THE FIRST LUAUThe Hawaiian Islands formed, scientists say, from volcanoes that erupted as a piece of Earth’s crust slid over a deep hot spot. A new study challenges the traditional notion that a thin plume of heat, flowing up from just outside the planet’s core, powered those early volcanoes. Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA GSFCLike a pig at a luau, the Hawaiian Islands get roasted from below. But — like novice cooks — scientists aren’t sure what kind of heat it takes to really get things cooking.
A new analysis questions the prevailing theory that the islands were formed in sequence by volcanic activity as the Pacific plate drifted over a thin, hot plume rising from deep inside the Earth. Instead, a shallower pocket of abnormally hot rocks could be powering the vacation spot’s famous volcanoes, researchers from MIT and Purdue University in West Lafayette, Ind., report in the May 27 Science.
Other scientists don’t think the hot plume is done for. “This result, like many results, isn’t the final word,” says Cecily Wolfe, a seismologist at the University of Hawaii at Manoa.
Study coauthor Robert van der Hilst and his colleagues used seismic data to dig through the mantle, the gooey but mostly solid region between the planet’s core and surface, looking layer by layer for sources of heat. The team couldn’t find a plume, but did find something strange to the west of the islands: a pancake-shaped pocket of heat about 650 kilometers underground and up to 2,000 kilometers wide. Rocks in this hot pocket were as much as 300 to 400 degrees Celsius hotter than scientists would expect at that depth.
In order to have formed the Hawaiian Islands, the pancake would need to somehow heat the surface, spurring volcanoes. It’s possible that molten rocks could be bubbling up from the easternmost edge of the pocket like a lava lamp, says van der Hilst, a geophysicist at MIT. But why molten material would rise there isn’t clear.
“Why there’s only one Hawaii and not five or 100, I don’t know,” he says.
A plume, on the other hand, would start much deeper, erupting from the boundary between core and mantle about 2,800 kilometers beneath the surface. In a study published in Science in 2009, Wolfe found a plumelike stream seemingly cooking Hawaii’s bottom.
In her study, Wolfe used sea-bottom sensors near Hawaii to record how shaking from earthquakes traveled through the mantle. Since seismic waves pass more slowly through a patch of hot rocks than a patch of cooler rocks, the waves can pinpoint heat sources directly.
The MIT team, in contrast, didn’t map out the mantle’s hot spots but its rock layers, analyzing earthquake data recorded by global networks far away from Hawaii. Like a parfait, the mantle contains distinct mineral layers determined by heat and pressure. If scientists can spot the transition from one mineral to another, they can roughly infer temperature at that depth.
Neither method is rock solid, Wolfe says, so it will take more work to truly expose Hawaii’s heat source. “Nothing in the earth sciences is perfect,” she says.
The truth may be part pancake and part plume, says Guust Nolet, a geophysicist at the University of Nice in Sophia Antipolis, France. He has proposed that plumes might not burst up like a flame thrower’s blast through the mantle. Instead, these streams of hot rock could get stuck between mineral layers, forming pockets of heat. Van der Hilst agrees that a deep plume could indeed be feeding Hawaii’s hot pancake from below.
Found in: Earth
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