Huge Impact Crater Uncovered in Canadian Forest

John Roach
for National Geographic News
November 25, 2008
About 1,100 years ago a space rock the size of a big tree stump slammed into western Canada, carving an amphitheater-like crater into the ground and littering it with meteorites, a new study found.

The rock that made the newly identified crater might have created a sky show similar to the one that tore across northern Alberta's skies in the early evening hours of November 20 (watch video above).

But unlike the recent fireball—which broke apart as it streaked through Earth's atmosphere—the meteorite that carved the newly announced crater would have stayed solid until impact.

"You need to have that wallop," said study author Christopher Herd, an associate professor of earth and atmospheric sciences at the University of Alberta in Edmonton.

"You need to have that big mass still going fast in order to make the crater."

(Related: "Giant Meteor Fireball Explodes Over Northwest U.S." [February 21, 2008].)

Hunting Spot

Meteorites, objects from space that hit Earth, often come from asteroids.

Only about 175 impact craters are known worldwide, and when a space rock does slam into Earth with enough force to create a crater, the rock almost always evaporates on impact.

"Here we have a crater and we also have meteorites associated with it," Herd said. "There are only a dozen impact craters in the world that have that association."

Until recently, the Canadian crater was little more than a dip in a thicket of aspen trees known among local hunters as a good spot to bag a deer.

That changed when two hunters hauled a metal detector to the site in July 2007 and found four metallic fragments on the depression's rim.

They called Herd, who also curates the university's meteorite collection.

"Based on the description, I was extremely skeptical," he said.

The hunters sent Herd one of the fragments, which testing confirmed was an iron meteorite. Herd and colleagues went to the site and recovered 17 more meteorites in a day.

"That told us right away that we are dealing with something quite interesting and quite unique," he said.

Crater Study

But the thicket of trees, especially in the midst of the summer growing season, made analysis of the crater next to impossible.

Duane Froese, one of Herd's colleagues at the University of Alberta, suggested that a type of existing aerial survey could help.

The technique is called LiDAR, a remote sensing technology that can be used to "strip" away vegetation and create images of the bare Earth beneath.

"The crater shows up beautifully as a nice, bowl-shaped structure" that measures 120 feet (36 meters) wide and 20 feet (6 meters) deep, Herd said.

Calculations based on the crater's size suggest the impact rock was about three feet (one meter) wide.

The researchers have been working with local politicians since August 2007 to secure government protection for the site.

The crater and its associated meteorites are described in the December issue of the journal Geology.

More Craters?

Only a handful of similar-size craters are known in the world, though calculations suggest meteorites like the one that caused the newfound crater should hit about once every five to ten years.

With the use of LiDAR, Herd said, researchers "should be able to find more of these craters" hiding beneath vegetation.

Phil Bland is a planetary scientist and meteorite expert at Imperial College London in England.

In an email, he agreed the LiDAR technique "would be a great way of finding more of the little guys."

Bland noted that due to the lack of weathering and sparse vegetation in Australia, for instance, researchers have already come close to finding the number of expected small craters there.

"This technique would give us a way of finding those little craters everywhere else. Very cool," he said.

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