Impact crater may lie beneath Lake Huron.

Impact crater may lie beneath Lake Huron

With the help of magnetic sensors, scientists have detected a rimmed circular structure, 30 miles in diameter, more than a mile beneath the floor of Lake Huron. They believe the magnetic ring marks a buried crater -- potentially one of the largest known -- blasted by a meteorite at least 500 million years ago.

Four researchers with the Geological Survey of Canada unexpectedly discovered the circular pattern while examining new, high-resolution magnetic images of a southern Ontario region previously labeled as "essentially featureless." They noticed that the patterns of gravitational and magnetic forces emanating from the structure resembled those of verified impact craters. The Ottawa-based researchers, led by David A. Forsyth, temporarily named their discovery the Can-AM structure because it straddles the Canadian-U.S. border. They describe the find in the August GEOLOGY.

Scientists have taken particular interest in impact craters over the last decade, in light of new theories linking meteor and comet strikes with ancient climate disruptions and mass extinctions. These theories suggest that massive meteorites collinding with Earth have periodically clouded the atmosphere with dust, triggering climate changes, photosynthetic failures and subsequent mass extinctions, including the great dinosaur demise some 65 million years ago (SN: 5/19/90, p.311).

But so far, the impact data have been too scattered to prove any correlation between the collisions and the species extinctions, says Jay Melosh, a planetary scientist at the University of Arizona in Tucson. With more craters like Can-Am to study, he says, "we can test those correlations."

Ahole the size of the Can-Am anomaly would have required a projectile about 3 miles in diameter, Melosh estimates. "We would expect 100 craters this size or greater during the last 500 million years," he says. "We know of about six."

Petroleum engineers might also want a closer look at Can-Am, since impact structures sometimes trap hydrocarbons. "The Can-Am structure may fall into this category," Forsyth says.

The only way to establish whether the Can-Am structure actually resulted from a meteorite collision is to examine the rock beneath it. "Impact pressure [approximates] the pressure in the core of the Earth," Melosh explains. Under the tremendous heat and pressure of a meteorite impact, rocks melt, shatter or metamorphose into strange forms that not even volcanic explosions can generate. Some of these forms -- like stishovite, a phase of quartz produced under extremely high pressure -- are "generally considered indisputable evidence [of an impact] when found near the surface," Melosh says.

"We need somebody to drill [Can-Am]," Forsyth says. At an average depth of only a mile or so, this could prove light work for modern drilling techniques. The U.S. scientific drilling program, however, has stalled in recent years, and the fledgling Canadian drilling program is just getting off the ground. Forsyth says he has heard of no plans to probe the secrets of the Can-Am structure.