Research Article

Bulletin of Volcanology

, Volume 73, Issue 2, pp 175-189

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

  • Juliet G. CriderAffiliated withDepartment of Earth and Space Sciences, University of Washington Email author 
  • , David FrankAffiliated with
  • , Stephen D. MaloneAffiliated withDepartment of Earth and Space Sciences, University of Washington
  • , Michael P. PolandAffiliated withHawaiian Volcano Observatory, U S Geological Survey
  • , Cynthia WernerAffiliated withCascades Volcano Observatory, U S Geological Survey
  • , Jacqueline Caplan-AuerbachAffiliated withDepartment of Geology, Western Washington University

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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.

Keywords

Quiescent degassing Thermal unrest Microgravity Volcano deformation Stalled intrusion Cascade Range