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Eruption Production: Cal researcher traces “heat fingers”

September 10, 2013

Many things—from relationships to assembling IKEA furniture—are not as simple as they seem on the surface. Turns out that’s the case for marine volcanoes as well. Most island chain and seamount volcanoes are found at the juncture of oceanic tectonic plates, where subduction, meaning one plate plowing under another, makes it relatively easy for magma to vent upward.  

But a significant number of volcanoes (think Hawaii) are mid-plate “hotspot” volcanoes. For a long time, it was assumed they formed through a relatively simple convection process. In other words, the mantle material under these volcanoes was for one reason or another hotter than the surrounding mantle. This temperature differential allowed magma to bubble upward in a plume, ultimately emerging as lava.

But that explanation seemed a little too pat for many volcanologists. Their suspicions were confirmed recently by a study conducted by scientists at Cal and the University of Maryland, who used a process known as seismic tomography to suss out what’s really going on.  As detailed in a paper published in the journal Science Express, the researchers crunched data gleaned from a global array of seismographs to obtain a kind of CT scan of Planet Earth. 

What they found is that the seismic waves resulting from earthquakes change in speed and shape as they encounter different materials and temperatures: most pertinently, the hotter the medium, the slower the wave.  A picture ultimately emerged that showed long channels of heated material in the upper mantle. That means mid-plate volcanoes can’t accurately be described as discrete entities fed by circumscribed plumes of hot mantle material. Rather, they are components in long, connected networks of heated material that move through the mantle, interacting with tectonic plate movement hundreds or even thousands of miles away.

“It adds a whole new layer of complexity,” Scott French, a UC Berkeley graduate student and lead author of the paper, told us. “Instead of going up to the base of the tectonic plate and punching through, the heated material is actually deflected, flowing in long-finger like streaks through the mantle.”

French adds that “flow” must be considered in relative terms. “I’m speaking on the order of geologic time,” he said. “The movement is on the scale of centimeters per year.”

The researchers’ work has focused exclusively on marine volcanoes, which raises the question: Can it be applied to terrestrial volcanoes as well?  Might it provide some insights to potentially troublesome hotspots like the Yellowstone super volcano?  Not really. It gets back to complexity again.

“The oceanic plates have a much simpler structure than the continental plates,” said French. “There’s not a great deal of topography to their bases. But the continental plates have deeper ‘roots’—a lot of thick structures that protrude into the mantle.  That makes modeling what’s going on much more difficult.”

For that matter, the research team has hardly concluded their work on oceanic plates. They have the broad strokes – now they want to get into the fine details. “We’re planning to extend the technique with higher frequency seismic data,” French said.  “We think that will give us models with much higher resolution.”

—Glen Martin

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