from Bowie Seamount PV 50 i s r e p r o d u c e d f r o m p u b l i s h e d r e s u l t s o f E n g e l and E n g e l (1963). The o t h e r s a r e v i s u a l e s t i m a t e s made by t h e a u t h o r .  -p-  46  rock with conspicuous plagioclase.  p h e n o c r y s t s o f o l i v i n e and  Black clinopyroxene phenocrysts are also  present b u t a r c n o t conspicuous because o f t h e i r  dark  colour.  than  5mm.  The p h e n o c r y s t s a r e g e n e r a l l y no l a r g e r  i n diameter. In t h i n  s e c t i o n , these b a s a l t s e x h i b i t  some  m i n e r a l o g i c a l v a r i a t i o n s and a g r e a t range  of textural  t y p e s , p r o b a b l y r e f l e c t i n g many d i f f e r e n t  environments.  All  have a m a t r i x o f e i t h e r  with i n t e r s t i t i a l labradorite  glass.  pure g l a s s o r m i c r o l i t e s  The m i n e r a l s a r e o l i v i n e ,  (An 6 2 - 7 0 ) , c l i n o p y r o x e n e , a n d m a g n e t i t e .  These may o c c u r i n d i v i d u a l l y as g l o m e r o p h e n o c r y s t s ,  as phenocrysts, t o g e t h e r  o r swarming i n t h e m a t r i x a s  m i c r o l i t e s and c r y s t a l l i t e s . ' p h e n o c r y s t s were i n a p p a r e n t and  are well  In general, the microe q u i l i b r i u m w i t h t h e magma  formed and unzoncd b u t t h e l a r g e r  phenocrysts  ( v i s i b l e i n hand s p e c i m e n ) show s t a g e s i n t h e i r when t h e r e was a g r o w t h  hiatus or p a r t i a l  history  resorption  by t h e magma ( P l a t e 7 ) . A unique  alkali  olivine basalt of apparently  r e s t r i c t e d a r e a l extent dominates the e a s t e r n h a l f o f the s u r v e y a r e a on t h e summit.  This rock i s distinguished  by an abundance o f l a r g e x e n o c r y s t s o f p l a g i o c l a s e (An 5 4 - 6 4 ) a n d h o r n b l e n d e ,  1 t o 2 cm. a c r o s s , a n d  PLATE 7 - Sonic m i c r o s c o p i c t e x t u r e s o f t h e olivine basalts.  alkali  ( a ) Zoned, a l t e r e d p h e n o c r y s t s and c u h e d r a l , unaltered microphenocrysts i n a matrix of i s o t r o p i c g l a s s . (Crossed n i c o l s ) . ( T h i n s e c t i o n 6? - 5 - C). ( x 2 ? ) (b) X e n o c r y s t o f b a s a l t i c h o r n b l e n d e w i t h a r e a c t i o n r i m o f m a g n e t i t e and orthopyroxene. (Plane l i g h t ) . (Thin s e c t i o n 67 - k). (x27) ( c ) Pounded x e n o c r y s t o f p l a g i o c l a s e , rimmed by a f r e s h o u t e r zone o f more c a l c i c composition. (Crossed n i c o l s ) . (Thin s e c t i o n 67 - k)• (x27)  48  coarse-grained, wholly or p a r t l y 2 t o 5 cm. i n d i a m e t e r .  (An 62 - 6 6 ) , u n d e r ^>mm.  d i a m e t e r , a r e more common i n t h e r o c k b u t a r e n o t  conspicuous. and  The h o s t r o c k , i n w h i c h  i n c l u s i o n s a r e f o u n d , i s an a l k a l i  s i m i l a r t o t h e o t h e r s on t h e seamount 12).  inclusions,  Phenocrysts of o l i v i n e ,  c l i n o p y r o x e n e , and p l a g i o c l a s e in  crystalline  I n i t , the great hornblende  which a r e fragmented,  olivine  x e n o c r y s t s , many o f  and o r t h o p y r o x e n e ,  l a r g o p l a g i o c l a s e x e n o c r y s t s bear  producing  and t h e  evidence o f similar'  i n t h e magma ( P l a t e 7b a n d c ) .  i n c l u s i o n s , most o f w h i c h  basalt  ( T a b l e 1, 67 - 6 -  have r e a c t e d s t r o n g l y ,  t h i c k rims o f magnetite  instability  the xenocrysts  The  are feldspathic or gabbroic  w i t h a l a r g e amount o f h o r n b l e n d e ,  were s i m i l a r l y i n  d i s e q u i l i b r i u m w i t h t h e magma a t t h e t i m e o f e r u p t i o n . The  o c c u r r e n c e o f a b u n d a n t i n c l u s i o n s i n an a l k a l i  o l i v i n e b a s a l t i s normal,  but t h e great predominance o f  f e l d s p a t h i c and g a b b r o i c t y p e s o v e r u l t r a m a f i c t y p e s i s unusual.  These i n c l u s i o n s w i l l be d i s c u s s e d i n a l a t e r  section. Andes_ite_s A n d e s i t i c r o c k s were f o u n d a t o n l y two l o c a t i o n s on B o w i e Seamount:  S t a t i o n s 6 7 - 7 and 68 - 5.  Sample 67 - 7 i s d i s t i n g u i s h e d i n hand s p e c i m e n by  49  its  t o t a l l a c k o f v e s i c l e s and h i g h degree  On a f r e s h , b r o k e n  surface,  fine needles of plagioclase  a r e c o n s p i c u o u s i n a g r e y groundmass. it  of jointing.  In thin  section,  j.s s e e n t o be made up d o m i n a n t l y o f more o r l e s s (An 4 5 - 5 0 ) w i t h  flov/-oriented plagioclase l a t h s relatively  few c r y s t a l s o f o l i v i n e ,  and a p a t i t e  (Plate 8 a ) .  pyroxene,  magnetite,  None o f t h e m i n e r a l s shows  any i n d i c a t i o n o f i n s t a b i l i t y  i n t h e magma.  The g r o u n d -  mass i s g l a s s y b u t c o n t a i n s s u c h an abundance o f m i c r o l i t e s t h a t i t i s a l m o s t masked.  The c o m p o s i t i o n  of t h i s r o c k i s seen i n T a b l e s 1 and 2 t o l i e between mugearite The  ( c f . M u i r and T i l l e y ,  196.1) a n d t r a c h y t e .  n o r m a t i v e p l a g i o c l a s e was c a l c u l a t e d t o be An 2 2 . 6 . The  r o c k h a s a p p a r e n t l y been c h i l l e d a s q u i c k l y a s  any o f t h e s u r f a c e l a v a s f o r a zone o f s i d e r o m e l a n c e x i s t s i n t h e ground while tachylyte  mass a l o n g one m a r g i n  of a block  f o r m s most o f t h e i n t e r i o r .  p o s s i b l e , t h e r e f o r e , t h a t t h i s r o c k may have i n a b l o c k l a v a f l o w (Washington,  It i s originated  1 9 2 6 and F i n c h ,  I t s h i g h l y d i f f e r e n t i a t e d c o m p o s i t i o n would  1933).  certainly  favour t h e f o r m a t i o n o f such a h i g h l y v i s c o u s f l o w (Macdonald,  1967).  the p o s s i b i l i t y  Without  still  remains  s a m p l e s may have f o r m e d dyke.  f u r t h e r e v i d e n c e , however, that these angular rock  p a r t o f a t a l u s n e a r an e r o d e d  PLATE 8 - P h o t o m i c r o g r a p h s o f t h e  anclosites  (a) N o n - v e s i c u l a r a n d e s i t i c r o c k f r o m Station 6 7 - 7 . Phenocrysts of p l a g i o c l a s e , m a g n e t i t e and c l i n o p y r o x e n e are v i s i b l e i n the m a t r i x of p l a g i o c l a s e m i c r o l i t c s and g l a s s . ( P l a n e l i g h t ) ( T h i n s e c t i o n 67 - 7 - A ) ( x 2 7 ) (b) Hornblende - b e a r i n g a n d e s i t e from Station 6 8 - 5 S m a l l diamond-shaped c r y s t a l s o f o l i v i n e and a d a r k c r y s t a l o f h o r n b l e n d e a r e s u r r o u n d e d by f l o w o r i e n t e d p l a g i o c l a s e and i n t e r s t i t i a l glass. The l i g h t a r e a s a r e v e s i c l e s ( P l a n e l i g h t ) ( T h i n s e c t i o n 68 - 5 ) (x27)  5o  51  The  s i n g l e d r e d g e h a u l on t h e n o r t h e a s t r i d g e  t u r n e d up t h e o n l y o t h e r a n d e s i t i c r o c k  found  on t h e  seamount.  I t i s u n i r a p o s i n g i n hand s p e c i m e n , b e i n g a  pale grey,  a p h a n i t i c , f i n e l y v e s i c u l a r rock.  microscope  ( P l a t e 8 b ) , I t i s s e e n t o be a l m o s t  composed o f f l o w - o r i e n t e d p l a g i o c l a s e l a t h s with i n t e r s t i t i a l  i s present  of magnetite  products,  Brown,  basaltic phenocrysts,  by a v e r y t h i n r e a c t i o n  and orthopyroxene.  apparently  hornblende,  (An 51 - 47)  as s c a t t e r e d euhedral  some o f w h i c l i a r e s u r r o u n d e d rim  wholly  g l a s s and r a r e , e u h e d r a l o r s k e l e t a l  c r y s t a l s o f o l i v i n e and pyroxene. hornblende  Under, t h e  The r e a c t i o n  formed by t h e d e h y d r a t i o n o f  may have formed a s a r e s u l t  o f a drop i n  w a t e r p r e s s u r e when t h e magma was e x t r u d e d . A n d e s i t e s o c c u r i n s m a l l p r o p o r t i o n s on many l a r g e oceanic volcanoes  and a r e thought  t o be h i g h l y  d i f f e r e n t i a t e d derivatives of the a l k a l i w h i c h makes up s u c h B•  olivine basalt  volcanoes.  Inclusions The  inclusions are c r y s t a l l i n e ,  2 t o 5 cm. a c r o s s .  Some o f t h o s e  ically  have t e x t u r e s t y p i c a l  I960),  igneous  accumulation  shapeless  examined  masses  petrograph-  o f c u m u l a t e s (Wager e t a l .  r o c k s w h i c h a p p a r e n t l y have formed by t h e  o f c r y s t a l s on t h e f l o o r o f a magma chamber.  52  Others are o f l e s s d i s t i n c t i v e  t e x t u r e a n d c o u l d be  either xenoliths of wall-rock or i n d i s t i n c t O n l y one u l t r a m a f i c i n c l u s i o n  cumulates.  ( a p y r o x e n i t e ) was f o u n d .  The r e s t a r c f e l d s p a t h i c o r g a b b r o i c a n d many a r e r i c h in  hornblende.  The c l i m o p y r o x e n e i n t h e s e i n c l u s i o n s i s  pale green i n t h i n s e c t i o n , u n l i k e the b u f f - c o l o u r e d c l i n o p y r o x e n c s common t o t h e l a v a s , (i)  Pyroxene  cumulate  This i n c l u s i o n  ( P l a t e 9 ) i s an a g g r e g a t e o f  zoned, c l i n o p y r o x e n c s a n d some o l i v i n e w i t h poikilitic  interstitial  p l a g i o c l a s e and m i c r o p o r p h y r i t i c s i d c r o m e l a n e .  The m a f i c m i n e r a l s w h i c h , f o r t h e most p a r t , a r c e u h e d r a l , are  randomly  o r i e n t e d and l o o s e l y packed i n i n t e r s t i t i a l  vesicular glass.  T h e r e i s no s i g n o f f r a c t u r i n g o r  r e s o r p t i o n on t h e e x p o s e d the  removal  g r a i n b o u n d a r i e s to' i n d i c a t e  o f p r e - e x i s t i n g m i n e r a l s from t h e s e  voids.  A p p a r e n t l y t h e n , t h i s u l t r a m a f i c i n c l u s i o n was a l w a y s an a g g r e g a t e o f e u h e d r a l c r y s t a l s w i t h  interstitial  magma a n d i s n o t t h e r e m n a n t o f a c o m p l e t e l y c r y s t a l l i n e igneous rock. cumulate  I t i s p r o b a b l e t h a t i t formed  on t h e f l o o r o f a magma chamber o r p e r h a p s a s  a nodule o f aggregated c r y s t a l s , the  as a  magma.  suspended  freely i n  The c l i n o p y r o x e n e a n d o l i v i n e ,  being the  most e u h e d r a l p h a s e s , were p r e s u m a b l y  the f i r s t to  PLATE 9 - P y r o x e n e  cumulate  (Thin s e c t i o n 6? - 3 -  C)  (a) P a r t i a l l y i n t e r g r o w n cumulus o f clinopyroxene crystals. The s p e c k l e d material i s microlite-rich, vesicular, i n t e r s t i t i a l glass. The l i g h t a r e a s a r e m a i n l y h o l e s and c r a c k s i n t h e s l i d e b u t some a r e p l a g i o c l a s e and o l i v i n e c r y s t a l s . (Plane l i g h t ) ( 2 ) X  (b)  Cumulus p y r o x e n e s and a few i n t e r s t i t i a l plagioclase crystals. Isotropic glass forms the b l a c k background. (Crossed n i c o l s ) (x27)  53  54  crystallize  and g r a v i t a t e t o t h e r e g i o n o f  Both apparently  accumulation.  a c c u m u l a t e d a s c r y s t a l s a t t h e same  time s i n c e n e i t h e r e x e r t s a dominant c r y s t a l l o g r a p h i c i n f l u e n c e on t h e o t h e r . few  o l i v i n e s continued  the p a r t i a l  Growth o f t h e p y r o x e n e s a n d t h e after  coalescence  deposition, resulting i n  of adjacent  crystals.  Although  some e a r l y p l a g i o c l a s e a p p e a r s t o have s e t t l e d o u t w i t h the  f e r r o m a g n e s i a n cumulus m i n e r a l s ,  of i t i s p o i k i l i t i c partly not  and i n t e r s t i t i a l ,  e n c l o s i n g pyroxene euhedra.  y e t gone t o c o m p l e t i o n  from i t s s i t e  the greater  of formation,  part  enclosing or  C r y s t a l l i z a t i o n had  when t h e c u m u l a t e was removed . i n c o r p o r a t e d i n t o t h e moving  magma, a n d e j e c t e d o n t o t h e s u r f a c e o f t h e v o l c a n o cored  bomb ( B r a d y  a n d Webb, 1 9 4 3 ) ,  p e r m e a t e d by q u e n c h e d (ii)  minerals  t h i n l y mantled and  siderornelane  Andesine - o l i v i n e The  as a  cumulate  m a k i n g up t h i s i n c l u s i o n a r c  andesine  (An 3 0 - 3 4 ) , f c r r o - h o r t o n o l i t e ( F a 8 0 ) ,  apatite,  and opaque i r o n o x i d e  (Plate 10).  The  composition  o f t h e o l i v i n e was d e t e r m i n e d by X - r a y d i f f r a c t i o n t h e method o f Y o d e r a n d Sahama ( 1 9 5 7 ) .  Such an a s s e m b l a g e  o f s o d i c p l a g i o c l a s e and i r o n - r i c h o l i v i n e represents the  using  probably  a d i f f e r e n t i a t e o f a d r y magma somewhere i n  s y s t e m o f magma r e s e r v o i r s b e l o w t h e s u r f a c e .  The  PLATE 10 - A n d o s i n o - O l i v i n e c u m u l a t e . 67 - 6 - E)  (Thin  section  ( a ) O l i v i n e Fa-80 ( u p p e r r i g h t ) , a p a t i t e ( c e n t r e ) , and opaque o x i d e ( l o w e r l e f t ) , s u r r o u n d e d by p l a g i o c l a s e f e l d s p a r An-30-34. Melting of the p l a g i o c l a s e a r o u n d t h e o l i v i n e and o x i d e g r a i n s i s r e a d i l y apparent. (Plane l i g h t ) (x27) (b)  I n t e r l o c k i n g grains of the plagiocla.se cumulus. (Crossed n i c o l s ) (x27)  ( c ) O l i v i n e ( c e n t r e ) on t h e m a r g i n o f t h e i n c l u s i o n , e x h i b i t i n g a strong reaction w i t h t h e magma ( l o w e r l e f t ) . The l i g h t e r c o l o u r e d g l a s s , p r o d u c e d by m e l t i n g o f the m i n e r a l s w i t h i n the i n c l u s i o n , i s v i s i b l e i n t h e space between t h e o l i v i n e and t h e l a r g e p l a g i o c l a s e ( u p p e r r i g h t ) (Plane l i g h t ) (x27)  c r y s t a l s a r e l a r g e and w e l l f o r m e d b u t have l o c k i n g grain boundaries  ( P l a t e 10b),  inter-  a texture  which  the  author b e l i e v e s  t o be t h e r e s u l t o f c o n t i n u e d  and  c o a l e s c e n c e o f a g g r e g a t e d p h e n o c r y s t s w h i c h had  s e t t l e d o u t o f t h e magma o n t o a s u r f a c e  of accumulation.  A change i n t h e e n v i r o n m e n t , e.g. a p r e s s u r e during  decrease  movement o f t h e o v e r l y i n g magma., c a u s e d i n c i p i e n t  melting the  growth  o f the m i n e r a l s i n t h e cumulate, e s p e c i a l l y o f  sodic  plagioclase  around o l i v i n e and i r o n o x i d e  g r a i n s , w h i c h do n o t a p p e a r t h e m s e l v e s t o have been so susceptible  to remelting  cumulate, transported during  an e r u p t i o n ,  ( P l a t e 10a).  to the surface  reacted  Pieces  of the  as i n c l u s i o n s  w i t h t h e magma a t t h e i r  m a r g i n s ( P l a t e 1 0 c ) b u t r e t a i n e d w i t h i n them t h e i n t e r stitial  liquid  olivine. liquid  f o r m e d by t h e m e l t i n g  Quenching o f b o t h t h e l a v a and the i n t e r s t i t i a l  on r e a c h i n g  different glasses; the  the surface,  p r o d u c e d two d i s t i n c t l y  a dark sideromelane, which  i n c l u s i o n a n d a much l i g h t e r  inclusion  o f t h e andesine and  surrounds  glass within the  (Plate 10c).  (iii)  Plagioclase  - hornblende  cumulate-  In t h i s cumulate, randomly o r i e n t e d plagioclase  grains  are enclosed i n large  calcic  ( 1 - 2 cm),  poikilitic  b a s a l t i c hornblende c r y s t a l s (Plate 11a).  Very small  amounts o f o l i v i n e ,  a p a t i t e and s k e l e t a l ,  P l a g i o c l a s e - hornblende cumulate S e c t i o n 67 - 6 - G)  (Thin  (a) P l a g i o c l a s e c r y s t a l s o f c u m u l u s s u r r o u n d e d by p o i k i l i t i c h o r n b l e n d e ( P l a n e l i g h t ) (x2) (b) and ( c ) M i c r o f r a c t u r c d and r o u n d e d s k e l e t a l p l a g i o c l a s e shOYri_ng d i s p l a c e m e n t o f a d j a c e n t s e g m e n t s o f t h e same c r y s t a l . P o i k i l i t i c hornblende surrounds the feldspar. ( C r o s s e d n i c o l s ) ( (b) x 2 7 , ( c ) x 80)  58  magnetite-rich poikilitic  clinopyroxene are also included  hornblende.  o c c u r s b o t h as s m a l l s k e l e t a l crystals.  is a slight  difference  of a s i n g l e p l a g i o c l a s e hornblende lamellae  latter  and  the  p l a g i o c l a s e , i s rounded  discrete grains  The  once been f r a g m e n t e d  The  by  and  and  as.large  show e v i d e n c e o f h a v i n g  then healed.  In places,  there  i n o r i e n t a t i o n of adjacent  units  crystal in a  ( P l a t e l i b and  c).  poikilitic  Misalignment of  twin  and a c o r r e s p o n d i n g d i s p a r i t y i n e x t i n c t i o n  p o s i t i o n between d i f f e r e n t a r e a s w i t h i n a  visibly  continuous p o r t i o n of a p l a g i o c l a s e g r a i n i s also T h e s e phenomena can b e s t be e x p l a i n e d and h e a l i n g w i t h i n t h e c r y s t a l . the p o i k i l i t i c  if  s e v e r a l a d j a c e n t u n i t s appear The  no m i c r o f r a c t u r i n g therefore  older  microfracturing  Each p l a g i o c l a s e  in  large crystal.  hornblende  by  hornblende, or z o n i n g .  on t h e o t h e r h a n d , shows The  plagioclase i s and a p p e a r s  by r e s o r p t i o n  u n f a v o u r a b l e magmatic e n v i r o n m e n t . microfractures by r e l a t i v e l y hornblende.  i n the f e l d s p a r  The  an  point,  i n an  open s p a c e s  and  filled  c r y s t a l s of b a s a l t i c  r e s u l t i s s i m i l a r to the  o f Wager e t a l . ( I 9 6 0 ) .  t o be  c u m u l u s were l a t e r  few, l a r g e , p o i k i l i t i c The  even  to.belong t o t h e same  a c c u m u l a t i o n o f s k e l e t a l c r y s t a l s w h i c h , a t one then rounded  unit  i s i n d i v i d u a l l y zoned,  than the hornblende  were b r o k e n and  apparen  heteradcumulate  Growth o f s u c h l a r g e , u n i f o r m  59  c r y s t a l s would n e c e s s a r i l y i n v o l v e the the  component i o n s o f h o r n b l e n d e  r e l a t i v e l y rare a p p a r e n t age  nucleii  i n the  difference  represents The  magma t o  intercumulus l i q u i d . plagioclase  possibility  that  a e u t e c t i c c r y s t a l l i z a t i o n of the  (iv)  the  Anorthosite  xenolith with  plagioclase  opaque i r o n o x i d e and  two  glass.  - 65),  be  drop or temperature i n c r e a s e p a r t l y r e s o r b e d and  to p l a g i o c l a s e  alone.  of clinopyroxene,  from the  on  i n the  the  surface.  the  by  degree,  Such a  a simple  system.  pressure  After  p l a g i o c l a s e was  a f r e s h o u t e r zone o f more c a l c i c  C r y s t a l growth during  of  to a s m a l l e r  explained  rounded, the  some  r e s o r p t i o n around  i n c l u s i o n and,  most s i m p l y  of  magmatic e n v i r o n m e n t  a l o n g g r a i n boundaries w i t h i n i t ( P l a t e 12).  nucleated  overgrowth  s m a l l amount  A change i n t h e  exposed margins of the  i s o l a t e d by  phases.  partly  harrisitic  (An 60  a relatively  some t i m e c a u s e d c o n s i d e r a b l e  change c o u l d  texture  core of t h i s type of i n c l u s i o n c o n s i s t s  crystalline  interstitial  the  plagioclase.  The coarsely  The  and  this  p o s s i b i l i t y r e m a i n s t h a t h o r n b l e n d e has  replaced  at  from t h e  between t h e  hornblende p r e c l u d e s the  free d i f f u s i o n of  being  finally  composition.  t h i s s e c o n d s t a g e - w a s not r e s t r i c t e d S k e l e t a l c r y s t a l s of o l i v i n e  shot through w i t h magnetite, surface  of the  i n c l u s i o n and  and  apparently grew away  Adjacent s k e l e t a l c r y s t a l s are  PLATE 12 - H a r r i s i t i c o v e r g r o w t h s on a n o r t h o s i t e xenolith. The p l a g i o c l a s e o f t h e i n c l u s i o n o c c u p i e s the l e f t corner of the photo. The minerals forming the h a r r i s i t e are c l i n o p y r o x e n e , which appears grey w i t h a r o u g h s u r f a c e , and o l i v i n e , w h i c h a p p e a r s white i n the photograph. Later-fromed p o i k i l i t i c p l a g i o c l a s e i s v i s i b l e among t h e s k e l e t a l o l i v i n e and c l i n o p y r o x e n e c r y s t a l s , a n d a s a l i g h t e r r i m on t h e p l a g i o c l a s e of the i n c l u s i o n . The b l a c k a r e a s w i t h i n t h e c r y s t a l s a r e composed o f g l a s s and m a g n e t i t e c r y s t a l s . The b l a c k background i s i s o t r o p i c v e s i c u l a r g l a s s . (Crossed n i c o l s ) ( T h i n s e c t i o n 67 - 1 - C) (x27)  60  61  t h o r o u g h l y intermoshecl  and i n p l a c e s  p l a g i o c l a s e i n c l u s i o n by p o i k i l i t i c zone o f p l a g i o c l a s e .  Small,  of the s k e l e t a l ferromagnesians.  tothe  growth o f the outer  probably l a t e  crystals also exhibit a p o i k i l i t i c  stage texture  are rooted  texture This  plagioclase around  intricate  parts second  a p p e a r s t o be a v a r i a t i o n o f t h e h a r r i s t i c  or c r e s c u m u l a t e t e x t u r e  o f Wager e t a l . ( I 9 6 0 ) and Wager  (1963) (v)  Plagioclase  - magnetite  i n c l u s i o n (Plate 13) consists of  This  intergrown plagioclase c r y s t a l s with The  has  (An 7 0 ) , m a g n e t i t e and a p a t i t e  a good d e a l  plagioclase i s highly  fills  many o f t h e c r a c k s .  taken place  during  xenolith  transport  of i n t e r s t i t i a l  sideromelane.  f r a c t u r e d and s i d e r o m e l a n e Resorption  a l o n g exposed g r a i n  of the plagioclase boundaries,  possibly  i n t h e magma p r i o r t o e r u p t i o n .  The  s i d e r o m e l a n e i n and around t h e i n c l u s i o n i n d i c a t e s r a p i d quenching took place melting in  after i t s ejection.  o f t h e m a g n e t i t e may a c c o u n t  that  Local  for a darker s t a i n  the sideromelane adjacent t o i t . (vi)  Hornblende gabbrc Coarsely  crystalline,  composed o f p l a g i o c l a s e clinopyroxene,  xenoliths gabbroic  xenoliths  (An 66 - 7 6 ) , b a s a l t i c h o r n b l e n d e ,  m a g n e t i t e a n d m i n o r a p a t i t e , a r c . common  PLATE  13 - P l a g i o c l a s e - m a g n e t i t e x e n o l i t h s e c t i o n 67 - 6 - H)  (Thin  (a) P l a g i o c l a s e ( w h i t e ) s u r r o u n d i n g a p o c k e t of magnetite ( b l a c k ) , a p a t i t e (white g r a i n s w i t h h i g h r e l i e f and " g r o u n d g l a s s " s u r f a c e ) , and i n t e r s t i t i a l s i d e r o m e l a n e (dark grey). The p l a g i o c l a s e i s s t r o n g l y r e s o r b e d on i t s m a r g i n s and s i d e r o m e l a n e i s v i s i b l e i n f r a c t u r e s w i t h i n i t (upper l e f t ) . (Plane l i g h t ) . ( x 2 7 ) (b) A d i f f e r e n t p a r t o f t h e same s e c t i o n viewed through crossed n i c o l s . Plagioclase a p p e a r s g r e y and w h i t e . M a g n e t i t e and g l a s s appear b l a c k . The r e s o r b e d m a r g i n s o f t h e p l a g i o c l a s e a p p e a r as d e n d r i t i c patterns. (x27)  62  63  in  the i n c l u s i o n - b e a r i n g  apparently  basalts.  The p l a g i o c l a s e h a s  b e e n r e s o r b e d by t h e magma a l o n g e x p o s e d  g r a i n b o u n d a r i e s and t h e r e a c t i o n m a r g i n i s m a n t l e d by a t h i n growth o f f r e s h p l a g i o c l a s e  (Plate 14).  h o r n b l e n d e , h o w e v e r , a p p e a r s t o have u n d e r g o n e c h a n g e s where i t was e x p o s e d t o t h e magma.  The profound  Those  grains  w h i c h l i e on t h e b o r d e r o f t h e i n c l u s i o n a r e p e r m e a t e d with  graphic  b l e b s o f magnetite and appear t o pass a t  t h e i r outer margin i n t o graphic clinopyroxene,  w h i c h r e t a i n s t h e o r i e n t a t i o n and m a g n e t i t e  t r a i n s of the hornblende.  Acicular, radiating crystals  of orthopyroxene occur with a  few p l a c e s .  arc present i n s t e a d  The  reaction  o r t h o p y r o x e n e and m a g n e t i t e  of t h i s unusual graphic  reaction  b u t t h e one t y p e seems t o g r a d e i n t o t h e o t h e r .  graphic  generally  described  a l t e r a t i o n z o n e s o f t h e h o r n b l e n d e arc-  f u r t h e r c o m p l i c a t e d w i t h what a p p e a r s t o be a  harrisitic  on  t h i s apparent a l t e r a t i o n i n  I n many g r a i n s , morj c o n v e n t i o n a l  rims o f fine, unoriented  texture  (or skeletal)  g r o w t h o f o l i v i n e and a u g i t e , previously, which probably  s i m i l a r t o that  formed  any o f t h e exposed c r y s t a l s u r f a c e s  independently  of theinclusion.  O r i g i n and s i g n i f i c a n c e o f t h e i n c l u s i o n s The that  great  v a r i e t y o f b a s a l t s and i n c l u s i o n s  a c o m p l e x s y s t e m o f magma r e s e r v o i r s  existed  suggests  PLATE Ik - P h o t o m i c r o g r a p h o f t h e g r a p h i c h o r n b l e n d e a l t e r a t i o n with intermeshed h a r r i s i t i c olivine. In the photographs, hornblende a p p e a r s brown, pyroxene appears grey w i t h a rough s u r f a c e t e x t u r e , o l i v i n e appears y e l l o w , p l a g i o c l a s e appears grey w i t h a smooth s u r f a c e t e x t u r e , and m a g n e t i t e a p p e a r s as s m a l l , b l a c k , g r a p h i c g r a i n s i n t h e o t h e r minerals. The e x t e n s i v e d a r k m a t r i x i s i s o t r o p i c g l a s s . The shape and o r i e n t a t i o n o f t h e m a g n e t i t e i s e v i d e n t l y c o n t r o l l e d by c r y s t a l l o g r a p h i c d i r e c t i o n s w i t h i n the hornblende. The p y r o x e n e , w h i c h i s s i m i l a r l y o r i e n t e d , i s a l s o t h o u g h t t o have been d e r i v e d f r o m t h e h o r n b l e n d e , and s m a l l amounts o f i t cavi-be s e e n a s g r e y a r e a s , w i t h i n t h e l a t t e r mineral. The s k e l e t a l o l i v i n e , h a s a p p a r e n t l y grown cat r i g h t a n g l e s t o t h e plagiocla.se of the i n c l u s i o n . (Crossed nicols) ( T h i n s e c t i o n 67 - 6 - F) ( M a g n i f i c a t i o n o f u p p e r p h o t o x.?.7; l o w e r photo x 80)  64  64  b e n e a t h Bowie Seamount b o t h i n t h e c r u s t , a n d i n t h e m a n t l e , where t h e magma must o r i g i n a l l y have been d e r i v e d . (1949)  Macdonald  i l l u s t r a t e d how l a v a s o f v a r i o u s  c o m p o s i t i o n s may o r i g i n a t e a t d i f f e r e n t l e v e l s w i t h i n a s i n g l e magma chamber  ( F i g u r e .13) a n d many o f t h e r o c k s  o f t h e seamount a r e p r o b a b l y r e l a t e d i n t h i s way. S e v e r a l e x t r e m e s o f d i f f e r e n t i a t i o n however a r e r e p r e s e n t e d by t h e i n c l u s i o n s .  Xenocrysts of hornblende,  x e n o l i t h s o f hornblende gabbro, and cumulates poikilitic  hornblende a t t e s t  containing  t o r e l a t i v e l y h i g h water  p r e s s u r e s t h a t must have p r e v a i l e d i n some p a r t s o f t h e magma chamber s y s t e m .  On t h e o t h e r h a n d , t h e u l t r a m a f i c  p y r o x e n i t e cumulate and t h e andesine - o l i v i n e r e s p e c t i v e l y r e p r e s e n t e a r l y and l a t e d i f f e r e n t i a t i o n o f d r y b a s a l t i c magma.  cumulate-  stage I t i s therefore  p r o b a b l e t h a t s e v e r a l magma chambers e x i s t e d a t a h i g h l e v e l i n t h e c r u s t and t h a t d i f f e r e n t i a t i o n  proceeded  more o r l e s s i n d e p e n d e n t l y i n e a c h . From t h e v a r i e t y o f c u m u l a t e t h a t t h e magma h a d s u f f i c i e n t  types, i t i s apparent  time t o d i f f e r e n t i a t e t o  a h i g h d e g r e e , a n d p o s s i b l y t o b u i l d up a c o n s i d e r a b l e t h i c k n e s s o f accumulated floors.  c r y s t a l s on t h e magma chamber  During e r u p t i o n there i sa decrease i n c o n f i n i n g  p r e s s u r e i n t h e magma, a c c o m p a n i e d  by t h e f o r m a t i o n a n d  65  FIGURE 13 - Diagram illustrating a possible manner of eruption of lavas of differing composition from the same mqgmo 'Ctasanber.- A, andesite; B , b a s a l t ; C, olivine basalt; D, p i c r i t e - b a s a l t ; E, trachyte. (After Macdonald, 1949.)  66  expansion  of vesicles.  the i n t e r s t i t i a l l i q u i d  V e s i c u l a t i o n also takes place i n w i t h i n t h e l o o s e l y bound  mush, a n d , i f t h e p r o c e s s were have b r o k e n of  crystal  v i o l e n t enough, i t c o u l d  up t h e c u m u l a t e beds a n d t r a n s p o r t e d  them upwards i n t o Corroded  fragments  t h e e r u p t i n g magma, a s i n c l u s i o n s .  w a l l - r o c k s , perhaps c o m p r i s i n g o l d cumulate  b e d s o r c o n v e n t i o n a l l y c r y s t a l l i z e d r o c k r e i n t r u d c d by a new column o f magma, c o u l d be b r o k e n into  t h e magma i n t h e same way.  up a n d i n c o r p o r a t e d  The d r o p i n p r e s s u r e i n  t h e w a l l - r o c k m i g h t a l s o be compounded by a V c n t u r i effoct The  c r e a t e d by r a p i d upv/ard movement o f t h e magma.  c o r r o d e d a n o r t h o s i ' t e and hornblende  which  are hosts to the unusual  gabbro  harrisitic  inclusions,  t e x t u r e , might  be t h e r e m a i n s o f f r a c t u r e d , magma-permeated w a l l - r o c k s or  chamber-floor  supported  rocks which,  harrisitic  t h e magma and t h e i r C.  crystal  f o r a t i m e , may have growths b e f o r e e r u p t i o n o f  consequent f r a g m e n t a t i o n and d i s p e r s a l  Tephra P y r o c l a s t i c m a t e r i a l i s more e a s i l y r e c o v e r e d by  d r e d g e and g r a b  sampler  than i s f l o w - r o c k and t h i s i s  r e f l e c t e d i n i t s occurrence  i n a l l b u t two sample  from  B).  t h e seamount  that the r a t i o  (Appendix  The a u t h o r  hauls  estimates  o f t e p h r a t o f l o w - r o c k on t h e seamount i s  p r o b a b l y a c t u a l l y a r o u n d 1:1,  67  (i) Volcanic  bombs  A c o l l e c t i o n o f v o l c a n i c bombs was along  a five mile  6 7 - 1 ,  6 7 ~ 6 , 6 7 - 5  t h e more f a m i l i a r which i s f u l l y  of i t s f l i g h t  o f t h e summit r i d g e a t s i t e s and 6 8 - 1  ( F i g u r e 2 ) . - . Among  t y p e s a r e t h e f u s i f o r m bomb  formed b e f o r e  ( P l a t e 15b)  bomb  strip  obtained  15a),  (Plate  l a n d i n g , and t h e cow-dung  which i s s t i l l  p a r t l y l i q u i d a t t h e end  and f l a t t e n s on i m p a c t .  A few s m a l l  cow-dung bombs a p p e a r t o have been f l a t t e n e d a n d t h e n folded before The  finally  solidifying  appearance o f t h e great  represented  completely  (Plato  m a j o r i t y o f bombs,  by t h e s u i t e f r o m s t a t i o n 67 - 5 ,  somewhere b e t w e e n t h e two t y p e s . range from s m a l l  lies  The bombs o f t h i s s u i t e  f l a t t e n e d forms r e s e m b l i n g  bombs t o l a r g e , g l o b o s e ,  cow-dung  h i g h l y deformed i n d i v i d u a l s  w i t h c o m p l e x i n v o l u t e s u r f a c e s b u t no d e f i n a b l e (Plate 16).  Although the deformation  v/as a p p a r e n t l y there  shape  o f t h o s e bombs  due t o i m p a c t w i t h a r e s i s t a n t s u r f a c e ,  a r e no i m p r i n t s o r a d h e r i n g  confirm  15c).  rock  fragments t o  t h a t t h e i m p a c t was w i t h a s o l i d  volcanic  s u b s t r a t e , n e i t h e r i s i t c l e a r w h i c h s i d e o f t h e bomb r e c e i v e d the impact.  T h e i r mode o f o r i g i n  remains  uncertain. All  t h e bombs have a q u e n c h e d s k i n o f s i d e r o m e l a n e  w h i c h may show some b r e a d c r u s t f r a c t u r i n g .  Normally,  PLATE 1 5 - V a r i e t i e s o f bombs f o u n d on Bowie Seamount ( a ) F u s i f o r m bomb ( S t a t i o n 6 8 -  1)  (b) Cow-dung bombs. The u p p e r bomb was found a t S t a t i o n 6 7 - 1 , t h e lower l e f t one a t S t a t i o n 6 7 - 6 , a n d t h e l o w e r r i g h t hand one a t S t a t i o n 67 - 5( c ) F o l d e d cow-dung bomb ( S t a t i o n 6 7 - 6 ) .  PLATE 16 - V a r i e t i e s o f bombs f o u n d on B o w i e Seamount (continued) (a) and ( b ) G r a d a t i o n f r o m l a r g e bombs w i t h deformed s u r f a c e s t o t h e s m a l l , f l a t , cow-dung v a r i e t y . A l l t h e s p e c i m e n s a r e from S t a t i o n 67  -  5.  70  v e s i c l e s a r e deformed near the s u r f a c e , towards the middle.  b e c o m i n g l e s s so  The v e s i c l e s o f t h e i n t e r i o r a r e  commonly s l i g h t l y l a r g e r  t h a n t h o s e .of t h e c h i l l e d  margin, and, i n extreme c a s e s , t h e i n t e r i o r i s v i r t u a l l y hollow  (Plate  17a).  Those bombs w h i c h a r c n o t f l a t t e n e d ,  are  c o n c e n t r i c a l l y and r a d i a l l y f r a c t u r e d  and  i n d i v i d u a l s have been f o u n d t h a t  (Plate  17b),  have c o m p l e t e l y o r  p a r t l y l o s t t h e i r q u e n c h e d s k i n due t o s p a l l i n g a l o n g these  fractures. The c o m p o s i t i o n s o f t h e bombs v a r y  p l a c e and are s i m i l a r t o t h a t immediate v i c i n i t y . are  from p l a c e t o  of the rocks i n the  C o r e d bombs c o n t a i n i n g  inclusions  rare. The p r e s e n c e o f bombs on t h e summit i m p l i e s  lava  fountaining  and e x p l o s i v e  that  eruptions took place  during the l a t e stages of a c t i v i t y  on t h e summit.  there In  v i e w o f t h e s u m m i t ' s p r e s e n t n e a r n e s s t o s e a l e v e l and its of  p r o b a b l e emergence i n t h e p a s t , many, i f n o t a l l , t h e bombs o f t h e summit w o u l d have been e j e c t e d  e i t h e r above o r j u s t b e l o w s e a l e v e l .  Many,  from  therefore,  probably t r a v e l l e d through the a i r along a t l e a s t part . of t h e i r t r a j e c t o r i e s , eventually onto t h e s u r f a c e o f young v o l c a n i c existed  f a l l i n g into the sea or i s l a n d s w h i c h may have  there at times during the past.  PLATE 17 - V a r i e t i e s o f 'bombs f o u n d on Bowie Seamount (continued) ( a ) H o l l o w cow-dung bomb ( S t a t i o n 6 7 - 6 ) (b) C o n c e n t r i c and r a d i a l f r a c t u r e s i n a f u s i f o r m bomb. The q u e n c h e d s i d e r o m e l a n e s k i n appears l i g h t e r than the dark core. The s p e c i m e n i s ' f r o m S t a t i o n 6 8 - 1 .  71  72  S i n c e t h e s e i s l a n d s a r c presumed t o have boon e r o d e d away, t h e p r e s e n t t h e seamount may  be  due  d i s t r i b u t i o n o f t h e bombs on not  o n l y to primary v o l c a n i c  d i s p e r s a l but a l s o t o secondary erosion.  I t i s the author's  the s u b a c r i a l l y  d i s p e r s a l by y/avc  o p i n i o n , h o w e v e r , that few  formed bombs c o u l d have s u r v i v e d i n t a c t  t h e wave a c t i o n w h i c h d e s t r o y e d t h e e p h e m e r a l i s l a n d s which they l a y .  The  m a j o r i t y o f t h e bombs now  the t e r r a c e s p r o b a b l y  found  Their present  s h o u l d be geographic  the r e s u l t  can be  Apparently  l o c a t i o n , i f t h i s i s the of primary  a s the  suite  cas., The  explained i n this  t h e n , bombs, w h i c h a r c  capable  of forming i n s h a l l o w water.  s u i t e o f bombs from s t a t i o n 6 7  6 7 - 5  from s t a t i o n  of as b e i n g s u b a c r i a l l y  way. traditionally  f o r m e d s t r u c t u r e s , may  ~ 5  Perhaps the  ( P l a t e 16)  b o t h a i r and w a t e r b e f o r e  on t h e s u r f a c e o f t h e T u f f s and Lapilli  finally  and  unusual  travel  coming t o  rest  volca.no.  t h e i r unconsolidated a s h , and  counterparts  w e a k l y cemented  be  i s typical  o n l y o f s h a l l o w w a t e r e r u p t i o n s where e j e c t a may  (ii)  under  volcanic dispersal.  thought  through  on  r e s t r i c t i o n o f l a r g e numbers o f o b v i o u s l y  r e l a t e d bombs, s u c h (Plate 16),  on  c o n s t i t u t e the remainder which  e i t h e r landed i n the sea or o r i g i n a t e d e n t i r e l y water.  of  tuffs  73  derived the  f r o m them, were f o u n d i n a l l b u t one l o c a l i t y  summit a n d a t t h r o e  (Appendix B ) . hyaloclastic  s t a t i o n s on t h e f l a n k s  Some o f t h e t u f f a c e o u s  m a t e r i a l may be  r a t h e r t h a n p y r o c l a s t i c , t h a t i s , f o r m e d by  shattering glass during (Rittman,  of the five  on  t h e aqueous c h i l l i n g  1 9 6 2 ) , r a t h e r t h a n by e x p l o s i v e  However, i t was d i f f i c u l t  of lava  vulcanism.  i n some c a s e s t o d i s t i n g u i s h  b e t w e e n t h e two o f them a n d t h e t e r m " p y r o c l a s t i c " , a s u s e d i n t h i s p a p e r , may t h e r e f o r e and  cover both true p y r o c l a s t i c  h y a l o c l a s t i c products. Lapilli  and L a p i l l i  Most o f t h e t e p h r a  tuffs form m a s s i v e l y  accumulations of l a p i l l i ,  bedded,  unsortcd  some o f w h i c h may be c e m e n t e d .  The p a r t i c l e s a r e g e n e r a l l y composed o f v e s i c u l a r hyalocrystailinc holocrystallino  s i d e r o m e l a n e , though t a c h y l y t e and f r a g m e n t s may be p r e s e n t  Cemented d e p o s i t s , formation on  i . e .t u f f s ,  on t h e s u r f a c e  t h e summit d u r i n g  there,  their  i n small  which a r e i n the process of  o f t h e s o a n o u n t were f o u n d  the present  survey  (Plate 21a). shallow  only  ( P I - t o 1 3 ) , and  occurrence i s sporadic.  The c e m e n t i n g a g e n t s a r e manganese o x i d e s i r o n oxides  amounts.  (Plato 2 0 ) , or carbonate of organic The c a r b o n a t e cement i s r e s t r i c t e d  (Plate 19), origin t o the  summit p e a k s , w h i c h a r e w e l l w i t h i n t h e p h o t i c  zone a n d a r e t h i c l l y  encrusted  w i t h s e s s i l e marine  life,  PLATE 18  - B o t t o m p h o t o g r a p h s o f a cemented t u f f d e o o s i t on t h e 130 f a t h o m t e r r a c e ( S t a t i o n 63"- 2) (a) The c e m e n t e d s u r f a c e c r u s t o f t h e t u f f , undercut at i t s margin, stands 6 inches above t h e d e p o s i t o f l o o s e t e p h r a i n the c h a n n e l t o the r i g h t . Sponges grow i n abundance on t h e s o l i d s u b s t r a t e , b u t n o t on t h e l o o s e m a t e r i a l i n t h e c h a n n e l . The d i a m e t e r o f t h e compass i s 3 i n c h e s and. t h e l e n g t h o f t h e vane i s 10 i n c h e s . (b) L i g h t c o v e r o f s e d i m e n t , d i s t u r b e d by t h e compass a s i t t r a v e l s o v e r t h e bottom. The s u r f a c e t e x t u r e o f t h e t u f f i s revealed quite clearly i n this photograph. The d i a m e t e r o- t h e , c o m p a s s i s 3 i n c h e s and t h e l e n g t h o f t h e vane i s 10 i n c h e s .  The n a t u r e o f c e m e n t a t i o n o f t h e t u f f s (a)  Hand, s p e c i m e n s h o w i n g b l a c k c o l o u r i m p a r t e d by c e m e n t i n g f e r r o - m a n g a n e s e o x i d e s (Sample f r o m S t a t i o n 67 - 5 ) .  (b)  T h i n s e c t i o n showing t h e b l a c k fer.romanganese cement among t h e l a p i l l i o f v e s i c u l a r , microioor'ohyrytic sideromelane. ( P l a n e l i g h t ) ( T h i n " s e c t i o n 67 - 5 - A) (x2)  c  PLATE 20 - The  nature of the cementation (continued)  of the  tuffs  (a) Hand s p e c i m e n s h o w i n g r e d c o l o u r a t i o n i m p a r t e d "by f e r r u g i n o u s cement. A g r o w t h of c o r a l i n e algae covers the o t h e r s i d e o f t h e r o c k and a p p e a r s a s a w h i t e r i m i n t h i s p h o t o g r a p h . (Sample from. S t a t i o n 67 - 3 ) . ~ (b) T h i n s e c t i o n s h o w i n g l a p i l l i , interstitial f e r r u g i n o u s cement ( b l a c k ) and g r o w t h o f c o r a l i n e algae ( b l a c k s t r i p at top of slide. (Plane l i g h t ) (Thin s e c t i o n 67 - 3 - B) ( ) x 2  76  PLATE 21 - The n a t u r e  of cementation (continued)  o f the  tuffs  (a) B i n d i n g m a t r i x o f o r g a n i c c a r b o n a t e and s i l i c a c o m p r i s i n g f o r a m i n i f e r t e s t s and sponge s p i c u l e s . ( C r o s s e d n i c o l s ) ( T h i n s e c t i o n 67 - 1 - A ) . ( x 2 7 ) (b) U n i f o r m l y  cemented l a p i l l i t u f f (Station 67-3)  (c) A s t r o n g l y i n d u r a t e d s u r f a c e c r u s t capping an o t h e r w i s e w e a k l y cemented l a p i l l i tuff (Station 67-2)  76  n o t a b l y t h e r e d c o r a l i n e a l g a LithotJmmnjL on, and v a r i o u s s i l i c e o u s  sponges.  The  barnacles,  ferruginous  and  m a n g a n i f e r o u s cements do n o t a p p e a r t o have any depth  such  limitation. Some s a m p l e s a p p e a r t o be u n i f o r m l y cemented  ( P l a t e 21b) top  b u t o t h e r s a r ^ s t r o n g l y cemented  few c e n t i m e t r e s  of the deposit  ( P l a t o 21c)  form a c r u s t o v e r l y i n g u n c o n s o l i d a t e d Loose d e p o s i t s o f l a p i l l i ( P l a t o 22) terrace.  may  and  simply  material (Plate  and s h e l l  18a).  fragments  The o r i g i n o f t h e s h e l l d e b r i s i s u n c e r t a i n , but t o the fauna p r e s e n t l y l i v i n g  f o u n d on t h e t e r r a c e .  on  Much o f t h e s h e l l m a t e r i a l  t h e r e f o r e have been . r e c e n t l y d e r i v e d from b o u l d e r s  rocky outcrops its  only i n the  a r e v e r y w i d e s p r e a d on and j u s t b e l o w t h e u p p e r  much o f i t i s s i m i l a r boulders  throughout  i n the v i c i n i t y  central pinnacles.  and  o f t h e u p p e r t e r r a c e and  C u r r e n t s , have, i n p l a c e s ,  reworked  t h e l o o s e d e p o s i t enough t o p r o d u c e r i p p l e marks i n t h e light  shell material (Plato  23).  S e d i m e n t s , b o t h l o o s e and lapilli  cemented,  a r e a s s o c i a t e d w i t h b o t h t h e 45  fathom t e r r a c e s ( P l a t e 2 ) .  These  of w e l l rounded f a t h o m and  130  are thought t o r e p r e s e n t  f o s s i l wave-washed d e p o s i t s , ( C h a p t e r I I I - B ) . Thin-bedded  Tuff  Thin-bedded  t u f f s composed o f v o l c a n i c a s h a r c n o t  PLATE 22 - The l o o s e l a p i l l i d e p o s i t o f t h e 1+5 f a t h o m tcrre.ce w i t h i t s c o n s p i c u o u s s h e l l d e b r i s . A thumb-tack a t the bottom o f the lower photograph provides the s c a l e . (Camera S t a t i o n 6 8 - 3 and sample S t a t i o n 6 ? - 12)  79  PLATE 2 3 - R i p p l e m a r k s v i s i b l e i n t h e l i g h t s h e l l m a t e r i a l o f t h e d e p o s i t s on t h e 4 5 f a t h o m terrace. The compass i s 3 i n c h e s i n d i a m e t e r and t h e vane i s 10 i n c h e s l o n g (Station 6 8 - 3 ) .  81  a s common a s l i p i l l i t h e 130 6 7 - 6  fathom and  tuffs.  The  t e r r a c e i n t h e a r e a c o v e r e d by d r e d g e and 6 7 - 4 .  t o u c h e d by 6 7 - 2  (Plate 24)  o n l y d e p o s i t f o u n d i s on  This  tuff  i s g e n e r a l l y d a r k brown b u t , i n p l a c e s , may  r e d d i s h brown o r o c h r e  c o l o u r e d , depending  amount o f f e r r u g i n o u s cement.  The  on t h e  i n thickness.  c h a r a c t e r i s t i c a l l y possesses  graded  s i z e s ranging mainly cm.  of the u n i t .  from  i n diameter  be  relative  samples are f r i a b l e  s l a b b y and none e x c e e d s 4 cm.  up t o 1  site  and  It  bedding w i t h  particle  c o a r s e t o medium sand b u t  lapilli  a r e f r e q u e n t l y embedded i n t h e base  Whether t h i s g r a d i n g c o n t i n u e s downward  b e l o w t h e l a y e r r e p r e s e n t e d by t h e s a m p l e s l a b s i s n o t known.  The  visibly  graded  p o r t i o n i s u s u a l l y capped  a t h i n n e r l a m i n a t e d u n i t o f medium t o f i n e s a n d . s e c t i o n , t h e r o c k i s seen (Pettijohn, sideromelane The  s h a r d s and  g l a s s o f the  w i t h a narrow range i n d e x was  determined  crystal  tuff  fragments.  grains i s exclusively  of r e f r a c t i v e  thin  vitrophyric  indices.  u s i n g immersion  sideromelane The  refractive  o i l s and t h e  l i n e method, t h e i n d e x o f e a c h o i l b e i n g immediately  In  t o be a v i t r i c - c r y s t a l  1 9 5 7 ) , w h o l l y composed o f  by  Bccko  checked  p r i o r t o use w i t h an Abbe r e f r a c t o m o t e r .  s h a r d s were c l e a n e d u l t r a s o n i c a l l y i n d i s t i l l e d w a t e r , simple time-saving technique which,  The a  for unaltered shards,  PLATE 2 4  Thin-bedded v i t r i c - c r y s t a l t u f f (Station 67-6) (a) S e c t i o n t h r o u g h the t u f f bedding  (bedded  shoving  ash)  graded  (b) Photonv c r o g r a n h o f t h e sane t u f f . The b l a c k m a t e r i a l between the shards i s the f e r r u g i n o u s cedent. (Plane l i g h t ) ( T h i n s e c t i o n 67 - 6 - D) (xoO)  82  83  was f o u n d t o work a s w e l l a s t h e c h e m i c a l method o f K i t t r i c k a n d Hope ( 1 9 6 3 ) .  Based  on o b s e r v a t i o n o f 2 2 5  g r a i n s , t h e g l a s s h a s an i n d e x b e t w e e n 1 . 5 7 a s t r o n g mode b e t w e e n I . 5 8 O and I . 5 8 6 . mode i s w i t h i n t h e l i m i t s e.sh f a l l  (Wilcox, 1965).  have o r i g i n a t e d The of  expect  fora single  T h i s d e p o s i t must,  therefore,  event.  s e d i m e n t a r y s t r u c t u r e s and p e t r o g r a p h i c p r o p e r t i e s tuff  The s e q u e n c e  f u r n i s h some i n s i g h t  into i t s  o f g r a d e d and l a m i n a t e d u n i t s  r e s e m b l e t h e A and 3 i n t e r v a l s o f t h e t u r b i c l i t c s  described  by Bouma ( I 9 6 2 ) , b u t t h i s d o c s n o t n e c e s s a r i l y mean the  t u f f was d e p o s i t e d by t u r b i d i t y  v o l c a n i c e j e c t a from a s i n g l e  flow.  explosive  The  event d u r i n g a  e r u p t i v e a s h c l o u d , o r may be c a r r i e d l a t e r a l l y  the  surge  (Moore,  of  the water.  i nthe  surge w i l l  spread out a c r o s s t h e s u r f a c e  I t i s e a s y t o i m a g i n e an a s h d e p o s i t  Straightforward v e r t i c a l  with  from any o f t h e s e p r o c e s s e s .  differential  column t h r o u g h a i r o r water w i l l the s i m i l a r  In  explosion which breaches the  graded bedding o r i g i n a t i n g  as w i l l  from t h e  1 9 6 7 ) down t h e f l a n k s o f t h e c m o .  case o f a submarine  s u r f a c e , a base  that  finer  p e r i o d o f e x p l o s i v e e r u p t i o n may r a i n v e r t i c a l l y  base  with  The spread, o f t h i s  during a single volcanic  this particular  origin.  one w o u l d  and 1 . 5 9 ?  s e t t l i n g of the ash  produce  f o u n d e r i n g o f a base  a graded  sequence,  s u r g e on t h e s e a  s u r f a c e when i t s e n e r g y i s s p e n t .  A submarine base  surge,  spreading  down t h e f l a n k s o f t h e a c t i v e cone s h o u l d  behave  much l i k e  a turbidity  tuff with turbidite  c u r r e n t and would g i v e r i s e  features.  a l s o be a h y a l o c l a s t i t c surface  Such a g l a s s y t u f f  to a might  d e r i v e d from t h e s h a t t e r i n g  s k i n o f a submarine flow of p i l l o w l a v a .  Pillow breccia A grain-supported large, pyramidal pillow-crust vitric  tuff  v o l c a n i c b r e c c i a , composed o f  pillow  fragments and s m a l l ,  angular  f r a g m e n t s , s e t in. an u n s t r a t i f i e d m a t r i x o f ( P l a t e 2 5 ) , was d r e d g e d f r o m t h e e a s t e r n  o f t h e summit r i d g e  ( S t a t i o n 68 - 1, A p p e n d i x B ) .  flank  The  b l a c k b a s a l t fragments c o n t r a s t sharply w i t h the greyg r e e n and creamy y e l l o w v a r i e g a t e d m a t r i x . s u r f a c e of t h e rock i s covered  The o u t e r  w i t h a t h i n l a y e r (0 -  o f b l a c k a n d brown f e r r o - m a n g a n e s e .  2mm.)  Recovered w i t h i t  were v a r i o u s i n d e p e n d e n t p i l l o w f r a g m e n t s and a more o r less intact pillow.  Bottom photographs i n the a r e a  ( P l a t e 26) r e v e a l e d a r u b b l y  substrate with v i s i b l e  f r a g m e n t s a n d what may be i n t a c t The and  matrix  pillow  pillows.  o f t h e b r e c c i a was e x a m i n e d p c t r o g r a p h i c a l l y  by means o f X - r a y d i f f r a c t i o n .  S a m p l e s were  f o r X-ray d i f f r a c t i o n u s i n g the chemical procedure of K i t t r i c k  and Hope  (1963).  cleaning  prepared  Pillow breccia (Station  68-1)  (a) P i l l o w fragment e x t r a c t e d from the p i l l o w breccia. The l i g h t e r c o l o u r e d m a t e r i a l on t h e u p p e r s u r f a c e i s p a l a g o n i t e . A c o l o u r photograph o f t h e upper s u r f a c e o f t h i s specimen appears i n P l a t e 32. (b) S e c t i o n t h r o u g h t h e p i l l o w b r e c c i a showing p i l l o w c r u s t fragments i n a t u f f a c eou s m a t r i x .  85  PLATE 26  - Bottom photographs i n the a r e a o f o u t c r o p of t h e p i l l o w b r e c c i a ( S t a t i o n 68 - 1 ) . The d i a m e t e r o f t h e compass i s 3 i n c h e s and t h e l e n g t h o f t h e vane i s 10 i n c h e s . ( a ) A b a s a l t p i l l o w ( ? ) . A wedge s h a p e d p i l l o w , fragment i s a l s o v i s i b l e t o the l e f t o f t h e compass. (b) U n i d e n t i f i e d b o u l d e r s i n t h e n e i g h b o u r h o o d of the p i l l o w b r e c c i a occurence.  PLATE 27 - Type o f r u b b l y s u b s r a t e w h i c h a p p e a r s t o be most common on t h e f l a n k s o f t h e summit r i d g e o f B o w i e Seamount. These p h o t o g r a p h s were t a k e n a t s t a t i o n 6 8 - 1 . The compass i s 3 i n c h e s i n ' d i a m e t e r and t h e vane i s 10 i n c h e s l o n g .  87  oQ  O O  The g r e y - g r e e n p a r t  of the b r e c c i a matrix  was f o u n d  t o be composed o f u n w c a t h e r c d , g r e e n g l a s s s h a r d s w i t h few  microlitcs,  tests. lie  sponge s p i c u l e s , a n d a few r a d i o l a r i a n  The r e f r a c t i v e i n d i c e s o f t h e s h a r d s were f o u n d t o  b e t w e e n 1 . 5 8 5 a n d 1.589?  indicate  very  a narrow range, which  t h a t t h e s h a r d s were d e r i v e d  period of volcanic  would  during a single  eruptions.  The creamy y e l l o w p o r t i o n o f t h e m a t r i x i s s e e n , under t h e microscope, t o c o n s i s t o f w h i t e , material, palagonite,  and a s m a l l e r  s p i c u l e s and rhombic z e o l i t e diffractograms  revealed  p r o p o r t i o n o f sponge  crystals.  X-ray  that t h i s portion of the matrix i s  l a r g e l y made up o f X-amorphous m a t e r i a l X-rays),  fine-grained  ( i . e . amorphous t o  w i t h l e s s e r amounts o f q u a r t z , p l a g i o c l a s e a n d  montmorillonite,  and t r a c e s o f c h l o r i t e ,  illite  m i c a and  amphibole. G l a s s , pala'gonite s p i c u l e s probably  and t h e o p a l i n e  make up t h e X-amorphous  z e o l i t e s and the montmorillonite  shards.  The p l a g i o c l a s e o c c u r s The q u a r t z  have c r y s t a l l i z e d  component.  The  a r e most l i k e l y t h e  a l t e r a t i o n products of the palagonite 1965)«  s i l i c a o f t h e sponge  ( B o n a t t i a n d Nayudu,  as m i c r o l i t c s i n the g l a s s  may be d c t r i t a l  o r i t may p e r h a p s  from g e l s and s o l u t i o n s w i t h i n t h e m a t r i x  R o c r y s t a l l i z a t i o n o f sponge s p i c u l e s m i g h t a l s o  contribute  89  some c r y s t a l l i n e  q u a r t z t o the system.  Whether t h e t u f f a c c o u o ' m e t r i x i s a syngenetic  hyaloclastite,  of the p i l l o w b r e c c i a  derived at the time of  e r u p t i o n f r o m t h e d e c r e p i t a t i n g s k i n s o f t h e p i l l o w s and p i l l o w f r a g m e n t s , w i t h w h i c h i t i s now a s s o c i a t e d , o r an explosion t u f f derived independently, However, t h e s i m i l a r i t y ,  i n thin  was n o t a s c e r t a i n e d .  s e c t i o n , o f the p r o p e r t i e s  o f t h e p i l l o w c r u s t f r a g m e n t s and t h e s h a r d s o f t h e m a t r i x would suggest t h a t t h e m a t r i x i s a c t u a l l y a hyaloclastite  ( c f . Rittman,  1962, C a r l i s l e ,  syngenetic 1963,  H o n n o r e z , 1963, Solomon, 1 9 6 9 ) . D.  V e s i c l e L i n i n g s and A m y g d u l c s 1.  Chemically  precipitated linings  V e s i c l e l i n i n g s and a m y g d u l e s a r e g e n e r a l l y restricted lapilli  t o m i c r o v c s i c l e s i n the s i d e r o m e l a n e o f  and c h i l l e d  f l o w o r bomb s u r f a c e s w h i c h c a n be  seen i n t h i n - s e c t i o n . Only i n r e l a t i v e l y a s "the d e e p l y  old rocks,  w e a t h e r e d b a s a l t on H o d g k i n s Seamount,  largo i n t e r i o r  v e s i c l e s l i n e d or  such a r e the  filled.  Many o f t h e a m y g d u l e s and l i n i n g s a r e composed o f rod-brown, i s o t r o p i c , i r o n o x i d e s  (Plate 28a), black,  opaque manganese o x i d e s , o r s p a r r y and f i b r o u s c a l c i t e (Plate 28b).  Most commonly,  however, they  consist  cither  of y e l l o w t o p a l e g r e e n i s h , a n i s o t r o p i c , low b i r c f r i n g e n t ,  PLATE 28 - P h o t o m i c r o g r a p h s o f v e s i c l e amygdules.  linings  and  (a) C o n c e n t r i c a l l y banded growth o f i r o n oxide beginning to f i l l a v e s i c l e ( P l a n e l i g h t ) ( T h i n s e c t i o n 68 - 5 ) ( x 2 0 0 ) (b) Amygdules o f s p a r r y c a l c i t e . The r o u n d , d a r k g r e y a r e a s a r e empty v e s i c l e s and the b l a c k background i s i s o t r o p i c g l a s s . ( P a r t i a l l y crossed n i c o l s ) (Thin section 67 - .3 - A ) . (x80)  90  91  fibrous minerals which are too fine-grained to i d e n t i f y ( P l a t e 29), or o f i s o t r o p i c , golden y e l l o w m a t e r i a l ( P l a t e 30a)  with a l l the p r o p e r t i e s o f p a l a g o n i t e , which i s  sometimes v i s i b l e  i n the glass of the v e s i c l e  wall.  Those s u b s t a n c e s were i n c l u d e d by P e a c o c k ( 1 9 2 8 ) u n d e r t h e term " p a l a g o n i t e " . fine and be  The f o r m e r , w h i c h may a c t u a l l y be a  fibrous zeolite the l a t t e r  g r o w t h , he c a l l e d  "gol-palagonito".  a form o f h y d r a t e d  basaltic  ; ;  fibro-palagonite"  P a l a g o n i t e i s thought to  g l a s s f o r m e d i n a subaquoou  e n v i r o n m e n t b u t i t s mode o f o r i g i n i s i n d i s p u t e .  It is  t h o u g h t t o form e i t h e r by t h e r e a c t i o n o f h o t l a v a  with  water  (Peacock and F u l l e r ,  1928),  o r by t h e g r a d u a l  h y d r a t i o n and a l t e r a t i o n o f s i d e r o m e l a n e Found i n t h e s e  replaces  m i c r o l i t c s and v e s i c l e s o f t h e g l a s s  (Plate 53a).  Several of the v a r i a t i o n s i n the composition a m y g d u l e s and v e s i c l e l i n i n g s p a r a l l e l  amygdules a r e found o n l y i n t h e r o c k s  capping  summit p e a k s , and l i n i n g s o f manganese o x i d e s where s u c h m a t e r i a l i s v i s i b l e station 6 8 - 3 ,  the o t h e r  Calcite  the shallow occur  only  i n q u a n t i t y on t h e s u r f a c e  Appendix E ) .  cavity-filling  of the  geographically the  v a r i a t i o n s i n t h e t u f f c e m e n t s d e s c r i b e d above.  (e.g.  1966).  ways, p a l a g o n i t e i s most commonly s e e n i n  among t h e u n d i s t u r b e d it  (Moore,  The d i s t r i b u t i o n o f  substances i s apparently  random.  PLATE 2 9 - P h o t o m i c r o g r a p h s o f v e s i c l e amygdules "(continued)  l i n i n g s and  (a) V e s i c l e l i n i n g s o f f i n e , f i b r o u s , lowbirefringent minerals (fibro-palagonite) ( P l a n e l i g h t ) ( T h i n s e c t i o n 67 - 5 - A ) ( x 8 0 ) (b) The same, t h r o u g h  crossed  nicols.  92  PLATE 30 - P h o t o m i c r o g r a p h s o f v e s i c l a amygdules (continued)  l i n i n g s and  ( a ) G e l - p a l a g o n i t o l i n i n g and f i l l i n g vesicles. The same m a t e r i a l l i n e s t h e s u r f a c e o f t h e r o c k f r a g m e n t i t s e l f and has p a r t l y r e p l a c e d t h e s i d e r o m e l a n e w i t h i n the rock. A b r e c c i a matrix of g l a s s shards forms t h e upper r i g h t h a l f o f t h e p i c t u r e ( P l a n e l i - . h t ) (Thin s e c t i o n 60 - 1 - C) (x27) (b) S e g r e g a t i o n v e s i c l e s i n t h e b a s a l t o f Rod.gid.ns Seamount. The s e c o n d a r y l i n i n g s a r e o f b l a c k t a c h y l y t e , drawn f r o m t h e matrix of the surrounding rock. A chin coating of fjbcous ' ' n e r a l s l i n e s t i e c a v i t i e s remaining i n the t a c h y l y t . ( P l a n e l i g h t ' ( T h i n s e c t i o n 6 0 - 8 -• A)  (x27)  94  2.  Segregation  vesicles  Some o f t h e b a s a l t s on the .summit o f H o d g k i n s Seamount c o n t a i n s e g r e g a t i o n v e s i c l e s those  d e s c r i b e d by S m i t h  (1967).  ( P l a t e 30b) such as  These a r c s e c o n d a r y  l i n i n g s o f g l a s s , which' a r c t h o u g h t t o be due t o a n increase i n c o n f i n i n g pressure The  on t h e l a v a a f t e r e x t r u s i o n .  s u b s i d i a r y s u r f a c e s o f t h e s e g r e g a t i o n v e s i c l e s do,  however, s u p p o r t . c h e m i c a l l y  precipitated linings of  manganese o x i d e s a n d f i b r o u s m i c r o c r y s t a l l i n e E•  Post-volcanic  Deposits  S u r f i c i a l material of non-volcanic only l o c a l l y  zeolites.  o r i g i n i s rare or  abundant i n t h e a r e a .  I c e - r a f t e d r o c k s a r e r a r e on Bowie Seamount a n d have so f a r b e e n f o u n d i n abundance o n l y among t h e r e l a t i v e l y old,  s p h e r o i d a l l y weathered b a s a l t s o f the n o r t h e a s t  ridge. this  The summit t e r r a c e s a p p e a r t o be a l m o s t d e v o i d o f  f o r e i g n m a t e r i a l , f o r o n l y one i c e - r a f t e d p e b b l e was  i d e n t i f i e d despite the extensive  sampling  i n that  area.  I n c o n t r a s t , i c e - r a f t e d m a t e r i a l on t h e summit o f H o d g k i n s Seamount seems t o be v e r y a b u n d a n t .  Glacial  d e b r i s c o n s t i t u t e d f u l l y 50% o f t h e s i n g l e d r e d g e taken  from t h e r e a n d r a n g e d f r o m s a n d - s i z e  boulders  30 cm. a c r o s s .  haul  detritus to  Copious q u a n t i t i e s o f f e r r o -  manganese a c c o m p a n i e d t h e g l a c i a l  d e b r i s i n t h e dredge.  95  The f e r r o - m a n g a n o s e i s f o u n d b o t h as c o n c r e t i o n s , p a r t l y blanketing altered basalt boulders s l a b s broken  ( P l a t e Jia)  and a s  from a r u b b l y s u b s t r a t e ( P l a t e 31b).  therefore probably  It  f o r m s a d i s c o n t i n u o u s pavement i n t h e  area of dredging.  of  It  apparently reached  i t s p r e s e n t maximum t h i c k n e s s  5cm.  before the a r r i v a l  o f the. i c e - r a f t e d m a t e r i a l , f o r  the g l a c i a l b o u l d e r s bear  little  more t h a n a brown s t a i n o r ,  at  most, a t h i n veneer  It  w o u l d a p p e a r from t h i s t h a t t h e f e r r o - m a n g a n e s c i s s t i l l  accumulating  a f r a c t i o n of a m i l l i m e t r e  but a t a very slow r a t e .  (1967) r e c o r d g r o w t h r a t e s o f 0 . 5 oceanic nodules.  B a r n e s and Dymond  - 40 mm./10^ y r . f o r  The r e s u l t s o f o t h e r a u t h o r s ,  favour slow a c c r e t i o n , are compatible w i t h these and  who figure,  t h e r a t e o f g r o w t h o f t h e f c r r o - r a a n g a n e s e c r u s t on  H o d g k i n s Seamount a l s o a p p e a r s t o f a l l If its  thick.  w i t h i n these  t h e f o r m a t i o n r a t e h a s been c o n s t a n t  growth h i s t o r y , the a l t e r e d r o c k s which  limits.  throughout support  t h i c k n e s s may be w e l l o v e r 1 m i l l i o n y e a r s o l d .  a 5  Apparently,  t h e n , t h i s p a r t o f t h e summit o f H o d g k i n s Seamount i s r e l a t i v e l y much o l d e r t h a n t h e summit r e g i o n o f B o w i e Seamount, w h i c h Pleistocene.  was a c t i v e a t l e a s t u n t i l  cm.  t h e end o f t h e  F e r r o - m a n g a n e s e n o d u l e s i r o n h o d g k i n s Seamount ( S t a t i o n 68 - 8) (a) A l a r g e f e r r o - m a n g a n e s e n o d u l e v i t h a nucleus of weathered b a s a l t . (b) The u n d e r s i d e o f a f r a g m e n t o f f e r r o manganese pavewent s h o v i n g a w e a t h e r e d r o u n d e d b a s a l t c o b b l e p i c k e d up f r o m the s u b s t r a t e .  96  97  F•  Chcmic a l The  Weathering  r o c k s and  v e r y f r e s h and  t e p h r a f o u n d on Bov.de Seamount  are  g e n e r a l l y show, a t most, r u s t y i r o n  but d e e p e r w e a t h e r i n g has a l s o been  found.  P a l a g o n i t c , f o r i n s t a n c e , may le p r e s e n t as a s k i n where c h i l l e d m a r g i n s P a l a g o n i t c may  of sideromelane  o c c u r as t h e i n i t i a l  a l t e r a t i o n product, very hot  (Peacock  t o be  exist.  g l a s s (Moore,  1966)  a s y n g e n e t i c h y d r a t i o n and  formed when t h e s i d e r o m e l a n e  and  thin  h y d r a t e d phase i n t h e  gradual chemical weathering of b a s a l t i c but i t i s a l s o thought  stains  Fuller,  is  still  1928).  In the a r e a of s t u d y , the o c c u r r e n c e of p a l a g o n i t c appears  t o be r e s t r i c t e d t o t h o s e l o c a l i t i e s where  rocks arc obviously r e l a t i v e l y  old.  For  the  example,  p a l a g o n i t c i s the dominant c o n s t i t u e n t o f the t u f f s H o d g k i n s Seamount suggested  ( P l a t e 32a),  which, a c c o r d i n g to  r a t e s of accumulation of ferro-manganese,  over 1 m i l l i o n y e a r s o l d .  On  on  may  be  the o t h e r hand, i n the r e g i o n  o f t h e summit t e r r a c e s and p e a k s o f Bowie Seamount, where t h e r o c k s a r e known t o be  v e r y y o u n g , some y o u n g e r  1 8 , 0 0 0 y e a r s , p a l a g o n i t i z a t i o n was  u s u a l l y absent  than or o n l y  incipient. T h i s p a t t e r n a l s o appears F o r i n s t a n c e , t h e p i l l o w s and  elsewhere  on Bowie Seamount.  p i l l o w b r e c c i a s of the cast  PLATE 32 - P a l a g o n i t e i n t h e b a s a l t i c  rocks  ( a ) P a l a g o n i t e t u f f f r o m - H o d g k i n s Seamount. G r a i n s o f p a l a g o n i t e and a f r a g m e n t o f weathered b a s a l t are c l e a r l y v i s i b l e i n a cementing m a t r i x o f ferro-manganese. ( S t a t i o n 68 - 8) ( b ) The p a l a g o n i t i z e d u p p e r s u r f a c e o f a p i l l o w f r a g m e n t f r o m S t a t i o n 68 -- 1.  98  99  f l a n k o f t h e summit r i d g o ( S t a t i o n 68 - 1) bo and  a r c thought  r e l a t i v e l y o l d , f o r they a r c coated w i t h they support  ferro-mangancse  the manganese-coated remains  of long  dead s i l i c e o u s s p o n g e s , an a p p a r e n t l y e x t e r m i n a t e d which a l s o appears  fauna  on t h e o l d r o c k s o f H o d g k i n s Seamount  and t h e n o r t h e a s t r i d g e .  Furthermore,  marked by t h e p r e c i p i t a t i o n o f f e r r i c  incipient  weathering,  o x i d e s , has l e f t  a core of f r e s h b a s a l t at the c e n t r e o f each p i l l o w Conversely, a s i m i l a r  d e p o s i t on t h e west f l a n k o f  summit r i d g e ( S t a t i o n 68 -  7)  i s thought  young, f o r i t i s f r e e o f fcrro-mangancso f r e e o f any  to  o r g a n i c growth  t o be and  only  fragment. the  relatively  virtually  or s i g n s of weathering.  Here  a g a i n , p a l a g o n i t e i s a p p a r e n t l y p r e s e n t o n l y on t h e o l d e r rocks,  f o r i t i s present i n the sideromelane  o f the  s k i n s and h y a l o c l a s t i c m a t r i x o f t h e s u p p o s e d l y rocks at S t a t i o n 6 8 - 1 absent  ( P l a t e s 30a,  32b  and  older  33a),  a l t o g e t h e r i n the younger r o c k s of S t a t i o n  (Plate 6).  pillow  but 68-7  I t i s the author's o p i n i o n , t h e r e f o r e , t h a t the  palagonite, i n these rocks i s a product of chemically unstable Evidence  of the  weathering  sideromelane.  f o r w e a t h e r i n g o t h e r t h a n p a l a g o n i t e and  iron staining i s rarer.  The  tuffaccous matrix of  the  p i l l o w b r e c c i a d e s c r i b e d above c o n t a i n s z e o l i t e s , montmorillonite,  c h l o r i t e and i l l i t e ,  w h i c h a r e most  100 likely  the p r o d u c t s  of weathering  of the g l a s s  W e a t h e r e d a n d e s i t c on t h e n o r t h e a s t  r i d g e was  shards. found  X-ray d i f f r a c t i o n to c o n t a i n a s i m i l a r mixture and  clay minerals.  deeply  and  Many o f t h e s e  of  recovered  of the n o r t h e a s t  ridge.  The  33b)  (Plate  f o u n d so f a r on Eowic Seamount a r e  or o n l y s l i g h t l y weathered, w i t h the e x c e p t i o n weathered r o c k s  quartz  s a m p l e s have w e a t h e r e d  as a r e s u l t have become q u i t e r o u n d e d  Most o f t h e r o c k s  by  of  fresh  the  samples  from i t s n e i g h b o u r , H o d g k i n s Seamount, a r c  relatively  deeply  altered.  In view of t h e i r apparent  t h i s a l t e r a t i o n i s probably  due  to weathering.  age,  The  f r a g m e n t s o f l a v a , some o f w h i c h have w e a t h e r e d  into  rounded shapes, are a r i c h tan c o l o u r , changing g r a d u a l l y to l i g h t  grey  towards the  centre.  not  a function of c r y s t a l l i n i t y ,  The  rocks  but  c o l o u r change i s  of degree of  e x a m i n e d a r c h o l o c r y s t a l l i n o and  g e n e r a l l y a b o u t 5cm.  weathered than the  primary  hematite,  The  zone,  grey  and  be  area.  revealed,  p l a g i o c l a s e : s e r p e n t i n e , magnesian  montmorillonito,  a p a t i t e and  tan  light  X-ray d i f f r a c t o g r a m of the t a n m a t e r i a l  besides  the  alteration.  t h i c k , i s seen i n t h i n - s e c t i o n to  s i m p l y more t h o r o u g h l y An  This  minor q u a r t z ,  calcite,  calcite,  illite.  t u f f s o f H o d g k i n s Seamount a r c composed o f  p a l a g o n i t c , rhombic z e o l i t e  c r y s t a l s and  altered  ( s i m i l a r t o t h e a l t e r a t i o n a b o v e ) cemented w i t h  yellow  lapilli varying  PLA ">] 33  - Two t y p o s o f a l t e r a t i o n o b s e r v e d i n t h e of study.  area  (a) P a l a g o n i t i z a t i o n . The s i d e r o m e l a n e m a t r i x o f t h e r o c k has b e e n a l t e r e d t o g o l d and r e d n a l a g o n i t e n e a r t h e m a r g i n . Note t h a t t h e p a l a g o n i t e d o e s not a f f e c t t h e m i n e r a l grains. (Plane l i g h t ) (Thin s e c t i o n 68 - 1 - C) (e;27) (b) W e a t h e r i n g . The l i g h t m a r g i n o f t h e r o c k has been l a r g e l y a l t e r e d t o c l a y . The t h i n outer s k i n , \ hich f l a k e s o f f r e a d i l y , i s composed e n t i r e l y o f c l a y . Such a l t e r a t i o n e v e n t u a l l y cause,:, t h e r o c k f r a g m e n t t o become r o u n d e d as i t i s s e e n here. ( S t a t i o n 63 - 5). r  102  (Plate 32a).  amounts o f h y d r a t c d manganese o x i d e s  Other  t h a n t h e p a l a g o n i t e and z e o l i t e s , m o n t m o r i l l o n i t e i s t h e o n l y major w e a t h e r i n g illibe  p r e s e n t , t h o u g h c h l o r i t e and  o c c u r i n tre.ee amounts ( c . f . p i l l o w b r e c c i a m a t r i x  from S t a t i o n 6 8 - 1  V. 1.  product  above).  SUMMARY AND CONCLUSIONS  Bowio Seamount i s a c o m b i n a t i o n  c e n t r a l and f i s s u r e  t y p e o f v o l c a n o , w h i c h h a s f o r m e d o v e r an a r e a o f c o m p l e x fracturing of the crust.  The assumed n o r t h e a s t  fracture  s e t h a s a p p a r e n t l y been t h e d o m i n a n t f e e d e r o f t h e s y s t e m and  e x t r u s i o n s a l o n g a t l e a s t 30 m i l e s o f i t s l e n g t h a r c  responsible The  f o r the present  elongate  shape o f t h e m o u n t a i n  main c e n t r e o f e r u p t i o n o c c u p i e s t h e s o u t h w e s t e r n  o f t h e r i d g e where i t c r o s s e s t h e n o r t h w e s t  half  trend of  H o d g k i n s a n d D a v i d s o n Seamounts and t h e m a j o r i t y o f t h e o t h e r seamounts on t h e P r a t t - W c l k c r  chain.  The  i n t e r s e c t i o n o f t h e two s t r u c t u r e s may have p r o v i d e d t h e magma w i t h a zone o f r e l a t i v e l y  easy a c c e s s t o the s u r f a c e  r e s u l t i n g i n the creation of a larger volcanic p i l e that area.  However, e r u p t i o n s were n o t r e s t r i c t e d  summit and n o r t h e a s t r i d g e s a l o n e . m o u n t a i n were p r o b a b l y  over t o the  Other l o b e s o f t h e  f o r m e d by a c t i v i t y  along  subordinat  10.3  f i s s u r e s i n t h e c r u s t , and  parasitic  f l a n k e r u p t i o n s must  a l s o have been common. 2. one  Two  t e r r a c e s e x i s t n e a r t h e sua: i t o f t h e o f 35  at a depth  depth  o f 120  t o 140  t o 55  f a t h o m s , and  fathoms.  They a r e  Pleistocene  f e a t u r e s , formed by  e r o s i o n and  shallow water vulcanism  stands.  The  u p p e r t e r r a c e was  t h a n 1 8 , 0 0 0 y e a r s ago  and  the other at apparently  d u r i n g low sea  a p p a r e n t l y formed t e r r a c e at  earlier Pleistocene interval.  The  e r u p t i o n s took p l a c e a f t e r the  f o r m a t i o n and  of the lower  probably  uninterrupted plateau capping The  peaks which r i s e  thought  t o be  level  less  an  f i n a l phase o f v o l c a n i submergence  originally  t h e seamount a t s e a  f r o m t h e 130  the p r o d u c t s  a  c o n t e m p o r a n e o u s wave  the lower  t e r r a c e w h i c h was  seamount,  an level.  fathom t e r r a c e a r e  of t h i s a c t i v i t y .  These  late  volcanic  c o n e s were a p p a r e n t l y a l s o t r u n c a t e d when t h e y  li-  built  to t h e new  of 13,000 to 18,000 years  a  up  b u t were l a t e r a f t e r t h e y had, to i n d i c a t e  The  s u r m o u n t e d by  further,  i n t u r n , submerged.  further activity after  A boulder a potassium  sea l e v e l  small tuff T h e r e i s no  fI  of 75,000 - 100,000 years  main b u l k o f Bowie Seamount, a c c o r d i n g t o  (1969),  s h o u l d be o l d e r t h a n 0 . 6 9  evidence  this epi--^e.  o f f r e s h b a s a l t f r o m t h e sum-.it  - a r g o n age  cones  -  ™  -  !  -  *  B.P.  michkofsky  m i l l i o n years,  having  104  formed p r i m a r i l y d u r i n g a p e r i o d o f m a g n e t i c 3.  The v o l c a n i c p i l e i s a p p a r e n t l y  reversal.  composed o f e q u a l  amounts o f f l o w r o c k a n d t c p h r a , w h i c h form n a t u r a l s l o p e s o f 10° t o 20°. pillow  The l a v a f l o w s f o r m e d a s b o t h  flows and as normal massive  b e i n g t h e most common.  Both types  u p p e r s u r f a c e s where t h e y  flows, p i l l o w  flows  have i n t e n s e l y c h i l l e d  came i n t o c o n t a c t  with sea water.  Many, i f n o t most, o f t h e f l o w s a r c b r o k e n up i n t o r u b b l e , some f o r m i n g  b r e c c i a s of p i l l o w fragments i n a  matrix of hyaloclastic lapilli  loose  vitric  tuff.  Unsortcd  deposits of  and l a r g e r l a v a b l o c k s a r c t h e dominant  d e p o s i t s on t h e s u r f a c e .  clastic  T u f f , a s h a n d v o l c a n i c bombs,  common i n t h e r e g i o n o f t h e summit, p r o b a b l y o r i g i n a t e d d u r i n g s h a l l o w w a t e r a n d s u b a o r i o . l e r u p t i o n s when t h e volcano 4.  was a c t i v e a t s e a l e v e l .  The f e e d e r s u n d e r t h e seamount open b e l o w t o what  must be a c o m p l e x s y s t e m o f magma chambers a n d c o n d u i t s i n t h e mantle a n d c r u s t .  The v a r i o u s r o c k t y p e s , w h i c h r a n g e  from a l k a l i o l i v i n e b a s a l t s t o a n d o s i t e s , n a y  owe t h e i r  o r i g i n s t o t h e t a p p i n g o f d i f f e r e n t i a t i n g magma a t d i f f e r e n t l e v e l s i n these  reservoirs.  U l t r a m a f i c and  f e l d s p a t h i c c u m u l a t e i n c l u s i o n s s u g g e s t t h a t t h e magma remained quiescent  i n reservoirs within the crust long  105  enough f o r e x t e n s i v e  d i f f e r e n t i a t i o n to take  C o r r o d e d f c l d s p a t h i c and have been d e r i v e d fresh h a r r i s i t i c w a l l s and  gabbroic  v/hile, at the  same t i m e ,  fresh mineral  5.  The  study  process  i s not  unstable  This suggests that  c l e a r , but i s the  weathering  i n the  areas of o l d e r r o c k s . f o r m e d as t h e i n i t i a l  cases, be 6.  glass i s apparently  a r e a s where r o c k  manganese o x i d e s  sideromelane  more common i n  of  the  fragments have  enough, r o u n d e d f o r m s a r e common. p r o d u c e d by w e a t h e r i n g  In a l l  appear  to  montmorillonite.  L i t h i f i c a t i o n of tephra taken  most  f i r s t m a t e r i a l t o be a f f e c t e d .  phase o f w e a t h e r i n g  In those  z e o l i t e s and  the  I t i s , t h e r e f o r e , t h o u g h t t o have  the main m i n e r a l s  apparently  aroaof  g e n e r a l l y a b s e n t on t h e c h i l l e d  weathered deeply  the  s e r v i n g as s u r f a c e s of n u c l e a t i o n  s u r f a c e s o f f r e s h e x t r u s i v e m a t e r i a l and  sideromelane.  within  s l o w l y a t t a c k e d by t h e magma  of chemical  very  was  the  phases.  phase and  Palagonite  may  f l o o r - r o c k s , bear  f l o o r s o f a magma chamber o r c o n d u i t  c r u s t were, p e r h a p s , b e i n g  for  x e n o l i t h s , which  from w a l l - r o c k s and c r y s t a l growths.  place.  p l a c e on t h e  d e p o s i t s t o form t u f f s surface.  commonly f o r m t h e  w a t e r , where b i o c h e m i c a l  Hydratcd  cements and  in  iron  has and  shallow  a c t i v i t y i s high, c a l c i t c i s also  106  a cementing agent.  The c o n t e n t s o f a m y g d u l e s and  m i c r o a m y g d u l c s i n t h e r o c k s may be s i m i l a r t o t h e c e m e n t i n g compounds i n t h e v i c i n i t y ,  t h o u g h most a r c  composed o f " g c l - p a l a g o n i t e " and " f i b r o - p a l a g o n i t o " . F e r r o - m a n g a n e s e f o r m s a d i s c o n t i n u o u s pavement o f 5 cm. t h i c k c o n c r e t i o n a r y s l a b s i n an a r e a o f r e l a t i v e l y o l d , w e a t h e r e d r o c k s on n e i g h b o u r i n g r a t e o f growth o f these  H o d g k i n s Seamount.  The  c o n c r e t i o n s a p p e a r s t o be s l o w and  o f t h e same o r d e r o f m a g n i t u d e a s t h a t r e c o r d e d f o r s i m i l a r open o c e a n o c c u r r e n c e s  by o t h e r a u t h o r s .  The  r o c k s on t h e summit o f H o d g k i n s Soamount, w h i c h a r c h o s t t o t h e f e r r o - m a n g a n e s e , may,  t h e r e f o r e , be w e l l o v e r  one  m i l l i o n years o l d . 7.  I c e - r a f t i n g i s an i m p o r t a n t  the area, e s p e c i a l l y  agent o f s e d i m e n t a t i o n i n  on t h e summit o f H o d g k i n s Seamount.  The summit o f Bowie Seamount i s a l m o s t d e b r i s and i t i s t h o u g h t shallowness  free of g l a c i a l  t h a t t h i s i s due t o i t s  o r exposure d u r i n g stages of the P l e i s t o c e n e  e p o c h , and t o b u r i a l volcanic deposits.  o f g l a c i a l m a t e r i a l by more r e c e n t  107  V I . REFERENCES B a r n e s , S.S., a n d Dymond, J.R., 1967, R a t e s o f accumulation o f ferro-mangancso nodules; N a t u r e , V. 213, pp. 1218 - 1219. B o n a t t i , E., and Nayudu, Y.R. 1965, The o r i g i n o f manganese n o d u l e s on t h e o c e a n f l o o r ; Am. J . S c i . , V. 263, pp. 17 - 39. Bouma, A.H., 1962, S e d i m c n t o l o g y o f some f l y s c h d e p o s i t s a graphic approach t o f a c i e s i n t e r p r e t a t i o n ; E l s e v i e r , Amsterdam. B r a d y , L . F . , a n d Webb, R. W., 1943, C o r e d bombs f r o m A r i z o n a and C a l i f o r n i a v o l c a n i c cones; J . G e o l . , V. 51, PP. 398 - 410. Carlisle,  D., 1963, P i l l o w b r e c c i a s and t h e i r a q u a g o n c t u f f s , Quadra I s l a n d , B r i t i s h Columbia; J . G e o l . , V. 7 0 , pp 48 - 71.  Cox, A., 1969, G e o m a g n e t i c pp 237 - 245-  reversals;  S c i e n c e , V.  163,  C u r r a y , J.R., 1965, L a t e Q u a t e r n a r y h i s t o r y o f t h e c o n t i n e n t a l shelves of the United States i n The Q u a t e r n a r y o f t h e U n i t e d S t a t e s ; P r i n c e t o n U n i v e r s i t y P r e s s , pp. 723 - 735. 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John  Ill  St.  Amand, P. , 19.57, G e o l o g i c a l and G e o p h y s i c a l s y n t h e s i s of the t e c t o n i c s of p o r t i o n s of B r i t i s h Columbia, t h e Yukon T e r r i t o r y and A l a s k a ; B u l l . G e o l . Soc. Am., V. 6 8 , pp. 1343 1370. J.A., 1969, Bowie Seamount - p r e l i m i n a r y s u r v e y f o r i n s t r u m e n t package placement; Defence Research Establishment P a c i f i c Technical Memorandum 69 - 7.  Scrimgor,  S h c p a r d , F.P. , 1963, Submarine Geology, Harper & Mew Y o r k - E v a n s t o n - L o n d o n . S m i t h , R.E., 1967, Segregation Am. J . S c i . , V. 2 6 5 ,  Row,  vesicles in basaltic lava; pp. 696 - 713.  Solomon, M., 1969, The n a t u r e and p o s s i b l e o r i g i n o f t h e p i l l o w l a v a s and h y a l o c l a s t i t e b r e c c i a s o f K i n g I s l a n d , A u s t r a l i a ; Q u a r t . J . G e o l . Soc. L o n d o n , V. 124, pp. 1.53 169Sutherland  Brown, A., 1968, G e o l o g y o f t h e Queen C h a r l o t t e I s l a n d s , B r i t i s h Columbia: British Columbia). D e p a r t m e n t o f M i n e s and P e t r o l e u m R e s o u r c e s B u l l . , V. 54.  T h o r a r i n s s o n , S., 1964, The s u b m a r i n e e r u p t i o n o f f t h e Vostmann I s l a n d s 1 9 6 3 - 6 4 , B u l l . V o l c a n o l . , V. 2 7 , pp. 435 - ^i45. .  , 1965, S i t t a f h v e r j u urn S u r t s e y j a r g o s i d , some f a c t s a b o u t t h e S u r t s e y e r u p t i o n . M a t t u r u f r a c d i n g u r i n n , V. 35, pp. 153 181.  Wager, L.R., Brown, G.M., and W a d s w o r t h , W.J. I 9 6 0 , T y p e s o f I g n e o u s c u m u l a t e s ; J . P e t r o l . , V. 1, up 73 - 8 5 . -  - , 1968, R h y t h m i c and c r y p t i c l a y e r i n g i n m a f i c and u l t r a m a f i c p l u t o n s , i n B a s a l t s ; Hess and P o l d e r v a a r t Ed., I n t r s c i c n c o P u b l i s h e r s ( J o h n W i l e y & S o n s ) , New Y o r k - L o n d o n - S y d n e y , pp. 573 622.  W a s h i n g t o n , H.S., 1926, S a n t o r i n i e r u p t i o n o f 1925: G e o l . Soc. Am., V. 37, pp. 349 384.  Bull.  112  Wilcox,  R.E., 1965? V o l c a n i c a s h c h r o n o l o g y i n The Q u a t e r n a r y o f t h e U n i t e d .States; P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n , Nov; J e r s e y , pp. 807 -- 8 1 6 .  Y o d c r , U.S. J r . , a n d Sahama, T.G., 1 9 5 7 , O l i v i n e X - r a y d e t e r m i n a t i v e c u r v e ; .4m. M i n e r a l o g i s t , V. 4 2 , PP. 475 - 4 9 1 . Y o d e r , H.S., a n d T i l l e y , C.E., 1 9 6 2 , O r i g i n o f b a s a l t magmas: an e x p e r i m e n t a l s t u d y o f n a t u r a l and s y n t h e t i c r o c k s y s t e m s ; J . P e t r o l . , V. 3 , pp. 342 - 5 3 2 . G . , I 9 6 0 , L ' c r u p t i o n du v o l c a n do C a p e l i n h o s , I l l e de F a i a l , A z o r e s , B u l l . V o l c a n o l . , V. 2 3 , pp. 77 - 1 0 0 .  Zbyszewski,  Equi-omont u s o d d u r i n g s h i p w o r k EDO  W e s t e r n Sonar - M o d e l l 8 p w i t h 12  Aldcn P r e c i s i o n Gifft  KHz T r a n s d u c e r  Graphic R e c o r d e r - Model  419  Depth R e c o r d e r - T r a n s c i e v c r  A l p i n e G e o p h y s i c a l X-band D e c c a Radar - M o d e l  r a d a r t r a n s p o n d e r - model 427-D  828  V a r i a n A s s o c i a t e s Marine P r o t o n P r e c e s s i o n Magnetometer model V - 4 9 3 7 E.G.  & G. I n t e r n a t i o n a l Camera - m o d e l 200, and s o u r c e - model  Ocean  R e s e a r c h Equipment  P o t t c r s s o n grab sampler Tv/elvo i n c h p i p e  dredge  D i c t z - Lafond snapper  210 P i n g c r - model 250B  light  -  APPENDIX B  Sample D e s c r i p t i o n s ^ f r o m E a c h S t a t i o n Station  67-1  Lapilli  T u f f - brown; u n s o r t e d ,  lapilli  and a s h ; w o a k l y cemented (cement o f i r o n  and  organic  calcium  carbonate);  a l g a L i t h o t h a m n i o n and s i l i c e o u s Cow-dung Bombs - f l a t ,  2 - 3  subangular t o rounded  m a n t l e d by t h e r o d c o r a l l i n e sponges.  cm. t h i c k ; b r e a d  crust  f r a c t u r e on u p p e r s u r f a c e ; i n t e r i o r more o r l e s s I n c l u s i o n - coarse,  oxides  hollow.  c r y s t a l l i n e p l a g i o c l a s e , o l i v i n e and  magnetite. Station  67-2  Volcanic  c o n g l o m e r a t e - y e l l o w i s h brown; p o o r l y  w e l l rounded l a p i l l i ,  f r o s t e d c r y s t a l g r a i n s , abraded  f r a g m e n t s ; s t r o n g l y cemented s u r f a c e and  sorted,  crust  shell  (red ferruginous  b l a c k m a n g a n i f e r o u s c e m e n t s ) , w e a k l y cemented b e l o w  c r u s t ; i c e - r a f t e d c o b b l e cemented i n s u r f a c e Pillow  fragments - roughly  b a s a l t ; quenched s u r f a c e  pyramidal;  crust.  fresh vesicular  o f s i d e r o m e l a n e w i t h no s i g n o f  palagonite. Vitric-crystal yellow;  t u f f - d a r k brown t o r e d d i s h b r o w n and  sand s i z e g r a i n s ; t h i n - b e d d e d , w i t h graded bed  t o p p e d by t h i n l a m i n a t e d  b e d , ( c . f . 67 - 6).  Unconsolidated m a t e r i a l - heterogeneous mixture of angular, unsorted cinders, l a p i l l i (c.f.  a s h and some s h e l l m a t e r i a l  67 - 10).  Indeterminate  - v a r i o u s a n g u l a r and c l i n k e r y b a s a l t  pebbles. S t a t i o n 67 - _^ Lapilli and  t u f f - r o d ; u n s o r t e d , angular t o rounded  lapilli  a s h ; s t r o n g l y cemented (cement i s f e r r u g i n o u s , hence  the r o d c o l o u r ) ; e n c r u s t e d w i t h r e d c o r a l l i n e  algae.  Lapilli  t u f f - d a r k brown; u n s o r t e d , a n g u l a r t o s u b r o u n d e d  lapilli  and a s h ; f r i a b l e ,  organic  carbonate).  weakly  I n c l u s i o n - coarse aggregate Indeterminate Station  cemented  o f pyroxene  (cement o f  crystals.  - various b a s a l t i c pebbles 1-4  cm. a c r o s s .  67-4  B a s a l t - flow top; fragments  probably spalled  from t h e  surface of a l a v a flow; dark grey, g l a s s y b a s a l t o f the same c o m p o s i t i o n a s t h e r o c k s o f 67 - 6; q u e n c h e d s u r f a c e of  sideromelane  weathered along Vitric-crystal  with thin palagonitc skin;  slightly  fractures. t u f f - ochre y e l l o w ; sand s i z e grains.;  f i n e l y laminated;  ( c . f . 67 - 6).  S t a t i o n 67 - 5 Lapilli  t u f f - black; unsorted  as a s l a b b y  lapilli  and a s h ;  c r u s t , cemented w i t h manganese  occurs  oxides.  Bombs - r a n g e f r o m f l a t t e n e d cow-dung v a r i e t y t o h i g h l y d e f o r m e d , g l o b o s e f o r m s with, i n v o l u t e a n d r o p y quenched s k i n o f Station  surfaces;  sideromelane.  6 7 - 6  V i t r i c - c r y s t a l t u f f - d a r k brown t o r e d d i s h brown a n d y e l l o w : sand s i z e g r a i n s ; thin-bedded,  w i t h graded bed  t o p p e d by f i n e h o r i z o n t a l l a m i n a t i o n s ;  ferruginous  imparts  the v a r i e t y of colours.  Lapilli  t u f f - red; t h i n crust only; unsorted  ferruginous  cement  fragments;  cement.  P i l l o w fragments - pyramidal  shape; b r e a d - c r u s t y ,  upper s u r f a c e o f sideromelane without  chilled,  palagonitc;  p l a g i o c l a s e and hornblende - r i c h v e s i c u l a r b a s a l t . Cow-dung bombs - f r e s h a n d b l a c k , w i t h b r e a d c r u s t u p p e r s u r f a c e ; a l s o weathered y e l l o w v a r i e t i e s with hollow I n c l u s i o n s - common; f S L d s p a t h i c a n d g a b b r o i c ;  centres.  coarsely  crystalline. Unconsolidated of ash,  m a t e r i a l - heterogeneous, unsorted  lapilli  Indeterminate  and c i n d e r s o f undetermined  - unconsolidated  mixture  affinities.  m a t e r i a l above;  nondescript  b o u l d e r s and c o b b l e s o f c o m p o s i t i o n s i m i l a r t o t h e p i l l o w f r a g m e n t s. Station  67-7  Unconsolidated m a t e r i a l - w e l l rounded, s o r t e d with frosted Andesitic  lapilli  c r y s t a l g r a i n s ( c . f . 67 - 2 ) .  rock - angular, blocky, i r r e g u l a r l y  jointed  b o u l d e r s of n o n - v e s i c u l a r rock s l i g h t l y weathered  along  fractures. Station  67-8 - u n s u c c e s s f u l ; hard  bottom.  S t a t i o n 67 - 9 U n c o n s o l i d a t e d m a t e r i a l - s m a l l amount o f c r y s t a l a n d vitric  sand;  insufficient  sample f o r use.  S t a t i o n 67 - 10 Unconsolidated m a t e r i a l - unsorted, subangular fresh, black l a p i l l i shell  and a s h ; some fre.gmented  t o rounded, b u t unworn  material.  Basalt - boulder; prominent  f r e s h , dark grey, v e s i c u l a r b a s a l t w i t h  o l i v i n e and e.ugite  phenocrysts.  S t a t i o n 67 - 11 Unconsolidated m a t e r i a l - brown-stained and  ash; i n s u f f i c i e n t  sample f o r u s e .  cinders, l a p i l l i ,  S t a t i o n 67  12  -  Unconsolidatcd  m a t e r i a l - p o o r l y s o r t e d , subrounded  r o u n d e d , brown l a p i l l i  and  ash w i t h abundant  s t a i n e d s h e l l m a t e r i a l ; s t a i n i n g of l a p i l l i m a t e r i a l i s duo S t a t i o n 67  S t a t i o n 67  yellowand  shell  oxides.  13  -  to i r o n  to  unsuccessful -  Ik  Unconsolidated  m a t e r i a l - p o o r l y s o r t e d , subrounded  rounded,  black l a p i l l i  fresh,  and  ash w i t h abundant  to shell  material. Basalt - boulder;  fresh,, dark grey,  basalt,  with calcareous  encrusted  S t a t i o n 67  -  ash;  m a t e r i a l - brown-stained cinders  q u a n t i t y too  Bombs - F u s i f o r m surface  quenched  30  cm.;  s m a l l t o work w i t h .  bombs, some undamaged, o t h e r s w i t h  s p a l l e d o f f ; a l s o bombs s i m i l a r t o 67  thin palagonite  Pillow  lapilli.  68-1  Station  outer  organisms.  15  Unconsolidated and  vesicular, olivine  s k i n sometimes p r e s e n t  on t h e  the -  5;  sideromelane  surface.  fragments - pyramidal  fragments w i t h r a d i i  a l s o c r u d e l y p i l l o w e d forms; rock  of 5  to  type i s a dark  grey,  f i n e l y v e s i c u l a r , o l i v i n e b a s a l t ; sideromelane  surface with w e l l developed palagonitc  skin;  outer  rocks  g e n e r a l l y weathered along f r a c t u r e s . • Pillow breccia - pillow pebble-size vitric  f r a g m e n t s , as a b o v e , and  smaller  p i l l o w c r u s t fragments, set i n a matrix  of  tuff.  Flow top - f l a t , pillowed  flow surface, apparently  from  non-  flow; bread crust surface of sideromelane  p a l a g o n i t c s k i n ; r o c k i s weathered as S t a t i o n 68  -  with  above.  k  Unconsolidated  m a t e r i a l - ponded s e d i m e n t ; 50  of small l a p i l l i  and  -  50  mixture  G l o b i g e r i n a o o z e ; sample t o o s a m l l  to  use. Station Andosite grey  68-5 - weathered cobbles  andesite,  of f i n e l y v e s i c u l a r , l i g h t  each e n v e l o p e d i n a s h e a t h  f r a g m e n t s a r e r o u n d e d due  t o deep  o f c l a y ; many  weathering.  I c e - r a f t e d d e t r i t u s - smooth, r o u n d c o b b l e s Station  68-6 - unsuccessful:  few  a s m a l l amount o f c a l c a r e o u s for  use.  of g r a n o d i o r i t e ,  chips of weathered r o c k clay; insufficient  and  material  6 8 - 7  Station Pillow  fragments - l a r g e fragments o f pyramidal  also pebble-size  shape,  fragments of p i l l o w c r u s t s ; f r e s h , dark  grey, h i g h l y v e s i c u l a r b a s a l t with sideromelane  chilled  s u r f a c e ; p a l a g o n i t e i s absent. Station 6 8 - 8 Pillow  (Summit o f H o d g k i n s Seamount)  fragments - pale  grey  t o tan; deeply  weathered,  vesicular basalt. Palagonito s  t u f f - yellow;  few w e a t h e r e d l a p i l l i  sand-size  bcisalt  and  boulder  c r y s t a l s a r e common.  fcrro-manganose - as nodules w i t h nuclei!,  g r a i n s and  cemented w i t h v a r y i n g amounts o f  ferro-mangancso; rhombic z e o l i t e Concretionary  palagonite  weathered  a n d a s s l a b s ; composed o f l a m i n a t e d  b o t r y o i d a l , d a r k brown f e r r o - m a n g a n e s e o x i d e s a n d  hydrated  o x i d e s ; maximum t h i c k n e s s 5 cm.  I c e - r a f t e d d e t r i t u s - s i z e range from s a n d - s i z e boulders  30 cm. i n d i a m e t e r ;  graded l i t h i c  wide range o f  schist, tuff, porphyritic  d a c i t e , g r a n i t e , d i o r i t e and m a g n e t i t e - r i c h rocks  compositions:  and a r k o s i c a r e n i t e s , p h y l l i t e , . a m p h i b o l i t i c  greenstone, t r e m o l i t e - garnet  of these  m a t e r i a l to  has a d i s t i n c t i v e  gabbro.  None  enough l i t h o l o g y t o t r a c e  to i t s o r i g i n . Indeterminate  - basalt pebbles of i r r e g u l a r  shape;  deeply  weathered to a tan colour;  many have w e a t h e r e d  into  r o u n d e d shapes'. Station Basalt basalt.  6 8 - 9 - boulder;  fresh, dark grey, v e s i c u l a r ,  olivine  

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