Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker,Washington, USA |
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Authors: | Juliet G Crider David Frank Stephen D Malone Michael P Poland Cynthia Werner Jacqueline Caplan-Auerbach |
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Institution: | (1) Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA;(2) Vashon, WA, USA;(3) Hawaiian Volcano Observatory, U S Geological Survey, Hawaii National Park, HI, USA;(4) Cascades Volcano Observatory, U S Geological Survey, Vancouver, WA, USA;(5) Department of Geology, Western Washington University, Bellingham, WA 98225, USA |
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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 ~30 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) long-period 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. |
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