Porosity and Textural Evolution of Bubbly Magma under High-Temperature Uniaxial Deformation

A densified, degassed plug is thought to fill the near surface conduit of the Soufrière Hills volcano (SHV) which facilitates pressure build up between Vulcanian eruptions. Pumice samples of a wide range of density (500-1200 kg.m-3) collected from the February 2010 eruption were deformed using a high temperature uniaxial press and analysed for porosity evolution and textural development at increasing strain. Deformation to incremental axial strains of 5-30%, at 5% intervals were investigated at 2.3 MPa axial stress at temperatures relevant to SHV (865 °C). We characterised the change in sample volume, density and pore evolution by helium pycnometry, gas permeability and ultrasonic wave velocity measurements both prior to and post deformation. We show that under relatively low axial stress and at magmatic temperatures a rapid ductile total porosity reduction is accommodated almost entirely by axial strain and only minimal radial strain (<5%). The bulk pore-collapse rate is proportional to the starting porosity for a given experimental temperature. In detail, connected porosity, which dominates the porous network, accommodates the majority of the pore closure. Isolated pore volumes close and dilate as pores pinch and coalesce. Using X-Ray computed micro tomography, optical microscopy and electron backscatter micrographs we present a 2D and 3D reconstruction of the evolving pore network and phenocryst anisotropy. We quantify the evolving vesicle number density and the size distribution and model the permeability by the lattice Boltzmann simulation. We compare these results to static measurements on Montserrat dome rock to explore the hypothesis that dome-forming magma is the compacted product of bubbly conduit magma. Our experiments show that magma is capable of reaching dome rock densities with a large reduction in porosity and permeability over the eruption repose intervals of approximately 9.5 hours displayed by the 1997 eruptions. We propose a scenario where gas flux from an actively degassing region beneath a densifying plug is a controlling parameter which, when permeability in the plug reaches a critical level, facilitates the development of high pore overpressure. The interplay between gas flux rates and rates of permeability collapse is key to understanding cyclic Vulcanian activity such that if one exceeds the other then transitions from effusive to explosively eruptible magma may be induced.