Letter abstract
Nature Geoscience 2, 785 - 789 (2009)
Published online: 25 October 2009 | Corrected online: 30 October 2009 | doi:10.1038/ngeo661
Subject Category: Structural geology, tectonics and geodynamics
Distribution of melt beneath Mount St Helens and Mount Adams inferred from magnetotelluric data
Graham J. Hill1,2, T. Grant Caldwell1, Wiebke Heise1,3, Darren G. Chertkoff4, Hugh M. Bibby1, Matt K. Burgess5, James P. Cull2 & Ray A. F. Cas2
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 conductivity1, 2, 3, 4, 5. Interconnected melt can increase the bulk conductivity of the region containing the melt6, 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 Helens8, and explains the distribution of seismicity observed at the time of the catastrophic eruption in 1980 (refs 9, 10).
- GNS Science, Lower Hutt, Wellington, New Zealand
- Australian Crustal Research Centre, Monash University, Melbourne, Australia
- Universidade de Lisboa, CGUL-IDL, Lisbon, Portugal
- Crystal Prism Consulting Inc., North Vancouver, British Columbia, Canada
- USGS California Water Science Center, San Diego, California, USA
Correspondence to: Graham J. Hill1,2 e-mail: g.hill@gns.cri.nz
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