Seismic Evidence of Bottom-Up Crustal Control on Volcanism and Magma Storage Near Mount St. Helens

The lower crust has long been recognized as a place where significant chemical evolution of magma occurs. Understanding the conditions that lead to long-term storage in or rapid transport through these lower crustal systems has remained a challenge. In this study, active-source seismic data are processed to image prominent reflectors in the lower crust and upper mantle near Mount St. Helens. Results show strong and weak near-Moho reflectivity located near previously inferred regions of magma storage and high-density bodies underlying volcanic centers, respectively. To explain these observations, we hypothesize that magmas intersecting the high-density lower crustal bodies experience enhanced vertical buoyancy forces/transport due to the increased density contrast between melt and host rock. The reduced reflectivity near high-density bodies is therefore a result of both decreased impedance contrasts between crust and mantle host rocks and diminished lower crust magma accumulation, which ultimately causes focusing of volcanism above these features.