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Heady theory for largest eruptions

The explosive eruption of Mount St. Helens in 1980 carpeted portions of eastern Washington and nearby states with a layer of loose ash several centimeters thick. Consider an eruption that could cover all of Washington and Oregon with a 5-meter-thick layer of solid lava. Roughly that amount of molten basalt poured out of cracks in India 66 million years ago, during a geologically brief period lasting less than half a million years. So-called flood basalt eruptions such as these represent the world's largest volcanic releases, but scientists have yet to agree on an explanation for the massive molten outpourings. One theory holds that plate tectonic forces near the Earth's surface generate flood basalts. But a group of geologists now implicates far deeper portions of the Earth.

According to the surface theory, when tectonic forces stretch and rip apart a continent, the mantle directly underneath can rise into an area of lower pressure. Such decompression causes portions of the solid mantle to melt and rise to the surface through fissures, where the molten rock pours out as flood basalts.

In the Oct. 6 SCIENCE, Mark A. Richards of the University of Oregon in Eugene, Robert A. Duncan of Oregon State University in Corvallis and Vincent E. Courtillot of the Institute of World Physics in Paris, France, offer support for an opposite model. In their scenario, hot rock rises from the deep mantle to trigger the massive eruptions. As it rises, it forms a plume with a "head" and a "tail" region, looking a bit like a balloon on a string. When the large head reaches the crust, it heats the surrounding solid rock, generating lightweight molten material that rises to the surface through cracks. The head region causes a period of massive volcanic outbursts, which then die out.

This model would also explain the linear tracks of volcanoes, such as the Hawaiian island chain, that appear across the globe. These chains form as a continental or oceanic plate passes over an abnormally warm region of the mantle called a hot spot. As the plate moves, the stationary hot spot spawns a line of volcanoes. Richards and his colleagues propose that the tail of the plume would feed the hot spot.

Others have suggested portions of the head-and-tail theory, but this model is the first to pull the entire scenario together, says Duncan. He notes that some flood basalts might be related to continental rifting, but he says the eruptions cause rifting -- an idea in direct opposition to the theory that rifting causes flood basalts. To help determine whether either of these two models is correct, geologists will have to perform detailed studies of flood basalt formations to learn whether the eruptions came before or after any associated rifting.