Hudson Bay Could Hold Giant Crater

Oct. 25, 2006 — Lessons being learned from massive impact craters on Earth and beyond could help settle the question of whether such a crater exists in the eastern Hudson Bay.

Any map of Hudson Bay shows a suspicious, semi-circular coastline in northwestern Quebec. The Belcher Islands, just to the west of that curved coast, might be the remnants of a central peak — a common feature of impact craters on the moon and elsewhere.

What's more, past work has suggested that when recent rifting of the land near the crater site is accounted for, even more of the circle — about two-thirds — would be visible.

But while the idea of a 280-mile-round (450-km diameter) Hudson Bay crater has been around for 40 years, there has been no conclusive evidence of its existence.

If such a crater exists, it would be the largest known on Earth. The current record-holder is the 112-mile-wide Chicxulub Crater on Mexico's Yucatan Peninsula.

Until recently, no one has been quite sure what evidence would point to the existence of a crater so big, or so old, in this case at least half a billion years.

Any telltale minerals for such a big, old crater would be scarce or at least hard to find, explained geologist Michael Brookfield of Guelph University. What's more, the region's remote location and harsh climate make it a difficult research site.

But recent studies of impact craters on Earth and other planets are helping zero in on what to look for in Hudson Bay — so much so that he visited the area on a recent "on the cheap" research excursion.

Brookfield summarized the trip and his ideas for further exploration in a poster presented this week at the Geological Society of America meeting in Philadelphia.

The crater may have multiple concentric rings, said Brookfield, as is the case in the recently discovered Vredefort Crater of South Africa, the famous Chicxulub Crater of the Yucatan, and on large craters on the moon and elsewhere in the solar system.

If so, there could be evidence in the form of minerals that have been altered by sudden, violent fault movements. There also ought to be jumbles of older, broken rocks called breccia.

Finding evidence of such rings would no doubt bolster the theory that an impact crater exists in Hudson Bay. But just what causes an impact crater to have multiple rings is not exactly clear, said Jay Melosh, a crater researcher at the University of Arizona's Lunar and Planetary Laboratory.

"The moon has multi-ring craters and Mercury has none," observed Melosh. The reason may be that the moon has a more rigid crust with a weak zone underneath.

A multi-ringed crater on Earth would indicate that the impact was felt as deep as the upper mantle — the softer rock zone below the crust. The moon's ringed craters could be from billions of years ago when the moon still had a hot, soft interior.

Jupiter's icy moons also have an abundance of multi-ringed craters and are suspected to have soft interiors under hard crusts, said Melosh.

As for the chances that Hudson Bay holds the largest impact crater on Earth, Melosh said the definitive evidence would be "shocked" minerals that prove extreme pressures, far beyond what any volcano can produce. But without such evidence, he said, it's far-fetched to suspect a crater in Hudson Bay.

Still, "it wouldn't be surprising if there were large impact craters on Earth that we don't know about," said Melosh. That's because the moon, right next door, retains many large craters because it has no weather or plate tectonics to erase them. Based on the moon, said Melosh, Earth should have 6,000 craters larger than 300 miles (500 km) in diameter.

"There's got to be some evidence of the big ones on Earth," said Melosh. "But so far there's no proof."