Thursday, September 13, 2007

Crater Mystery Cracked in Michigan

Eighteen hundred million years ago, an area that now spans the U.S.-Canadian border near Lake Huron was battered by a rain of molten debris and mega-tsunamis caused by what is thought to be the second largest impact in Earth's history. But the source of that collision has long been a mystery.

Now, telltale signs of what's called the Sudbury impact of southern Ontario — including shocked quartz, once-molten rock spherules and extraterrestrial iridium — are ruling out a comet and making a strong argument that it was an asteroid that struck southern Canada all those eons ago.

"It was a Himalaya-sized object that slammed into the Earth," said geologist Peir Pufahl of Acadia University in Nova Scotia.

The asteroid was about a dozen miles across and flew through space at 45,000 miles per hour, he said. The crater it left behind is estimated to have been about 150 miles across and is a very unusual and important deposit of valuable metals ores. The largest on Earth is the Vredefort Crater in South Africa, at 190 miles across.

In the millions of years since then, the crater itself has been eroded and buried, and much of the more remote debris from the impact has done the same. Even the shape of the crater has changed as the crust has been pushed and pulled.

"The impact crater has been squashed into an east-west oval," Pufahl told Discovery News. "Some of [the fallout] is preserved a kilometer below the surface of the Earth."

In northern Michigan, across Lake Huron from the impact zone, Pufahl and his team drilled down into the Earth to find rocks containing signs of the impact. The most obvious were long, teardrop-shaped blobs in rocks that were once molten rock flying through the air just after the impact. They also found that the fallout material was quite jumbled.

"It looks like the impact caused a mega-tsunami that reworked deposits of the impact," said Pufahl. He and his team published their discovery in the September issue of the journal Geology.

No comments: