Porosity-viscosity relationships during compaction of pumice Gainer, Daniel
During compaction of pyroclastic material, strain is mainly accommodated by volume reduction from the closing of pore space. At temperatures above the materials softening point, this deformation is achieved through viscous relaxation. The effective viscosity of the material increases with loss of porosity and increased viscosity can lead to brittle deformation. Here, high temperature experiments are conducted on pumice cores of diameter 4 cm and length 8 cm, sampled from Mount Meager’s Pebble Creek Formation using the Volcanic Deformation Rig in the Centre for Experimental Study of the Lithosphere (CESL) at the University of British Columbia. Cores were deformed at 875 °C under a constant displacement rate of 2.78*10-⁴ or at constant loads of 1334 N and 3559 N. Initial and final porosities were measured for each core. The acoustic emissions of brittle deformation were measured with an Acoustic Emission System (PCI-2 AE) where high acoustics indicated fracturing from brittle deformation. Samples showed a strain dependent rheology where the effective viscosity (ηe) increased with increased strain and reduced porosity until viscous deformation was replaced by viscous and brittle deformation. Results show that at this point of viscous-brittle deformation, the stress required to continue straining the material at the same rate increases exponentially. With a constant applied stress, the strain rate will rapidly decrease across this point. At 875 °C Log10(ηe) increases linearly with porosity loss to a point of zero porosity between 10¹¹˙⁵ to 10¹⁴˙⁵ Pa s. This relationship is important in the welding of ignimbrites and for volcanic recharge, as pyroclastic material can seal a volcanic conduit. The porosity will affect the materials permeability, which will dictate its ability to either effusively release gases or build up pressure leading to an explosive eruption.
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