Gaunt, HE; (2014) Experimental study of permeability under simulated volcanic conditions on lava dome rocks from mount St. Helens: constraints on degassing and eruption style. Doctoral thesis , UCL (University College London).

Abstract

The research presented in this thesis integrates field observations and sampling, data from controlled laboratory experiments with a radially-symmetric fluid flux model to investigate permeability controls on degassing processes operating during lava dome eruptions at silicic volcanoes. Laboratory permeability measurements were made on samples collected systematically across the marginal shear zone of a lava spine at Mount Saint Helens (MSH), Washington, USA, from the 2004-08 eruption, at room temperature and confining pressures up to 85 MPa. Permeability experiments were also performed on samples of massive dacite from the spine core at a range of temperatures up to 900°C and a confining pressure of 10 MPa. A simplified radially-symmetric fluid flux model was developed which incorporated the observed structure of the conduit at Mount Saint Helens and the measured permeability profile to show the effects of the permeability on the magnitude and direction of fluid flow. Permeability was found to be essentially isotropic in the conduit interior but highly anisotropic in the marginal shear zone rocks. Vertical permeability in the marginal rocks is increased by two orders of magnitude, through the formation of an aligned shear fracture network. Ultracataclasite shear bands in the fault core also decreased the horizontal permeability by two orders of magnitude. Permeability was also seen to decrease by around four orders of magnitude as temperature was increased to 900°C. In contrast to previous work that assumed significant lateral transport of gases, modeled fluid flow using the experimentally determined permeability data shows that the vertical volatile volumetric flow rate will be orders of magnitude greater than the horizontal rate into the wall rock; more than 90% of volatile flow would have been partitioned vertically within the conduit-margin fault zone, consistent with the location of gas and ash venting during the eruption. Changes in permeability with temperature indicate that magma rising in the conduit becomes progressively more permeable to gas escape during ascent and crystallisation. When applied to gas sealing processes, the intrusion of molten lava under a solidified plug and the associated heating may cause the permeability of the overlying rock to decrease sufficiently that gas escape is inhibited, allowing for the build up of pressure and potentially an explosive eruption

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