If we could see all the holes gouged in the Earth by asteroids, we'd run screaming for cover.
- By Tony Reichhardt
- Air & Space magazine, May 2004
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A key bit of evidence found at Canyon Diablo is a type of mineral called shocked quartz, which has since become the most accepted proof for identifying impact craters. Under a microscope, the rock grains are arrayed in a distinctive criss-cross pattern, the lattice structure of the quartz having been knocked off kilter by a sudden, intense blast of pressure. In only two places can you find rocks that have been so profoundly crunched. One is a meteor crater. The other is the bottom of a nuclear test pit.
Middlesboro's status as a confirmed, rather than suspected, crater comes largely from the discovery of shocked quartz there in the 1960s. Geologists also have found another telltale sign of impact-"shatter cones," caused when the shock wave from a sudden blast moves through rock at supersonic speeds. Milam shows me one. I'm not sure I would be able to tell its subtle striations from all the other ripples you see in rocks. He's always on the lookout for more, and even his seven-year-old, Zac, who's along on our field trip today, is on the hunt. Every so often he holds up a pebble. "Is this a shatter cone?" No, says Milam, smiling.
He's still puzzling out some of the complex evidence himself, trying to reconstruct the details of the impact, which partly depend on what the impactor was made of (there's a big difference between the wallop of a loose iceball and that of a chunk of rock or iron), and how big it might have been (100 to 500 yards across, he guesses). He's starting to think the crater might not be as neatly circular as it appears, that it may be more elongated.
The Middlesboro impact was big enough to form what's called a complex crater. The initial excavation was followed by a rebound of material from the center, creating a central uplift feature, the way a pebble dropped in water splashes up a jet of water. Fractured material then slumped back into the crater, and the outer rim became terraced, unlike the neat bowl you find at the smaller (and fresher) Meteor Crater.
In this case the central uplift feature-ground zero-is on the grounds of the Middlesboro Golf Course, which dates back to 1889 and bills itself as the oldest golf club in the country. On this unseasonably warm Tuesday afternoon in November, the course is mostly empty. I watch a twosome approach the second green, right behind me. A guy who looks like he just stepped from the Land's End catalog whacks stiffly at a golf ball, which rolls about 10 feet. His next shot dribbles onto the green, and two undistinguished putts later, he picks up his ball, fuming. He looks to be having a bad day.
Of course, bad days are relative. Right next to the first tee (a 273-yard par 4) is an old, weathered block of sandstone that shouldn't be there. By rights this particular rock should be buried 1,300 feet below, with the rest of the Lee Sandstone formation. The asteroid that hit some 300 million years ago yanked it up in a horrifying instant, the central peak uplifted from underneath the now empty bowl.
Out at the perimeter of the original crater, Milam shows me rock beds turned completely upside down, a disorder that can't easily be explained by trivial events like earthquakes. "When I bring professional geologists here who aren't familiar with impacts, they just scratch their heads," he says. In the normal course of geological research, they would never encounter forces like the ones on display here.
Driving north from Middlesboro on Highway 25 late that afternoon, I see that the exposed rock on either side of the road returns to its normal pattern. Stacks of beds, the once-muddy floors of ancient seas, are as flat and regular as the layers of a cake. Order is restored. Leaving this place of past violence, I see a hand-drawn sign off the highway that says "Prepare to Meet God."