Thirty‐year gravity change at Mount Baker Volcano, Washington, USA: Extracting the signal from under the ice

Abstract

Mount Baker in the Cascade Volcanic Arc displayed an unexplained period of increased fumarolic activity in 1975 and has since been quiescently degassing. We reoccupied gravity stations near the active crater and on the south flank of the volcano, initially measured in 1975–1981. We observe 1800 ± 300 μ Gal gravity increase at the crater since 1977. Estimates of snow and ice volume change suggest these environmental factors significantly mask gravity change due to magmatic and hydrothermal sources; correcting for environmental factors could double the observed gravity increase. Hydrothermal recharge, magma intrusion, and significant deflation are rejected as explanations for the source of the gravity change. Densification of a magma body emplaced in 1975 is consistent with our gravity observations and with deformation and degassing data. Shallow pyrite precipitation may also contribute to the gravity increase.