Petrology of the 2004-2006 Mount St. Helens lava dome -- implications for magmatic plumbing and eruption triggering

Abstract

5 km. Viewed in the context of seismic, deformation, and gas-emission data, the petrologic and geochemical data can be explained by ascent of a geochemically distinct batch of magma into the apex of the reservoir during the period 1987-97, followed by upward movement of magma into a new conduit beginning in late September 2004. The question of new versus residual magma has implications for the long-term eruptive behavior of Mount St. Helens, because arrival of a new batch of dacitic magma from the deep crust could herald the beginning of a new long-term cycle of eruptive activity. It is also important to our understanding of what triggered the eruption and its future course. Two hypotheses for triggering are considered: (1) top-down fracturing related to the shallow groundwater system and (2) an increase in reservoir pressure brought about by recent magmatic replenishment. With respect to the future course of the eruption, similarities between textures and character of eruption of the 2004-6 dome and the long-duration (greater than 100 years) pre-1980 summit dome, along with the low eruptive rate of the current eruption, suggest that the eruption could continue sluggishly or intermittently for years to come.

Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems

“…In early July 2005 the dome was 2365 m high , so the maximum dome elevation was more than 350 m above the imaged crack. On the basis of this maximum dome height, we find the lithostatic pressure on the subjacent crack is on the order of 8 MPa, assuming dome rock with a density of 2300 kg/m 3 [e.g., Pallister et al, 2008]. This is nearly an order of magnitude larger than the maximum amplitude of the pressure fluctuation estimated above.…”

“…In particular, Iverson et al [2006] ascribe them to stick-slip motion between the extruding lava and conduit walls [see also Iverson, 2008;Harrington and Brodsky, 2007]. A gouge layer 1 -2 m thick on the outside of the lava spines [e.g., Cashman et al, 2008;Pallister et al, 2008], the regularity and similarity of the shallow earthquakes, and a fairly regular extrusion rate [e.g., Schilling et al, 2008] seem consistent with a stick-slip model for these earthquakes. However, despite a deliberate effort to capture stick-slip motions of the spine geodetically, no stick-slip events have been documented (D. Dzurisin, personal communication, 2007).…”

“…The exterior of the extruded spines are composed of fault gouge 1 -2 m thick [e.g., Cashman et al, 2008;Pallister et al, 2008]. This has been attributed to seismogenic stick slip of the dome and conduit [Iverson et al, 2006], although gouge could form during aseismic slip.…”

“…The corner in the conduit implied by the VLP source could promote gouge formation. Occasionally, curvilinear bands of discolored gouge that parallel the contact between the spine and debris apron at the base of the spine have been observed [e.g., Cashman et al, 2008;Pallister et al, 2008]. These ''bathtub rings'' may represent accumulations formed during transient sagging of the dome related to the collapse of the crack.…”

“…[94] Mineralogical indicators in the dome rocks, such as the presence of tridymite in most samples, indicate groundmass crystallization at low pressures (10 -25 MPa) and high temperatures (900°C) Pallister et al, 2008]. Cashman et al [2008] and Pallister et al [2008] argue that relatively little crystallization occurs until the magma is <1 km from the surface, where it is driven by extensive degassing rather than cooling.…”