Prehistoric Asteroid "Killed Everything"

Newswise/Science News
August 23, 2002

Scientists have found traces of an asteroid-collision event that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the mountains. The coastline of the continents was changed drastically and almost all life on land was exterminated.

This cataclysm some 3.5 billion years ago is the earliest known meteor strike to hit the Earth, and one of at least four that have been identified in a geologically brief 300-million-year period. The strike is the subject of an article published in the current issue of the journal Science by Louisiana State University geologist Gary Byerly and others.

Byerly and Xiaogang Xie, also of LSU, and Donald Lowe and Joseph Wooden of Stanford, identified traces of the event in some of the oldest known rocks on Earth—in South Africa and northwest Australia.

When the asteroid hit, it was vaporized by the extreme energy of the impact. Condensation of this vapor produced droplets of melt, called spherules, which dropped into the roiling sea over the next few days and were deposited in layers on the sea floor.

Byerly said it was not known where the meteor hit, but it was probably some distance from where they found the spherules and probably in water rather than on land. He deduced this because the composition of the spherules lacked the mineral composition that would have been expected from vaporization of the continental crust, and because there was even more water covering the surface of the Earth then than there is today.

Byerly illustrated with a slab of grayish rock about the size of a large hand. In it could be seen layers of spherules interlaced with layers of finer sand or silt. "It would take about 30 hours from impact for the tsunami to travel all the way around the world. Then, of course, it wouldn't stop, but bounce all the way back till it met itself 30 hours later, then bounce the other way again, setting up a harmonic." Several thin layers of mud within the spherule layer represent periods of quiet sedimentation between the arrival of tsunami waves.

The water would likely have inundated everything but the mountains, Byerly said, and drastically eroded the continental land masses, changing their coastlines dramatically. The heat of the impact would have evaporated the upper 30 to 300 feet of water in the oceans. It would also have killed everything, or almost everything, that was alive at that time on land or near the ocean surface.

"There was almost certainly life at this time. Primitive, bacterial life, and if the impacts were made by a meteor 20 miles in diameter, they would have killed everything on the surface of the Earth," he said. First a hot steam of molten rock and water would have withered most life, then the massively destructive tsunamis would have destroyed even more. After that, years of incredibly cold winters, caused by particles in the atmosphere blocking out the sun, would have conspired to kill nearly everything else.

"Anything that survived would have been in deep rocks or below the surface of the Earth," he said.

Byerly first came upon evidence of these impacts by chance in 1984 while he was studying ancient volcanism in Australia and South Africa. He published his first paper on them in 1986. This year alone he and his team will have four papers published on the subject. It is now generally accepted that the inner solar system was battered twice by massive meteor impacts of mysterious origin.

"It is assumed that the solar system was created by a cloud of dust and rocks that condensed into the sun and planets, with larger and larger chunks falling in near the end. This process would have finished about 4.5 billion years ago.

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