Distribution of melt beneath Mount St Helens and Mount Adams inferred from magnetotelluric data

The cause of high electrical conductivity in the middle crust beneath the Pacific Northwest region of the US is not clear. New electrical-resistivity data reveal a connection between this regional conductor and a localized conductor beneath a prominent volcano in the region, suggesting that the anomalous conductivity is due to the presence of partial melts. Three prominent volcanoes that form part of the Cascade mountain range in Washington State (USA)—Mounts St Helens, Adams and Rainier—are located on the margins of a mid-crustal zone of high electrical conductivity 1 , 2 , 3 , 4 , 5 . Interconnected melt can increase the bulk conductivity of the region containing the melt 6 , 7 , which leads us to propose that the anomalous conductivity in this region is due to partial melt associated with the volcanism. Here we test this hypothesis by using magnetotelluric data recorded at a network of 85 locations in the area of the high-conductivity anomaly. Our data reveal that a localized zone of high conductivity beneath this volcano extends downwards to join the mid-crustal conductor. As our measurements were made during the recent period of lava extrusion at Mount St Helens, we infer that the conductivity anomaly associated with the localized zone, and by extension with the mid-crustal conductor, is caused by the presence of partial melt. Our interpretation is consistent with the crustal origin of silicic magmas erupting from Mount St Helens 8 , and explains the distribution of seismicity observed at the time of the catastrophic eruption in 1980 (refs 9 10 , 10 ).