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Mystery of Why Mount St. Helens Is Out of Line With Other Volcanoes Solved

Some of the clearest, most comprehensive images of the top several miles of the Earth’s crust have helped scientists solve the mystery of why Mount St. Helens is located outside the main line of the Cascade Arc of volcanoes. A giant subsurface rock formation some 20-30 miles in diameter, known as the Spirit Lake batholith, appears to have diverted magma and partially melted rock outside of the arc and to the west, forming the region’s most active volcano.

Results of the study, which was supported by the National Science Foundation and carried out in collaboration with the U.S. Geological Survey, was published in Nature Geoscience. Previous imaging studies have primarily utilized seismic methods. During natural earthquakes and artificially induced tremors — by setting off explosions — scientists can image some of the properties of subsurface rocks by tracking the sound waves.

Southern Washington Cascades Volcanic Range (Image: Oregon State University)
Southern Washington Cascades Volcanic Range. (Image: Oregon State University)

This method provides clues to the structure, density, and temperature of the rocks. More recently, researchers are using “magnetotelluric,” or MT data, which measures the Earth’s subsurface electrical conductivity.

Variations in the geomagnetic and geoelectric fields can reveal much about the subsurface structure and temperature, as well as the presence of fluids, such as magma. Adam Schultz, an Oregon State University geophysicist who is the principal investigator on the NSF grant to OSU and co-author, said:

Understanding the formation of Mount St. Helens begins with plate tectonics. Similar to the present day, where the Juan de Fuca plate is being subducted beneath North America, in the past, crustal blocks with marine sediments were “slammed into the continent, where they accreted,” Schultz said, noting:

Mount St. Helens experienced a major eruption in May 1980 and since has gone through periods of dome-building (2004-08) and dormancy. A study in 2006 by researchers from the University of Canterbury in New Zealand provided some images of the volcano’s subsurface. During the next year, Schultz and the author of the 2006 study will use magnetotelluric technology to gather new and hopefully crisper images to see how much has changed since that study.

Schultz said that the images from the latest study are clear enough that by continuously monitoring the geoelectric and geomagnetic fields, they may be able to detect changes in the movement of magma beneath Mount St. Helens, and perhaps other volcanoes, Schultz added:

Provided by: Oregon State University [Note: Materials may be edited for content and length.]

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  • Troy was born and raised in Australia and has always wanted to know why and how things work, which led him to his love for science. He is a professional photographer and enjoys taking pictures of Australia's beautiful landscapes. He is also a professional storm chaser where he currently lives in Hervey Bay, Australia.

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