Volcanic eruptions more massive than outflows from a million Mount St. Helens tore through the earth and skies some 250 million years ago, disgorging enough lava in what is now Siberia to have covered the whole earth with a 10-foot layer. The Siberian eruptions came from deep within the earth, near the boundary of its lower mantle and molten iron core some 1,800 miles beneath the surface, scientists have determined. The discovery is published in the August 11, 1995 issue of Science.
By showing that rock samples from the Siberian Flood Basalts contain an isotope of helium gas present when the earth was formed 4.5 billion years ago, scientists at the University of Rochester and the Berkeley Geochronology Center (BGC) in Berkeley, CA, have found the "fingerprints" that suggest the sources of eruption came from an area near the earth's core. Scientists believe it to be rich in these primordial gases even today, billions of years after the earth's formation. Beginning as a plume, a long column of gas-bearing rock, the molten material slowly pushes its way toward earth's surface, depositing enormous quantities of lava -- more than 1 million cubic kilometers in the case of the Siberia Flood Basalts.
"This study presents definitively the source of the most catastrophic volcanism in the history of our planet," said Asish R. Basu of the University of Rochester, the study's first author.
Until recently, scientists had debated the source of such huge outpourings of lava. Some believed that such volcanoes were formed within the earth's upper mantle or just beneath the moving lithospheric plates. Other scientists believed their origins were far deeper, some 1800 miles beneath the surface at the base of the lower mantle. The Science paper gives fresh, compelling evidence that such volcanism begins within the lower mantle near the earth's core. Asish R. Basu and Robert J. Poreda of the University of Rochester and Paul R. Renne of the BGC believe that other flood basalts, such as ones that covered India, parts of Brazil, the Karoo Province in southern Africa, and the northwestern United States, also have a plume origin.
At least twice, such outpourings of lava have coincided with the most severe environmental crises on earth, when life was practically extinguished from the planet for periods lasting thousands of years.
To determine the origin of the Siberian Flood Basalts, Basu and colleagues analyzed the ratio of certain isotopes of helium, neodymium and strontium in rocks brought up during volcanic activity, looking for the isotopic "fingerprints" characteristic of the lower mantle. They found them.
"These results help establish the traits common to all flood basalt provinces," said Renne. "What is remarkable is that helium-3 is so well preserved in these rocks after 250 million years. It's like finding complete DNA preserved in dinosaur fossils."
The isotopic fingerprints of the Siberian volcanic rock are also seen in rocks from several "hot spots," volcanic areas beneath earth's plates responsible for forming chains of volcanoes as the plate gradually slides above. Reunion Island in the Indian Ocean is thought to be the hot spot that formed the Deccan Traps. Other hot spots with a lower mantle source are believed to have formed Hawaii and Iceland. Basu, Renne, and Poreda, along with many other earth scientists, believe that flood volcanism such as that in Siberia marks the initiation of a hot spot.
The helium isotopic composition was measured by Robert Poreda, associate professor of geology at Rochester. "The high helium-3 to helium-4 ratio points to a lower mantle source for the Siberian flood basalts," says Poreda. "The helium-3 is a remnant from the formation of the earth 4.5 billion years ago; such high ratios are found only in plumes from the lower mantle."
Working on the project besides Basu, Poreda, and Renne were graduate student Friedrich Teichmann at Rochester and Russian scientists Yurii Vasiliev and Nikolai Sobolev of Novosibirsk, and Brent Turrin of the BGC and U.S. Geological Survey, Menlo Park, CA. The work was supported by the National Science Foundation.