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Diamonds May Unlock Secrets of Hot-Spot Volcanoes
14 July 2010 5:26 pm
Volcanic hot spots like the Hawaiian Islands have puzzled geologists for decades. Volcanoes usually form where tectonic plates crash into each other or spread apart, but the Hawaiian chain sits in the center of one of these plates. Now, diamond-bearing rocks from beneath Earth's surface may help reveal where these volcanoes came from.
Scientists trying to explain so-called hot-spot volcanoes have invoked mantle plumes. The idea is that large upwellings of hot, partially molten rock break through the crust and begin spewing lava. There's just one problem: Although most scientists think mantle plumes exist, no one has incontrovertible proof. That's because the enormous heat and pressure beneath Earth's crust make it impossible for researchers or their equipment to go down there.
Geophysicist Trond Torsvik of the University of Oslo in Norway thought diamond-bearing volcanic rocks called kimberlites could mark the presence of these plumes. Kimberlites form at depths of 150 kilometers or lower in the mantle, but plumes could bring them to the surface. Torsvik and his colleagues needed to figure out where they erupted and if they erupted at places where plumes—including two suspected "superplumes," one below the south-central Pacific Ocean and one beneath southern Africa—are supposed to be brewing.
Torsvik's team analyzed kimberlites that were only a few hundred million years old—toddlers compared with Earth—from Africa, North America, and other continents. Enough time had passed since the kimberlites reached the surface that the continents had shifted, so Torsvik's team had to create maps of the continents as they were hundreds of millions of years ago, essentially rewinding plate tectonics. Once they figured out where the continents were located when one of the diamond-dotted rocks erupted, they could figure out how close the rock was to a potential mantle plume.
What the researchers found seemed to confirm the existence of at least one superplume—the one supposedly bubbling beneath Africa. Eighty percent of the kimberlites they analyzed that erupted in the past 320 million years lined up along the plume-generation zone that scientists suspect gave rise to the African superplume and several hot spots. The correlation suggests that a plume could have brought them to the surface. The team reports its findings in the 15 July issue of Nature. The results' accuracy hinges on the assumption that the plume-generation zones have stayed put for 320 million years. But that's such a short geologic time span that Torsvik says he thinks it's a safe bet.
Not all researchers are convinced. Gillian Foulger, a geophysicist at Durham University in the United Kingdom and a long-time opponent of mantle plume theory, takes issue with the idea of using a "completely unrelated phenomenon like diamonds" as a marker for plumes. She also disagrees with the study's approach of concluding that there is a superplume beneath Africa without definitive proof. "It assumes plumes exist and assumes where they exist without evidence," she says.
Instead, Foulger advocates the plate hypothesis, an alternative that she says could explain hot spots such as Hawaii without relying on mantle plumes. The idea is that Earth's plates can deform from within, opening cracks that could give rise to volcanoes far away from plate boundaries without requiring extra activity from the mantle.
But Torsvik says it's unlikely that so many kimberlites would line up with a plume-generation zone by chance. The odds fall somewhere between 0.1% and 1%, according to the paper.
That's good enough for Yale University geologist David Evans, who says the study lends "further support for lower mantle origin of mantle plumes."
"Somebody that totally does not believe in plumes would dismiss it," adds geophysicist Norman Sleep of Stanford University in Palo Alto, California, but it provides good evidence of how these plumes work.