The volcanic mantle plume of the Galápagos Islands may not be where geologists thought it was, new evidence suggests.
The finding is based on images gathered by University of Oregon scientists using seismic waves. Professor Douglas R. Toomey, coauthor of the study published in the journal Nature Geoscience, says the findings “help explain why so many of the volcanoes in the Galápagos are active.”
The newly discovered anomaly is at a depth of 250 kilometers (155 miles) about 150 kilometers (about 100 miles) southeast of Fernandina Island, the westernmost island of the Galápagos Archipelago.
The suspected plume—consistent with partial melting, extraction, and remixing of hot rocks that define such volcanic pockets—is spreading north toward the mid-ocean ridge instead of, as projected, eastward with the migrating Nazca plate on which the island chain sits.
The archipelago covers roughly 3,040 square miles of ocean and is centered about 575 miles west of Ecuador, which governs the islands. Galápagos volcanic activity has been difficult to understand, Toomey says, because conventional wisdom and modeling say newer eruptions should be moving ahead of the plate, not unlike the long-migrating Yellowstone hotspot.
The separating angles of the two plates in the Galápagos region cloud easy understanding. The leading edge of the Nazca plate is at Fernandina. The Cocos plate, on which the islands’ some 1,000-kilometer-long (620-miles) hotspot chain once sat, is moving to the northeast.
The suspected plume’s location is closer to Isabella and Floreana islands. While a dozen volcanoes remain active in the archipelago, the three most volatile are Fernandina’s and the Cerro Azul and Sierra Negra volcanoes on the southwest and southeast tips, respectively, of Isabella Island, the archipelago’s largest landmass.
The plume’s more southern location, Toomey says, adds fuel to his group’s findings, at three different sites along the globe encircling mid-ocean ridge (where 85 percent of Earth’s volcanic activity occurs), that Earth’s internal convection doesn’t always adhere to modeling efforts and raises new questions about how ocean plates at the Earth’s surface—the lithosphere—interact with the hotter, more fluid asthenosphere that sits atop the mantle.
“Ocean islands have always been enigmatic,” says co-author Dennis J. Geist of the geological sciences department at the University of Idaho. “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.”
‘We don’t know why’
The Galápagos plume, according to the new paper, extends up into shallower depths and tracks northward and perpendicular to plate motion. Mantle plumes, such as the Galápagos, Yellowstone, and Hawaii, generally are believed to bend in the direction of plate migration.
In the Galápagos, however, the volcanic plume has decoupled from the plates involved.
“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 says. “It is going opposite than expected, and we don’t know why.”
The answer may be in the still unknown rheology 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.
Co-authors of the study are doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, California; Emilie E.E. Hooft of University of Oregon; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.
The National Science Foundation supported the research.
Source: University of Oregon