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

Abstract

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.

Segmentation and Radial Anisotropy of the Deep Crustal Magmatic System Beneath the Cascades Arc

“…Similarly, the locations of active crustal fault systems, such as the MSH Seismic Zone and West Rainier Seismic Zone, have long been suggested to influence the locations of major vents (Stanley et al, 1996). In contrast, Kiser et al (2021) proposed a "bottom-up" mechanism that emphasizes the influence of lower crustal structure in modulating the relative buoyancy of melt compared to the surrounding crust, which could focus vents above areas of higher lower crustal velocity and density. Future geodynamic studies of melt migration in actively deforming crust may be able to provide new insights into the relative importance of shallow and deep influences on melt ascent and vent locations.…”

Double Reservoirs Imaged Below Great Sitkin Volcano, Alaska, Explain the Migration of Volcanic Seismicity

“…The conduits or fractures produced by the explosive eruption of the L2 reservoir would make it easy for the magma in the L1 reservoir to flow out. Nevertheless, the lateral transport of magma is commonly seen or proposed at other volcanoes (e.g., Kiser et al, 2021;Lerner et al, 2020;Tibaldi, 2015). This stage lasted about 1 year (Global Volcanism Program, 2022a).…”

“…The geometry and dynamics of magma plumbing systems play an essential role in controlling the eruption behavior. Magma plumbing systems possess a wide range of complexity in terms of the connection and interaction between multiple magma chambers (e.g., Huang et al., 2015; Kiser et al., 2021; Tibaldi, 2015), the lateral offset between reservoirs and the edifice (e.g., Lerner et al., 2020; Tibaldi, 2015), the geochemical evolution of the magma (e.g., Spera, 2004), and the development of eruptive activity in an eruption cycle (e.g., Chaussard et al., 2013; Paulatto et al., 2022; Roman & Cashman, 2018; Tibaldi, 2015). Among these complexities, it is important to understand how magma moves in space and time during an eruption and between eruptions and what controls this process.…”