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.
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.
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.