January 21, 2014
Galapagos Islands’ Volcanic Plume Not Where It Should Be
Brett Smith for redOrbit.com - Your Universe Online
A new analysis from a team of American researchers has found the volcanic plume that created the Galapagos Islands isn’t where models have projected it. The study team said their findings could also explain volcanic activity around the islands.
According to the study, which was published on Sunday the journal Nature Geoscience, the Galapagos plume is at a depth of 155 miles, approximately 100 miles southeast of the westernmost island of the archipelago, where geologists and computer-generated mantle convection models had placed the plume. The study team reached their conclusions using seismic waves penetrating to a depth of nearly 200 miles.
"Ocean islands have always been enigmatic," said study author Dennis J. Geist, a geologist at the University of Idaho. "Why out in the middle of the ocean basins do you get these big volcanoes? The Galapagos, Hawaii, Tahiti, Iceland – all the world's great ocean islands – they're mysterious."
The plume's location according to the new study is closer to Isabella and Floreana islands, where the three most volatile volcanoes in the island chain are located. This new location adds support to the study team’s other finding – Earth's interior convection doesn't always follow modeling efforts. The study also raises new queries about how ocean plates at the Earth's surface – referred to as the lithosphere – relate to the hotter, more fluid asthenosphere that rests atop the mantle.
The team’s paper describes the Galapagos plume rising up into shallower depths than previously described and streams both northward and perpendicular to plate motion. Mantle plumes, also found at Yellowstone and Hawaii, are generally believed to curve along with plate migration – meaning the Galapagos plume may have 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," said study author Douglas R. Toomey, a geology professor at the University of Oregon. "It is going opposite than expected, and we don't know why."
The explanation for this finding may lie in the fluid dynamics of the viscous asthenosphere on which the Earth's plates sit, Toomey said. In the paper’s conclusion, the study authors theorized that the plume material is dragged to the mid-ocean ridge by a deep return flow found in the middle of the asthenosphere, instead of along the base of the lithosphere as some modeling projections have shown.
The Galapagos archipelago covers over 3,000 square miles of ocean and sits around 580 miles west of Ecuador, which has dominion over the islands. Volcanic activity on the island chain has been particularly difficult to understand because theories and models have projected newer eruptions moving ahead of the plate, similar to activity seen at the hotspot around Yellowstone.
There are two main plates related to the Galapagos, the Nazca and Cocos plates. The leading edge of the Nazca plate is at the island of Fernandina. The Cocos plate, which used to hold the islands' 620-mile-long hotspot chain, is said to be moving to the northeast.