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Welding in pyroclastic deposits

University of British Columbia

The process of welding in pyroclastic deposits involves compaction sintering and flattening of hot glassy particles. Pronounced changes in physical properties attend welding; as welding intensifies, for example, primary porosity is reduced, density increases and the deposit becomes progressively more foliated. Consequently, welding intensity in individual deposits varies with stratigraphic depth. This thesis comprises field, laboratory and experimental studies aimed at understanding the conditions necessary for welding, the rheology and mechanisms of welding the prediction of welding intensity and timescales of the welding process. Changes in welding intensity and the accumulation of strain in single pyroclastic flow cooling units are studied using physical property measurements. Combined with petrographic indicators, these measurements are used to develop a classification scheme for welding intensity. The scheme has eight indices, demarcated by specific petrographic features correlated to a range of normalized density values used to calculate strain in welded deposits. The physical mechanisms by which strain accumulates are analyzed through deformation experiments on analogue glass beads and natural pyroclastic materials. The experiments use a new deformation apparatus capable performing hightemperature, low-load deformation experiments and collecting high-resolution rheological data. Total strain is partitioned into axial (porosity loss) and radial (bulging) components. The relative amount of each is dependent on initial porosity, temperature and strain rate. In all cases experimental cores showed a strain-dependent rheology that is more strongly affected by temperature than by load or strain rate. Results from these experiments are used to develop a relationship in which the effective viscosity (ŋe) of the experimental cores is predicted by: ŋe = ŋo exp – α (Φ / 1-Φ) where ŋo is melt viscosity, Φ is sample porosity and α is a constant dependent on material properties. This predictive, rheological model provides insight into the relative roles of emplacement temperature, load and glass transition temperature on welding intensity. The model is used to predict strain accumulation with time during welding and the timescales of the welding process.

Thesis/Dissertation

application/pdf

2009-12-01

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http://hdl.handle.net/2429/16048

Oceanography

University of British Columbia

UBCV

DSpace