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

Nature Geoscience volume 2, pages 785–789 (2009)Cite this article

An Erratum to this article was published on 01 December 2009

This article has been updated

Abstract

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 910, 10).

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Figure 1: Locations of magnetotelluric measurement sites, Mount St Helens and nearby Cascades volcanoes.
Figure 2: Phase-tensor ellipses and induction vectors (real parts) at a period of 85.3 s.
Figure 3: Phase-tensor ellipse pseudo-section for the east–west line of measurements across the SWCC.
Figure 4: Resistivity models.

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Change history

  • 30 October 2009

    In the version of this Letter originally published, in Fig. 2 the elongated closed contour line should have been labelled +4. This error has been corrected in the HTML and PDF versions.

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Acknowledgements

We would like to thank W. Siripunvarporn for the use of his 3D inverse modelling code WSINV3DMT. B. Schaefer is thanked for the many discussions and support he lent to the completion of the work. The Yakama Nation, Weyerhaeuser, Olympic Resource Management and the US Forest Service are thanked for allowing us access to their land during the course of data collection. The logistical support of L. Mastin (Cascades Volcano Observatory—USGS), E. Rose and G. Barker is gratefully appreciated. We would like to thank E. Castleberry, J. Ellison, A. Fisher, T. Fisher, A. Green, J. Hill, Q. Jordan-Knox, V. Maris, M. McKenna, M. McLean, N. Olivier and L. Wan for help with data collection during the two field campaigns. H. Brasse and G. Jiracek provided reviews of the manuscript.

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Authors and Affiliations

  1. GNS Science, Lower Hutt, Wellington, New Zealand

    Graham J. Hill, T. Grant Caldwell, Wiebke Heise & Hugh M. Bibby

  2. Australian Crustal Research Centre, Monash University, Melbourne, Australia

    Graham J. Hill, James P. Cull & Ray A. F. Cas

  3. Universidade de Lisboa, CGUL-IDL, Lisbon, Portugal

    Wiebke Heise

  4. Crystal Prism Consulting Inc., North Vancouver, British Columbia, Canada

    Darren G. Chertkoff

  5. USGS California Water Science Center, San Diego, California, USA

    Matt K. Burgess

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  1. Graham J. Hill
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Contributions

G.J.H. designed the experiment. G.J.H., T.G.C. and D.G.C. wrote the paper. G.J.H. and W.H. reduced the observed time series. G.J.H., T.G.C., W.H. and H.M.B. carried out the modelling. G.J.H., M.K.B., D.G.C., W.H. and R.A.F.C. carried out the field campaign. All authors contributed to the interpretation and manuscript editing.

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Correspondence to Graham J. Hill.

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Hill, G., Caldwell, T., Heise, W. et al. Distribution of melt beneath Mount St Helens and Mount Adams inferred from magnetotelluric data. Nature Geosci 2, 785–789 (2009). https://doi.org/10.1038/ngeo661

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