There are roughly 20 major volcanoes in the Cascade Range, and while scientists might know about them individually, they don’t know as much about why volcanoes in a chain might vary in composition, their eruption histories or how they erupt.
With millions of people living in at-risk areas around the volcanoes of the Cascade Range, and hundreds of millions more around the larger Pacific Ring of Fire, a group of geologists is arguing for more “big picture” research to answer those questions.
“The basic idea is that, when we look at volcanoes that are in the same tectonic setting – so, formed by the same overarching processes – our model as scientists tends to predict they would all behave similarly,” said Christy Till, a geologist and professor at Arizona State University.
The Cascades formed through the subduction of the denser Juan de Fuca tectonic plate beneath the North American plate.
Subduction is the process where an oceanic tectonic plate slides underneath another, continental plate. The process causes the plates to buckle and fold, creating – over millions of years – mountains and arcs of volcanoes. The oceanic plate is full of water, lowering the melting point of the mantle as the plate slides deeper into the Earth. Great pressure squeezes water from the plate into the mantle, and the mantle melts, forming magma below the continental plate that ascends to form an arc of volcanoes roughly parallel to the subduction zone.
The Cascade Range is one such volcanic arc. But despite having a shared geologic history, its volcanoes have different eruption histories and different compositions.
Till and Adam Kent, a geology professor at Oregon State University, were teaching a class on subduction zones at University of California, Berkeley. Over those few weeks they started work on researching what creates those differences, and brought on other scientists as the project grew.
The Cascades are home to towering peaks such as Mount Hood and Mount Rainier, along with groups of smaller peaks with smaller volcanoes.
Co-author Kent, in an news release, pointed to Mount Hood’s history of relatively smaller, localized eruptions, and compared that with Crater Lake, where the catastrophic eruption of Mount Mazama that formed it 6,000 to 8,000 years ago spread ash over much of North America.
“What we would like to know is why one volcano turns out to be a Mount Hood while another develops into a Crater Lake, with a very different history of eruptions,” he said. “This requires us to think about the data that we have in new ways.”
As they got going, they brought in other earth scientists, and started looking at bigger data sets from volcanic sites up and down the Cascades.
They took data from hundreds of sites and applied different statistical analyses to reconcile the different data sets.
The approach was somewhat novel, Till said. Much of this data became available only over the last 10 or so years.
“What we did that was new and different was to look at these data sets that can download from resources online now,” she said. “We’re not the first people to do it, but it’s still in its infancy in geology and volcanology.”
The researchers analyzed seismic wave speed to analyze the composition of the crust, and also found different volcanoes have different amounts of magma below.
“That’s the big new finding,” Till explained. The subduction process produces different amounts of magma beneath a given volcano, she said, and that appears to be the fundamental thing that drives the differences in their behavior.
Their work was published earlier this month in the journal Nature Communications.
“Part of the point of our paper is to make the case that we should be working on this scale of questions as sort of an under-addressed area of science,” Till said.
She said they hope to apply the techniques and lessons they used to other subduction zones.
“This is an important study that helps us understand why volcanoes behave differently from each other, but there’s a lot more exciting work to do on that question going forward.”
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