Of all the questions that swirl around the iconic Mount St Helens, which erupted so explosively in May 1980, one of the most curious is also deceptively simple: Why is the volcano situated where it is?
Located in the southwest of Washington state, the main Cascade arc of volcanoes (formed by the subduction of the Juan de Fuca Plate beneath North America over the past 40 million years) is a fairly straight line featuring such peaks as Mount Adams, Mount Rainer and Goat Rocks. ‘St Helens, however, sits about 60km to the west, in a region where scientists don’t expect deep magma to surface,’ explains Paul Bedrosian, research geophysicist at the United States Geological Survey (USGS). ‘Not only is St Helens “out-of-line”, but it is also the most active volcano in the Cascades.’ The reason for this paradox has been historically unclear.
Now, by tracking seismic sound waves and magnetotelluric data (electrical conductivity beneath the surface), USGS and Oregon State University geophysicists have identified a vast underground rock formation – a ‘batholith’ – near the Cascadia arc, that obstructs the rise of magma. ‘Magma in the lower crust is spread out over a relatively wide area – within this broad zone there are places more or less favourable for it to ascend and ultimately erupt,’ says Bedrosian. ‘Below the batholith, where ascent is difficult, magma is deemed to “stall” in the crust. In contrast, where there exist crustal flaws, such as beneath Mount St Helens, magma can ascend more readily.’ Due to what he calls ‘an ancient tectonic scar’ below Mount St Helens, the volcano acts as a release valve on the pressure that builds up beneath the batholith.
Predicting future eruptions – which has generally depended on observations of the bulging dome and earthquake activity that preceded the 1980 event – could be more accurate now, as experts monitor the movement of magma below the surface.
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