Sept. 6, 2007 — 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.

As for that extraterrestrial iridium — the sign of an asteroid rather than a comet — it is found both near and far from the impact zone. It is, however, just one of the rare metals concentrated in the impact zone when the asteroid hit, explained geologist John Spray of the University of New Brunswick.

"The impact fused the earth — lock, stock and barrel," explained Spray, referring to ground zero. It converted the ground there into a 75-mile-wide, 1.5-mile-deep pool of molten rock. Trace metals in the ground, like nickel, copper, platinum, palladium, osmium and iridium, from the asteroid itself were separated out and dropped to the bottom of the pool, concentrating there. Today there remain some extremely valuable deposits.

"I would argue that Sudbury contains some of the biggest ore bodies on Earth," said Spray.

Pufahl, for his part, is interested in the effects of the impact even beyond Michigan. For instance, at about the same time as the impact, there was a global change in ocean chemistry, he said. A natural question to ask was whether the impact caused it.

"Did it shut down ocean photosynthesis?" Pufahl asked. Finding out is no easy task, however, because the farther we go back in geologic time the harder it is to resolve the timing of different events. "It's like a book with 90 percent of the pages missing," he said.