
                         Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: WEDNESDAY, March 6, 1991                   TAG: 9103060062
SECTION: NATIONAL/INTERNATIONAL                    PAGE: A-3   EDITION: METRO 
SOURCE: Newsday
DATELINE:                                 LENGTH: Medium

CRATERS MAY FIT EXTINCTION THEORY/ SCIENTISTS PONDER CAUSE OF DINOSAURS' DEATH

The long, difficult search for a "footprint" - a crater big enough and old enough to explain the demise of the dinosaurs - is focusing now on a massive blemish in northern Siberia, where some scientists suspect the ancient cataclysm occurred.

If their evidence is correct, the huge size and strange geology of the remote Popigay crater could resolve the decades-old argument over the death of the dinosaurs. Theorists argue that a massive event of some kind - a meteor impact or a volcanic cataclysm - occurred 65 million years ago, changing the environment and destroying the animals. Excitement about the Popigay crater is based on a sample of glassy rock from the center of the crater supplied by Soviet scientists. The latest tests suggest strongly that the crater is 66.3 million years old.

Speaking of the impact, planetary geologist Jim Garvin said "We believe it occurred 66 million years ago, and we have a 100-kilometer - or perhaps bigger - crater" of the correct size and age to fit the impact theory. The exact size is unclear because of erosion. The crater is named for the nearby town of Popigay, north of the Arctic Circle east of the Ural Mountains.

Garvin, at NASA's Goddard Space Flight Center in Greenbelt, Md., said his new evidence, which is still controversial, will be released next month at a lunar and planetary science meeting in Houston. But in a telephone interview, he explained that age measurements of glassy rocks from Popigay show an impact occurred there 66.3 million years ago. The crater's size indicates it was the point where a meteorite five to 10 miles in diameter hit the Earth. It was probably going about 45,000 mph when it hit.

Similar excitement is being voiced about a potential site on the Yucatan Peninsula in Mexico, where evidence of a huge impact has also been found. It is possible that a crater 100 miles in diameter lies buried under thick sediments there.

An impact capable of creating such craters resembles an atomic explosion of 10 million megatons. That is large enough to cause a worldwide event by pushing a huge cloud of dust and other debris into the air. Such a cataclysm might have darkened the entire globe long enough to interrupt plant photosynthesis, created acid rain, caused a greenhouse effect that warmed the air and the seas and burned huge forests, causing extinction of vulnerable species, both on land and in the sea.

Evidence collected from sediments around the world suggests such an event did occur some 65 million years ago. But until now a crater of the correct size and age had not been identified. Some scientists argue strongly that massive volcanic eruptions may have been the cause instead, although opinion now favors the impact theory.

The impact scenario was proposed by geologist Walter Alvarez. In 1974 he reported discovery of a strange layer of material - enriched with metallic iridium - buried in sediments at several widely separated locations. The iridium layer deposit is about 65 million years old, coincident with the sudden, worldwide die-off of many species. It has been found in 105 locations around the world.

by CNB