Hybrid rhyolitic eruption at Big Glass Mountain, CA, USA

Jonathan Castro,Sebastian Walter

doi:10.30909/vol.04.02.257277

Abstract

Eruptive dynamics of the 1060 CE rhyolitic eruption of Big Glass Mountain (BGM), USA, are investigated with field observations, hydrogen isotope and H2O content analysis of pyroclastic obsidian chips and lavas. Field relations at BGM reveal evidence for hybrid eruption, defined as synchronous explosive venting and effusive emplacement of vast obsidian lava flows. This activity is particularly well manifested by extensive breccia zones implanted within the BGM obsidian lavas, which may represent rafted tephra cones, in addition to remnants of airfall tephra on the lava. Rhyolitic obsidians collected from a 2.5-m-thick fall deposit and co-eruptive lava flow were studied by FTIR and TCEA methods to elucidate the eruption’s degassing history. The data, along with VolcDeGas program simulations, demonstrate a correlation between H2O content and H-isotopic composition (δD) that likely reflects ever-increasing amounts of volatile loss via repetitive close-system steps, best described as batched degassing.

Highlights

IntroductionRhyolite eruptions, is critical to assessing volcanic threat [e.g. Eichelberger 1995; Castro and Dingwell 2009; Watt et al 2009]
Studying silicic volcanism, rhyolite eruptions, is critical to assessing volcanic threat [e.g. Eichelberger 1995; Castro and Dingwell 2009; Watt et al 2009]
A first-order question in the context of volcanic hazard mitigation is, How will an eruption progress once underway? Early work on Holocene rhyolites— a rare yet physically impactful form of volcanism— recognized that these systems are both “explosive and effusive” in nature, this assessment being drawn from field observations of obsidian lava flows perched atop pyroclastic fall deposits [e.g. Heiken 1978; Eichelberger and Westrich 1981; Taylor et al 1983]

Summary

Introduction

Rhyolite eruptions, is critical to assessing volcanic threat [e.g. Eichelberger 1995; Castro and Dingwell 2009; Watt et al 2009]. After initial week-long explosive episodes, eruptions at both Chaitén (2008) and Cordón Caulle (2011), Chile, exhibited rarely seen “hybrid” activity, comprising simultaneous pyroclastic fountains (100s to 1000s m) and lava eruptions from the same vents [Schipper et al 2013; Castro et al 2014; 2016]. The latest activity of MLV, about 950 years ago [Donnelly-Nolan et al 2007], formed Big Glass Mountain, a broad (~5 km diameter) silicic landform comprising pyroclastic fall deposits, near-vent breccia and tephra cones, and a voluminous obsidian flow. Numerous studies have elucidated the general progression of the eruption, including its trigger via magma injection [Eichelberger 1975] and ensuing eruptive activity, which presumably began with early pyroclastic venting followed by late-stage effusion of compositionally mixed silicic lava [e.g. Numerous studies have elucidated the general progression of the eruption, including its trigger via magma injection [Eichelberger 1975] and ensuing eruptive activity, which presumably began with early pyroclastic venting followed by late-stage effusion of compositionally mixed silicic lava [e.g. Heiken 1978; Eichelberger and Westrich 1981]. Heiken [1978] suggested that the volumetric dominance of lava over tephra (~9:1 at BGM), is characteristic of moderate-volume rhyolite eruptions, including the Little Glass Mountain complex on MLV, the Inyo Domes [Heiken 1978; Castro and Gardner 2008], and the Rock Mesa and Devils Hill rhyolites on South Sister Volcano, Oregon [e.g. Scott 1987]

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