Field-trip guide to Mount St. Helens, Washington - An overview of the eruptive history and petrology, tephra deposits, 1980 pyroclastic density current deposits, and the crater

Abstract

Our hope is that future generations of scientists as well as the general public will use these field guides as introductions to these fascinating areas and will be enticed toward further exploration and field-based research.

Unravelling the complexity of magma plumbing at Mount St. Helens: a new trace element partitioning scheme for amphibole

“…and low-pressure fractionation dominated by plagioclase ± MgHbl. It is notable that our calculated AEM do not record any melts more primitive than andesite, and this is consistent with observations that mingled basalts and basaltic andesites typically contain an assemblage of olivine + plagioclase + clinopyroxene ± orthopyroxene (Wanke et al 2019;Pallister et al 2017); and this may reflect insufficiently high water concentrations to stabilise amphibole in these primitive melts (e.g. Leeman & Smith 2019;Rea et al 2012;Gardner et al 1995).…”

“…These trace element patterns are generally not consistent with generation of the more evolved rocks by low-pressure fractional crystallisation from primitive parental magmas, which would result in incompatible behaviour for these elements Leeman 1987, 1993;Gardner et al 1995;Blundy et al 2008;Pallister et al 2008;Sisson et al 2014). Although the erupted magmas contain various cumulate gabbro inclusions (Scarfe and Fujii 1987;Heliker 1995), U-Pb dates for zircons from these samples show that they are not cognate to the Mount St Helens system but derive from an older (Miocene) intrusion (Pallister et al , 2017. Amphibole equilibrium melts from MgHst-Tsch also show approximately constant (though scattered) HFSE and HREE, concentrations, and slightly increasing La/Yb, constant Sm/Yb and decreasing Dy/Yb from 55-70 wt% SiO 2 ( Figure 6).…”

“…Solidification of evolved dacites at within the crust under Mount St. Helens would generate diorites and granodiorites with low-temperature minerals such as biotite, zircon and apatite, as shown by the presence of these as interstitial phases in plutonic inclusions in Mount St Helens magmas (Heliker 1995;Wanke et al 2019). Although the plutonic inclusions typically have older (Tertiary) zircon ages (Pallister et al 2017), similar mineralogy is observed in the oldest (Ape Canyon) deposits from Mount St Helens, which included low-temperature dacites and rhyodacites with quartz and biotite (Clynne et al 2008). Therefore, partial remelting of plutonic residua from earlier intrusive events or magmatic stages would generate disequilibrium melts enriched in Zr and HREE (from zircon), LREE (from apatite) and Ba, Rb and Nb (from biotite) (Villaros et al 2009;McLeod et al 2012).…”

Deposit‐Derived Block‐and‐Ash Flows: The Hazard Posed by Perched Temporary Tephra Accumulations on Volcanoes; 2018 Fuego Disaster, Guatemala

“…Perched temporary tephra stores range from segments of gently sloping ignimbrite sheets (e.g., Mount Pinatubo; Torres et al., 1996), stacked pumiceous density current deposits on a fan (e.g., Mount St. Helens; Pallister et al., 2017), to more protracted pyroclastic accumulations from frequent small explosive eruptions on proximal volcanic flanks (e.g., El Fuego, this study). Loose, partly aerated pyroclastic deposits have low strength and, being insulative, may remain hot for several years: loose ignimbrite at Pinatubo remained above 500°C for longer than 2 yr after the 1991 eruption, despite heavy tropical rainfall.…”

Joint Inversions of Ground Deformation, Extrusion Flux, and Gas Emissions Using Physics‐Based Models for the Mount St. Helens 2004–2008 Eruption

“…The largest plinian eruption in the Holocene produced 2.3 km 3 of material (Nathenson, 2017). However, given that Mount St. Helens has more frequently produced lava flows and domes, the more common eruption volumes are smaller (Clynne et al., 2008; Pallister et al., 2017). Earthquake hypocenters in the 1980s show distinct lobes surrounding an earthquake‐free zone, thought to be the semiliquid magma body, of about 10–20 km 3 (Scandone & Malone, 1985).…”