Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker, Washington, USA

Abstract

Mount Baker volcano displayed a short interval of seismically-quiescent thermal unrest in 1975, with high emissions of magmatic gas that slowly waned during the following three decades. The area of snow-free ground in the active crater has not returned to pre-unrest levels, and fumarole gas geochemistry shows a decreasing magmatic signature over that same interval. A relative microgravity survey revealed a substantial gravity increase in the~3 0 years since the unrest, while deformation measurements suggest slight deflation of the edifice between 1981-83 and 2006-07. The volcano remains seismically quiet with regard to impulsive volcano-tectonic events, but experiences shallow (<3 km) low-frequency events likely related to glacier activity, as well as deep (>10 km) longperiod earthquakes. Reviewing the observations from the 1975 unrest in combination with geophysical and geochemical data collected in the decades that followed, we infer that elevated gas and thermal emissions at Mount Baker in 1975 resulted from magmatic activity beneath the volcano: either the emplacement of magma at mid-crustal levels, or opening of a conduit to a deep existing source of magmatic volatiles. Decadal-timescale, multi-parameter observations were essential to this assessment of magmatic activity.

Failed magmatic eruptions: late-stage cessation of magma ascent

“…For shallow intrusions that stall (our definition of a failed eruption), the balance is tipped even further in favor of resisting forces, resulting instead in plugs, dikes, and perhaps elevated geothermal activity. The papers in this volume provide a variety of case histories (Crider et al 2011;Gardine et al 2011;Nishimura and Ueki 2011;Roman and Power 2011;Werner et al 2011) and perspectives from modeling (Taisne et al 2010) that we hope will be a significant resource for scientists grappling with the issue of whether or not a future episode of unrest will lead to a magmatic eruption.…”

“…Examples include Soufrière Guadeloupe, 1975-1976(Hirn and Michel 1979Dorel and Feuillard 1980;Feuillard et al 1983;Beauducel and Besson 2008), Akutan, Alaska, 1996(Lu et al 2000Power et al 2008), Mount Baker, Washington, 1975(Crider et al 2011), Iliamna, Alaska, 1996(Roman et al 2004Roman and Power 2011), Iwate, Japan, 1998(Nishimura and Ueki 2011), Parícutin, Mexico, 2006(Gardine et al 2011), Fourpeaked, Alaska, 2006(Gardine et al 2010), Mount Spurr, Alaska, 2004(Power 2004, and others. A few cases are well-studied, but many are poorly documented in the literature and so details of such events are often unavailable for comparison during an unrest episode.…”

“…To address this question, many factors are taken into account, including seismicity, deformation, gas emissions, the geologic and/or historic record of past eruptions, and analogues from the global record of volcanic unrest and eruptions. In several instances, two of which are detailed in this special volume (Crider et al 2011;Nishimura and Ueki 2011), indicators of unrest have become significant enough to prompt scientists to issue public statements; to engage with the media, local land managers, public officials; and to engage in other activities in preparation for a potential eruption, only to have the unrest de-escalate without magma reaching the surface. Such instances can threaten the credibility of scientists in volcano observatories and public officials responsible for evacuations and access restrictions, and can also bring unwanted economic and social disruption.…”

Shear wave splitting,vP/vS, and GPS during a time of enhanced activity at Aso caldera, Kyushu

“…We suggest that monitoring data from the region surrounding Aso caldera are revisited and checked for evidence of diffuse emission of magmatic gases or temporal changes in gravity as observed in combination with DLFEs and potential magma injection for example at Mt. Baker [e.g., Werner et al , 2009; Crider et al , 2011; Nichols et al , 2011] or at Long Valley caldera [ Hill , 2006].…”

Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles)

“…Long term monitoring of geochemical and geophysical parameters of shallow hydrothermal systems has been undertaken at numerous active volcanic areas (Newhall and Dzurisin, 1988), such as Flegrean Fields (Chiodini et al, 2003), Long Valley caldera (Sorey et al, 2003), Yellowstone (, Lowenstern et al, 2006, Hurwitz et al, 2007, Mt Baker (Crider et al, 2011) and La Soufrière of Guadeloupe (Villemant et al, 2005). In these volcanic areas, unrest events have been interpreted as driven by magmatic processes at depth, involving the transfer of heat and fluids from magma intrusions to the shallow hydrothermal system.…”