Sierra Negra is one of the most active volcanoes in the Galapagos and the most voluminous.
Credit: Douglas Toomey
The volcanic plume that gave birth to the Galapagos Islands is not where scientists thought it was, a new study finds.
Three-dimensional seismic images suggest the plume lies southeast of the chain's Fernandina Island, the spot where computer models place it. What's more, the plume is not being bent eastward by the migrating Nazca tectonic plate (atop which the Galapagos Islands sit), but appears to be moving northward.
The Galapagos Islands, the locale where Charles Darwin developed his famous theory of natural selection, lie about 575 miles (925 kilometers) west of Ecuador. Unlike other volcanic island chains, like Hawaii, the Galapagos have many volcanoes active at once. [The World's 5 Most Active Volcanoes]
"Having the plume relocated helped explain why [many of] the Galapagos volcanoes are active," said Douglas Toomey, a geologist at the University of Oregon and leader of the study, detailed online Jan. 19 in the journal Nature Geoscience. But it remains a puzzle why the plume isn't heading in the same direction as the Nazca plate. "We're still trying to figure that out," Toomey told LiveScience's OurAmazingPlanet.
Volcanoes often occur along tectonic-plate boundaries, either at subduction zones, where one tectonic plate is moving under another, or at midocean ridges, where two plates are spreading apart.
But sometimes, volcanoes form far from the boundaries of plates. Hot plumes from the mantle, the thick layer just beneath Earth's crust, fueled by primordial heat from the planet's core, could send a plume of molten rock upward — like a Lava-lamp bubble — to create ocean islands.
Models suggest that as the tectonic plates move over the mantle, they should "blow" mantle plumes, similar to the way the wind blows smoke from a smokestack.
Galapagos gets 'CAT scan'
Toomey's team, including the aptly named Darwin Villagómez, created a kind of "CAT scan" of the Galapagos region by putting sensors on the Earth's surface and listening for distant earthquakes. By mapping the way the seismic waves from those quakes moved through the ground below the Galapagos, the model generated 3D pictures of the Earth's interior that show the plume's location and movement.
"It wasn't where we thought it would be," Toomey said. They expected the plume to be west of Fernandina Island, but instead, it was 155 miles (250 km) down and about 93 miles (150 km) southeast of Fernandina Island.
The plume should have been bending west to east, following the motion of the Nazca plate, according to the models. But instead, it was bending south to north toward a midocean ridge — perpendicular to the movement of the plate, Toomey said.
The location of the plume makes sense in light of the strange pattern of volcanism in the Galapagos. The area contains about a dozen active volcanoes, and the most active ones are those on Fernandina Island and the Cerro Azul and Sierra Negra volcanoes on Isabella Island, the largest landmass in the archipelago.
The direction in which the plume is moving poses more of a mystery, shaking up current notions of how the lithosphere, the rigid layer that consists of the crust and the top of the mantle, interacts with the asthenosphere, the "gooey" layer beneath it. The researchers believe that a deep flow in the asthenosphere may be carrying the plume toward the midocean ridge, rather than a shallow flow along the base of the lithosphere, as models suggest.
Toomey expects that some scientists might find the results controversial. But Christopher Kincaid, an oceanographer at the University of Rhode Island who was not involved in the research, called it an excellent study.
The study offers "further evidence that plumes do not always behave in simple ways, particularly when they encounter complexities in flow […] in the shallow mantle beneath plates," Kincaid told LiveScience's OurAmazingPlanet.