Pathways for degassing during the lava dome eruption of Mount St. Helens 2004–2008

H Elizabeth Gaunt,Peter R Sammonds,Rosanna Smith,John S Pallister,Philip G Meredith

doi:10.1130/g35940.1

H Elizabeth Gaunt, Peter R Sammonds + Show 3 more

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https://doi.org/10.1130/g35940.1

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Abstract

The ability of volatiles to escape rising magma regulates the explosivity of a volcanic system. During silicic lava dome eruptions, strain localization at the conduit margin occurs during magma ascent, creating a damage halo with implications for gas escape. Here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the A.D. 2004–2008 lava dome at Mount St. Helens (Washington State, USA). The results show increasingly large permeability anisotropy of as much as four orders of magnitude (over ∼4 m) moving from the interior of the spine through the damage halo. We find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core. Our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers. Here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall.

Highlights

The presence of gas in magma ascending to the Earth’s surface has a strong control on the timing and style of volcanic eruptions (Woods and Koyaguchi, 1994; Sparks, 1997)
It has been proposed that shear-induced fracturing at the conduit margins creates a damage halo, which acts as a permeable pathway for vertical gas escape (e.g., Holland et al, 2011; Schipper et al, 2013)
In this paper we present steady-state flow permeability measurements of a suite of rocks collected from the margin of spine 4 in two perpendicular orientations

Summary

Introduction

The presence of gas in magma ascending to the Earth’s surface has a strong control on the timing and style of volcanic eruptions (Woods and Koyaguchi, 1994; Sparks, 1997). Permeable fracture networks can develop and allow the gas phase to escape vertically through the magma column (e.g., Holland et al, 2011; Schipper et al, 2013) or horizontally through the conduit walls (Stasiuk et al, 1996; Jaupart, 1998). It has been proposed that shear-induced fracturing at the conduit margins creates a damage halo, which acts as a permeable pathway for vertical gas escape (e.g., Holland et al, 2011; Schipper et al, 2013).

Results

Conclusion