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Isaac Asimov

Theory Has Impact on Dinosaur Doom

April 14, 1989|Isaac Asimov

For nine years now, scientists have been arguing over a new explanation for the disappearance of the dinosaurs 65 million years ago. But that matter may finally have been settled at last.

In 1980, it was reported that in a 65-million-year-old thin layer of sediments there was an unusual concentration of the rare metal iridium. The suggestion was made that it could have come from a collision or impact with the Earth of a sizable asteroid or comet. The impact would have punctured the crust, set volcanoes to exploding, caused huge fires and tidal waves, and sent so much dust into the stratosphere as to cut off the sunlight for a long time. This would have brought about the death of much of Earth's life, including all the dinosaurs.

There's no question that 65 million years ago there was a "Great Dying" and that a catastrophe occurred, but not all scientists were ready to accept that it was the result of a large impact. In 1987, for instance, it was pointed out that if the Earth suddenly underwent a period of explosive volcanism, with many volcanoes erupting more or less simultaneously, that would be enough to produce a catastrophe of the size that could have caused the mass extinctions.

So things have boiled down to competing theories of "impact versus volcanism."

The question isn't just academic, since we may face one or the other catastrophe again someday (although, in the case of an object striking the Earth, we may someday know how to prevent the impact). We need to know as much as we can about the effects of these events, so we can try to plan some kind of emergency steps to take should we face them in the future.

So, scientists have been busy trying to find evidence to support these two theories.

In 1961, a Soviet scientist named S. M. Stishov found that if silicon dioxide (very pure sand) was put under huge pressure, its atoms were forced closer together, and it became very dense. A cubic inch of this squashed sand weighed considerably more than a cubic inch of ordinary sand. This squashed sand has been called "stishovite" ever since.

Stishovite isn't really stable. The atoms are too close together and they tend to move apart and become ordinary sand again. However, they are held together so tightly that this change takes place extremely slowly, so that stishovite can remain as it is for millions of years.

The same thing happens with diamonds. The carbon atoms in diamonds are pressed unusually close together and there is a tendency for them to spread out and become ordinary black carbon but that too takes millions of years under ordinary conditions.

You can hasten the change, however, if you raise the temperature enough. That adds energy to the atoms and allows them to pull away from their neighbors and resume their usual configuration. Thus, if you heat stishovite at 850 C. (1,560 F.) for 30 minutes, it will turn into ordinary sand. (You can also make black carbon out of a diamond by heating it in the absence of air, but who would want to?)

Stishovite was made in the laboratory. Does it also occur in nature? Yes, but only under conditions where great pressure has been placed on the soil.

For instance, stishovite has been found at places where there is evidence that a sizable meteorite once slammed into the ground. The great pressure of the impact formed the stishovite. Stishovite has also been found in places where experimental nuclear explosions have taken place. The huge pressures of an expanding fireball formed it.

It seems certain that stishovite must also occur deep under the Earth's crust where the pressures are extremely high. In that case, it might be brought to the surface by volcanic eruptions. However, those eruptions are extremely hot and the rock is liquefied. Any stishovite that emerges from a volcano would be converted into ordinary silicon dioxide. And, as a matter of fact, no stishovite has ever been detected at sites of volcanic activity.

You can say, then, that the presence of stishovite indicates that an impact must have taken place and that volcanic action must not have taken place.

Well, just this past month, John F. McHone and several co-workers at Arizona State University studied rocky layers in Raton, N.M., layers that were 65 millions years old and therefore date back to the time the dinosaurs disappeared.

They made use of modern techniques of determining atomic arrangements in solid materials--nuclear magnetic resonance, as well as X-ray diffraction--and on March 1, 1989, they reported they had definitely detected the kind of atomic arrangement found in stishovite.

That makes it look as though there was a vast impact 65 million years ago that formed tons of stishovite, which was kicked up into the stratosphere before settling down to Earth. It wasn't volcanic action that killed the dinosaurs, then; it had to be the impact.

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