Tuesday, December 7, 2010

Middlesboro Impact Crater

Take a close look at the Appalachian Mountains in the southeast corner of Kentucky. Mile after mile of long, high ridges and narrow valleys carved by streams. And then, just at the junction of Kentucky, Virginia, and Tennessee, there's a very strange shape.

It resembles a giant smooth bowl:

View Larger Map

It's a very strange and out-of place shape. It's got nicely circular walls in a few spots. Yet, there's no volcanic activity for a few thousand miles. How do you get a crater in the middle of folded ridge-and-valley mountains?

From space.

No kidding.

It's a three-mile-wide impact crater, formed sometime in the last 300 million years. (Wikipedia article | Entry in Earth Impact Database) The crater size estimator at the University of Arizona's Lunar and Planetary Laboratory estimates an impactor diameter somewhere around 180 meters (600 feet). That's like getting hit with a ball of rock or iron the size of Fenway Park.

And that's a small crater. There are several confirmed craters on Earth with diameters greater than 100 km (60 miles), indicating an impactor over one mile in diameter. That's like getting smacked with the entire city of Boston instead.

But it gets crazier. Look carefully at that map again. The southeast corner of the crater rim touches one of the mountainous ridges. Right where it touches, there's a little gap in the ridge. You might vaguely recognize the name from your last American history course. It's Cumberland Gap. It's where Daniel Boone crossed the mountains to bring settlers through. It was where 250,000 settlers passed through on their way to the fertile Ohio Valley.

Now, that gap was, it seems, created by the impact. Which means that that anonymous space rock played an important role in American history.

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