Deadliest volcano in the US has a heart of cold, wet stone

Seismologists set off explosives to map the plumbing beneath Mt St Helens – and may have solved the mystery of the missing magma. Belinda Smith reports.

Mt St Helens' magma source isn't directly below – so where is it?
Steven Hansen

Logic says the volcano responsible for the deadliest eruption in the US should be sitting atop a huge chamber of bubbling magma ... but it turns out Mt St Helens in Washington State is perched over a big wedge of cold rock.

Steve Hansen from the University of New Mexico and US colleagues mapped the crust and mantle beneath Mt St Helens and found it to be cold, wet stone, so magma must be funnelled in from another site – probably from the south-east.

The work, published in Nature Communications, challenges assumptions of active volcano plumbing.

During Mt St Helens’ largest eruption on 18 May 1980, its entire north face slid away. Hot rock within, suddenly exposed to low pressure, exploded. Lava and rock sprayed at supersonic speeds, filling surrounding valleys with landslide deposits. The blast killed more than 50 people.

And despite being constantly monitored since, the volcano is a bit of an enigma. It lies some 50 kilometres west of the Cascade arc of volcanoes that extends more than 1,000 kilometres from southwestern British Columbia to Northern California.

So where does it get its fuel?

To answer this, in summer of 2014 Hansen and his crew set off 23 explosions around Mt St Helens.

While it sounds like a death wish, they wanted to see how waves generated by the explosions bounced off the boundary between the Earth’s crust and underlying mantle, and measure the speed and amplitude (or height) of those waves as they made their way through the material.

Different materials produce different waves and from the data, they generated a 3-D picture of the crust and mantle below the volcano.

Instead of a big hot chamber of magma, the smaller amplitude waves they detected pointed to a giant cold, wet wedge of serpentine minerals in the mantle.

So what is Mt St Helens’ magma source?

The likely spot, the researchers write, are south-east of the volcano towards the main arc. Recent clusters of deep earthquakes, around 23 to 44 kilometres below the surface, are probably due to magma moving around.

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