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Unearthing the Galápagos Story

Geology professor Dennis Geist studies the surprising geology deep beneath the Galápagos Islands

By Tara Roberts
Photos courtesy Dennis Geist and the University of Oregon 

University of Idaho geologist Dennis Geist is digging deep into the mysteries of one of Earth’s most unusual islands chains, the Galápagos. He and a team of researchers from across the nation have published a paper that challenges some of the conventional explanations of how the islands’ geology has formed.

Geist describes the rock layer deep below the Galápagos as a sponge. Traditional views held it was a “dry” sponge, just hot rocks. But Geist and his colleagues’ data indicate the sponge is actually “wet,” filled with hot magma.

“What my colleagues are seeing with the seismic waves is the rocks are actually starting to melt, and they’re starting to melt much, much deeper than anyone ever imagined before, and in locations that no one suspected,” Geist said.

Their research, reported in the February issue of the journal Nature Geosciences and released online Jan. 19, is key to a fundamental scientific understanding of how the Galápagos – and other islands – formed.

“Ocean islands have always been enigmatic,” Geist said. “Why out in the middle of the ocean basins do you get these big volcanoes? The Galápagos, Hawaii, Tahiti, Iceland, all the world’s great ocean islands – they’re mysterious.”

Geist, who has worked in the Galápagos for more than 30 years, partnered with a University of Oregon team led by Douglas R. Toomey and his student Darwin Villagomez, who gathered seismic data from the Galápagos. Toomey is a pioneer in using seismic tomography to explore the Earth’s interior. Much like a CAT scan sends x-rays through tissue to create images of the inside of a human body, their technique uses seismic waves that pass through rock to map what’s beneath the ground.

The team’s data reveal that seismic waves pass through the rocks beneath the Galápagos particularly slowly and in a chimney-like structure that they interpret as a mantle plume. The team identified the deep, magma-bearing rocks as the cause.

Mantle plumes, such as the Galápagos, Yellowstone and Hawaii, generally are believed to bend in the direction of tectonic plate migration – they are smeared by the moving plate. In the Galápagos, however, the volcanic plume has decoupled from the plates involved and moves both “upstream” and perpendicular to plate motion.

A volcano erupting on a Galapagos island
A volcano erupting on a Galapagos island

"Here's an archipelago of volcanic islands that are broadly active over a large region, and the plume is almost decoupled from the plate motion itself," Toomey said. "It is going opposite than expected, and we don't know why."

But, he theorized, the answer may be in the still unknown movement of the gooey asthenosphere on which the Earth's plates ride. In their conclusion, the paper’s five co-authors theorize that the plume material is carried to the mid-ocean ridge by a deep return flow centered in the asthenosphere rather than flowing along the base of the lithosphere as in modeling projections.

In addition to Geist and Toomey, the paper’s co-authors include former University of Oregon doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, Calif.; Emilie E.E. Hooft of the UO Department of Geological Sciences; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.

Birds nesting on a Galapagos island
Birds nesting on a Galapagos island


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