Giant Comet Hit Nevada 370M Years Ago

©The Associated Press

SALT LAKE CITY (AP) - New evidence suggests a comet slammed into Earth 370 million years ago, blasting a huge crater into the sea floor and triggering 1,000-foot waves that led to the extinction of many species, scientists said Monday.

The crash may have been the first in a series of comet strikes that forever changed life on Earth, including the extinction of dinosaurs millions of years later.

Charles Sandberg, a geologist emeritus with the U.S. Geological Survey, said Monday at a conference that the comet hit roughly 130 miles northwest of Las Vegas in southern Nevada when the region was covered by ocean.

The force of the blast, dubbed the ``Alamo Impact,'' created a crater on the sea floor that was 30 to 50 miles in diameter, ripping apart a reef on what was then the continental shelf, Sandberg said at the annual meeting of the Geological Society of America.

Waves 1,000 feet high spewed chunks of reef as large as a half-mile-wide over an area in southern Nevada that was 120 miles in diameter, he said.

Sandberg said he has roamed the area for several years and found shattered blocks of the reef, turned into a kind of rock named Alamo breccia. He said he and two other geologists - John Warme of the Colorado School of Mines and Jared Morrow of the University of Colorado - found evidence in June of the mass destruction stretching some 60 miles farther than previously believed.

The crash, named after a town in the area, happened 3 million years before one of the five greatest extinctions of life in Earth's history at the end of the Devonian Period, when most organisms lived in the ocean, the researchers said.

The evidence includes: crystals of shocked quartz that are sand grains shattered by the force of the impact; a rock layer rich in iridium, which is an element rare on Earth but common in asteroids and comets; and sphere-shaped pieces of limestone-like material created when small pieces of reef were blasted skyward and melted, then fell to Earth, Sandberg said.

The area is rich in fossils that are 370 million years old.

Researchers in recent years also found a crater and similar breccia rocks in other countries, suggesting the Devonian extinction was caused by a series of comet strikes, Sandberg said.

He said a comet that hit Jupiter in 1994 is evidence that collisions by comets or asteroids are more than just theory.

On the possibility that a future comet could strike Earth, Sandberg said. ``It's something that we all have to think about.''

He added: ``There were impacts throughout time and mass extinctions. ... And we can expect this again.''

Some scientists are skeptical that cosmic impacts caused mass extinctions, citing gradual die-offs of species as evidence that climate changes due to other factors were to blame.

Paleontologist Kevin Padian of the University of California, Berkeley told The Salt Lake Tribune that the timing of the Nevada impact and the Devonian extinction are ``close enough that you want to look at it further. ... It's reasonable.'' He was not involved in the research.

AP-NY-10-20-97 1941EDT