Space Daily- Staff Writes Sept 11 2013,Berkley CA (SPX)
This is a 3D view of the top 1,000 kilometers of the earth's mantle beneath the central Pacific showing the relationship between seismically-slow "plumes" and channels imaged in the study. Green cones on the ocean floor mark islands associated with "hotspot" volcanoes, such as Hawaii. Credit: Berkeley Seismological Laboratory, UC Berkeley.
Scientists at the University of California, Berkeley, have detected previously unknown channels of slow-moving seismic waves in Earth's upper mantle, a discovery that helps explain "hotspot volcanoes" that give birth to island chains such as Hawaii and Tahiti.
Unlike volcanoes that emerge from collision zones between tectonic plates, hotspot volcanoes form in the middle of the plates. The prevalent theory for how a mid-plate volcano forms is that a single upwelling of hot, buoyant rock rises vertically as a plume from deep within Earth's mantle the layer found between the planet's crust and core and supplies the heat to feed volcanic eruptions.
However, some hotspot volcano chains are not easily explained by this simple model, suggesting that a more complex interaction between plumes and the upper mantle is at play, said the study authors.
The newfound channels of slow-moving seismic waves, described in a paper to be published Thursday, Sept. 5, in Science Express, provide an important piece of the puzzle in the formation of these hotspot volcanoes and other observations of unusually high heat flow from the ocean floor.
The formation of volcanoes at the edges of plates is closely tied to the movement of tectonic plates, which are created as hot magma pushes up through fissures in mid-ocean ridges and solidifies. As the plates move away from the ridges, they cool, harden and get heavier, eventually sinking back down into the mantle at subduction zones.
But scientists have noticed large swaths of the seafloor that are significantly warmer than expected from this tectonic plate-cooling model. It had been suggested that the plumes responsible for hotspot volcanism could also play a role in explaining these observations, but it was not entirely clear how.
"We needed a clearer picture of where the extra heat is coming from and how it behaves in the upper mantle," said the study's senior author, Barbara Romanowicz, UC Berkeley professor of earth and planetary sciences and a researcher at the Berkeley Seismological Laboratory. "Our new finding helps bridge the gap between processes deep in the mantle and phenomenon observed on the earth's surface, such as hotspots."
