Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high‐resolution tomographic images of the crust and upper mantle. Rayleigh‐wave phase‐velocity maps and three‐dimensional images of shear velocity (Vs), generated from ambient noise and earthquake surface waves, show that west of MSH the middle‐lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0–4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, middle‐lower crust Vs is low (3.45–3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wave speeds, producing a strong Moho. The sharp boundary in crust and mantle Vs within a few kilometers of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes.

Mount St. Helens (MSH) is anomalously 35–50 km trenchward of the main Cascade arc. To elucidate the source of this anomalous forearc volcanism, the teleseismic‐scattered wavefield is used to image beneath MSH with a dense broadband seismic array. Two‐dimensional migration shows the subducting Juan de Fuca crust to at least 80‐km depth, with its surface only 68 ± 2 km deep beneath MSH. Migration and three‐dimensional stacking reveal a clear upper‐plate Moho east of MSH that disappears west of it. This disappearance is a result of both hydration of the mantle wedge and a westward change in overlying crust. Migration images also show that the subducting plate continues without break along strike. Combined with low temperatures inferred for the mantle wedge, this geometry greatly limits possible source regions for mantle melts that contribute to MSH magmas and requires lateral migration over large distances.

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Yellowstone National Park’s plumbing system linking deep thermal fluids to legendary thermal features is virtually unknown. Prevailing concepts of Yellowstone’s hydrology and chemistry are that fluids flow laterally from distal sources and emerge at the edges of lava flows and that spring chemistry reflects varying fluid source regions1,2. Here we present the first view of Yellowstone’s hydrothermal system derived from electrical resistivity and magnetic susceptibility models of airborne geophysical data3,4. Groundwater and thermal fluids containing total dissolved solids or low pH significantly reduce resistivities of porous volcanic rocks5. Low susceptibility clay sequences mapped in thermal areas6,7 and boreholes8 typically form over fault-controlled thermal fluid and/or gas conduits9-12. We show that most thermal features are located above high-flux conduits along buried faults and flow paths are similar irrespective of spring chemistry. Lateral outflow from the conduits mixes with upflow and groundwater at shallow levels in the thermal basins. Similarities between our models and those from the Taupo Volcanic Zone highlight the implication of our work beyond Yellowstone and suggest that hydrothermal systems worldwide are vertically-driven and surface geochemical variations are controlled at depth by mixing of local and distal thermal fluids and groundwater and more locally, by shallow permeability.

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