July 24, 1998 Despite the trove of summer blockbusters suggesting that the greatest threats to humanity are asteroids lurking in outer space, scientists now know that an equally great threat can lie roiling under our feet. They've linked some of the greatest mass extinctions the world has ever known with rock that originated near the Earth's core and crept upward, lying in wait just beneath the surface before dousing the Earth with scalding lava.
Researchers at the University of Rochester and Harvard University have now gained the clearest picture yet of the underpinnings of these most catastrophic of eruptions by analyzing the chemical composition of lava disgorged in one such massive period of volcanism in Siberia 250 million years ago. The findings by geochemists Asish Basu, Robyn Hannigan, and Stein Jacobsen, published in a recent issue of Geophysical Research Letters, indicate that the lava that erupted at the time arose from the pristine lower mantle, material that harks back to the birth of our solar system.
Scientists believe this particular volcanic outpouring played a role in the greatest mass extinction the world has even seen. Up to 95 percent of all plant and animal species were wiped out; a similar series of eruptions 65 million years ago in India coincided precisely with the sudden extinction of the dinosaurs.
The work plays into a long-running debate among geologists about the floods of flaming rock that sporadically well up and burn through the Earth's crust, oftentimes with disastrous consequences worldwide. Scientists have long debated whether flood basalts, the vast expanses of rock that result from such volcanic outpourings, have their origins in rumblings deep within the Earth or closer to its surface. Now, by studying the lava's major chemical components, such as sodium, magnesium, and iron, as well as rare isotopes of neodymium, strontium, lead, and helium, Basu and Jacobsen have found that the Siberian flood basalt's chemical composition matches that of magma found deep within the Earth.
"This indicates beyond any doubt that the lava that's now hardened into the Siberian flood basalt originated far beneath the Earth's surface, between the lower mantle and the molten iron core," says Basu, a professor of earth and environmental sciences at Rochester.
The work, funded by the National Science Foundation, is the latest in a series of findings by Basu and other geologists that point to the lower mantle as the source of such volcanism in Siberia and other parts of the world. Three years ago in the journal Science, Basu and colleagues suggested that the presence of an unusual concentration of helium-3, an ancient gas common deep within the Earth, could serve to trace the Siberian flood basalt's lineage to the lower mantle.
"Members of the opposing school, which held that the flood basalts arose from the melting of lithospheric rock right under the Earth's crust, weren't persuaded by those results," says Jacobsen, a Harvard professor of geochemistry who joined Basu and Hannigan, a former Rochester doctoral student, in the more recent research. "These geochemists said that while it was possible that a portion of the rock came from deep within the Earth, the majority of the rock probably wasn't formed that way. They discounted the helium-3, suggesting that it may have been recycled from cosmic dust falling to Earth and making its way into the rocks."
Jacobsen and Basu say the current research, which examined the distribution of more than a half-dozen chemical elements and rare isotopes found throughout the basalts, should put those theories to rest. The team analyzed rocks culled from the Siberian flood basalt for compounds that are rare on the Earth's surface but common in parts of the lower mantle that originated soon after the birth of our solar system. They also drew upon experiments conducted at other institutions where researchers melted rocks in the lab, much as happens deep within the Earth's interior. They found that the Siberian flood basalt arose from super-heated, buoyant rock that rose in a narrow column from a depth of 1,800 miles into a huge mushroom-shaped mass of hot rock just 40 to 50 miles below present-day Siberia. Then, some 250 million years ago, 12 to 16 percent of this rock suddenly melted and broke through the Earth's crust, resulting in a vast flood of lava.
"It is surprising that most of this melting occurred so close to the Earth's surface, really just beneath the crust," Jacobsen says. While it's easy to envision rocks melting in the massive crucible at the Earth's core, the lava that spilled into Siberia 250 million years ago melted at the much lower temperatures and pressures found just beneath the surface. Basu says it's an example of decompressional melting, where a large decrease in pressure can actually depress the melting point of a substance.
These overwhelming periods of flood basalt volcanism make ordinary volcanoes look like child's play, releasing up to a million times the material belched out by an eruption such as that of Mount St. Helens. In fact, the one million cubic kilometers of molten volcanic rock that bubbled up to the surface -- now a Siberian plateau the size of California -- would be enough lava to cover the entire Earth to a depth of 10 feet. "This was, almost unquestionably, the most catastrophic volcanism in last half-billion years of the Earth's history," Basu notes.
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