Great Extinction


July 12, 1991

In the July 12 issue of the distinguished journal Science, two scientists present the strongest evidence yet linking precisely the largest extinction of animals in Earth's history with rapid and extensive volcanic activity in Siberia. The strong link between the two events suggests the possibility that the extinction was perhaps a result of volcanic activity.

The work by Paul R. Renne of the Institute of Human Origins in Berkeley, Ca., and Asish R. Basu of the University of Rochester indicates that the volcanic activity 248 million years ago occurred very rapidly, perhaps in less than one million years -- just an eyeblink in Earth's history.

Scientists have known about both the global faunal mass extinction and about the extensive volcanic activity in Siberia for decades. While a few scientists have recently suggested there may be a link between the two, Renne and Basu are the first to offer convincing evidence showing that the events occurred at precisely the same time on a geologic time scale.

Their experiments show that volcanic activity began approximately 248.3 million years ago and ended approximately 247.5 million years ago. Scientists believe that as many as 95 percent of all animal species on Earth were wiped out in an extinction that occurred approximately 250 million years ago, at the boundary between two geologic time periods, the Permian and the Triassic. More than half of all marine families and most groups of mammal-like reptiles were killed off.

"This eruption occurred over an extremely short period of time, geologically speaking," says Renne. "We believe we've nailed the date down fairly precisely. It's even conceivable that it occurred over an interval as short as 200,000 years."

The volcanic activity in Siberia did not resemble spectacular eruptions such as Mount St. Helen's or Mount Pinatubo currently erupting in the Philippines. Rather, the volcanism of the Siberian Traps, as the area is known, was marked by large amounts of lava oozing out of cracks in the ground. Such huge lava flows are known as flood basalts, and they are characterized by different layers of lava stacked much like pancakes.

Such rapid, profuse volcanic activity is characteristic of other huge flood-basalt lava deposits, such as the Deccan Traps in India (which erupted at the time of another mass extinction), the Karoo Province in southern Africa, and the Columbia River basalt in the northwestern United States.

Basu and Renne say the rapidity of the eruption shows that the Siberian volcanic activity originated deep within the Earth, at the boundary between the Earth's molten outer core and its mantle. "We believe the Siberian Traps have a 'mantle plume origin,'" says Basu. "Picture a giant mushroom with a very, very long stem, forming beneath the Earth's crust. The source of the lava is deep within the Earth -- nearly 3,000 kilometers beneath the crust -- and flows upward through a narrow channel, until it collects in a large area just beneath the Earth's crust. Eventually the crust bursts open and the lava flows out." Basu, Renne and many other scientists believe that other major lava deposits, such as the Deccan Traps and the Columbia River basalts, have a plume origin as well.

The Siberian flood basalts, located south of the Kara Sea and east of the Ural Mountains in the Soviet Union, represents one of the largest outpourings of lava the world has ever seen: Scientists believe its volcanoes poured out some 1.6 million cubic kilometers of lava (if this lava had spread evenly over the continents and oceans, the entire surface of the Earth would have been covered with a lava layer 10 feet thick). The area is now a plateau covering 337,000 square kilometers.

Renne and Basu dated rocks from several layers of the lava using the "argon-argon" technique, one of the most precise dating techniques available to scientists. Scientists determine how old a rock is by determining how much argon has been produced by the radioactive decay of potassium in that rock.

Two features of the argon-argon technique employed by Renne and Basu help make the results highly accurate. First, the heating system used to release the gas from a rock sample is a laser, which is an extremely clean method of heating. Also, the scientists used an automated laser system to reduce user error. Renne and Basu believe this is the first time anyone has applied the argon-argon technique to samples from the Siberian Traps using both automated equipment as well as a laser to heat the samples.

"With the argon-argon technique, you use incremental heating, and you analyze the gases that are released each time you go to a higher temperature," says Renne. "If the results from a series of different heating steps agree, then that strongly confirms the date. Our results are very consistent." Renne and Basu dated both the rocks and, separately, some of the individual crystals within the rocks containing gas; the results were nearly identical.

The selection of rock samples was a key feature of this work: Rocks had to be in pristine condition, virtually unchanged from their condition 248 million years ago. The rocks were provided by G.V. Nesterenko of the Vernadsky Institute of Geochemistry of the Soviet Union's Academy of Sciences, who selected them according to specifications laid out by Basu. "The rocks we studied show very little alteration. It's not easy to find rocks 248 million years old that are unaltered," says Basu.

Several scientists have suggested how extensive volcanic activity could cause a large extinction. Volcanoes release tons of sediment and ash, as well as massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere. The dust could have blocked out sunlight across the Earth, preventing plant photosynthesis and causing food chains to collapse. Or, carbon dioxide could have trapped the sun's heat, sending temperatures on Earth soaring and killing many forms of life.

This work was supported by the National Science Foundation and the Institute of Human Origins.


Paul R. Renne

Paul Renne, a research associate at the Institute of Human Origins Geochronology Center in Berkeley, CA, is a geochronologist who specializes in dating events and processes in Earth history.

Renne is well known for his work dating the magnetization acquired by rocks from the Earth's magnetic field. By studying the fields as they are recorded in ancient rocks, geologists can track the movements of the plates that make up the Earth's crust. Renne has used the technique to date rocks up to one billion years old.

The Institute of Human Origins (not affiliated with the University of California at Berkeley) was home to the world's first fully automated laser laboratory and the world's first automated argon-dating lab. Renne has used the argon-argon dating technique, which he calls the most versatile dating technique available, to study several different kinds of rocks.

Renne earned his bachelor's degree and Ph.D. in geology at the University of California at Berkeley. He was a post-doctoral associate at Princeton University before joining the institute in 1990.

Asish Basu

Asish R. Basu, professor and chair of the Department of Geological Sciences at the University of Rochester, studies the origin and evolution of the Earth's crust and mantle.

A petrologist and geochemist, Basu has contributed significantly in understanding the geochemical and isotopic evolution of the oceanic and continental lithospheric plate, the outer mobile shell of the Earth. He is well known for his work on kimberlites, a volcanic explosive rock of deep origin that brings up diamonds to the Earth's surface. His current work on Siberian and Hawaiian basalts focuses on the dynamic and chemical state of the Earth's deep interior, and of the interior's effect on the structures and processes observed at the Earth's surface.

Basu is a graduate of Calcutta University and earned his master's degree in geophysical science from the University of Chicago and his Ph.D. degree in geology from the University of California at Davis. He carried out post-doctoral research at the University of Minnesota and the U.S. Geological Survey in Denver before joining the faculty at Rochester in 1978.

CONTACT: Tom Rickey, (716) 275-7954