Strain localization in vesicular magma: Implications for rheology and fragmentation
Heather M.N. Wright and Roberto F. Weinberg
School of Geosciences, Building 28, Monash University, Clayton, VIC 3800, Australia
The rheology of two- or three-phase magmas has been the focus of much interest because it controls magma ascent and eruption behavior. Research on magma rheology has typically considered homogeneous flow. Here we demonstrate, based on natural examples, that strain resulting from viscous fl ow preceding explosive fragmentation localizes into shear zones at a microscopic scale. Strain localization affects the
rheological behavior of magmas, modifying predictions based on experiments. Localization leads to high-strain-rate shear zones, where elongated, sheared vesicles and shear heating have a weakening effect, surrounding regions of relatively low strain rate, where subequant vesicles have a strengthening effect. Thus, energy is dissipated more efficiently into localized bands, where strain rate increases through feedback effects and can lead to melt fragmentation.
Physical and Chemical Signatures of Degassing in Volcanic Systems
Heather Michelle Wright (A Dissertation)
The physical and chemical effects of degassing of silicates melt are important volcanic processes, beginning prior to eruption and continuing after deposition of erupted pyroclasts. Volatile species, such as water and carbon dioxide, exsolve to form bubbles within rising volcanic melt. The ability of gas to migrate out of this melt affects eruptive style. To determine controls on permeable gas migration within a volcano, physical properties of bubble network geometry are quantified using #-D x-ray tomographic and 2-D SEM BSE image analysis as well as laboratory measurements of porosity, permeability, and electrical conductivity. The same techniques are used to examine welding compaction in a pyroclastic flow deposit. The relationship between measured properties is addressed; permeability is controlled by porosity, but is also a function the geometry of the pore pathways. This dependence is particularly evident in highly anisotropic samples such as tube pumice and welded tuffs, where permeability can be orders if magnitude higher along tubes and foliation planes, respectively.