Oblique 3D view of a map of the Decorah crater area. Image credit: Adam Kiel, USGS

Nov 19, 2014 What lies beneath... Iowa?

Sen—If you’re an object in the Solar System, it’s just a fact of existence that you’re going to get hit by some other object in the Solar System. The evidence for this is as simple as looking up: Check out the Moon. It’s covered in craters, in some places saturated with them. They’re packed so tightly together that any big impact these days is likely to obliterate more craters than it creates.

But when you look down, craters are few and far between. Sure, there’s Barringer Crater in Arizona, and a few dozen others that are peppered here and there on Earth. But that’s nothing compared to the Moon, and the Earth’s a lot bigger target than its satellite. So where are our craters?

Gone, mostly. Erased by wind, water, geologic upheaval, and time. Most of the ones we still see are either young, in places where erosion is light, or so big it takes a long time to get rid of them.

… or to bury them. When 20 million tons of rock impacts the ground at a few of dozen kilometers per second, it does a lot of geologic damage. It makes a crater, of course, which pushes stuff out of the way (violently, ejecting it at high speed in all directions), even compressing and displacing the bedrock underneath. Over time water can flow into the crater, dropping sediment into it, which can get compacted and form shale. Eventually the crater gets buried, and can’t be seen from the surface.

Which is precisely the case in the town of Decorah, located in northeast Iowa. You could drive on Route 52 right by town and never notice that you were, in fact, over a 470-million-year-old impact crater.

But it’s true. The existence of the crater had been suspected for a while; unusual shale was found there, and seemed to have been deposited in a circle over 5 kilometers across. More investigation found shocked quartz—a clear sign of an impact; quartz is a tough mineral, and it takes a hypervelocity impact of an extraterrestrial body to generate the shock wave needed to alter its crystalline pattern.

More evidence came in 2013 when a gravimetric survey—literally a map of the strength of gravity in the area—showed a circular region of lower density than surrounding rock. The electric resistance of the rock also showed a distinct circular pattern.

All the evidence points to the same thing: A rock, probably 250 or so meters across, slammed into the area nearly half a billion years ago, exploding with the yield of a billion tons of TNT. Over time, it got buried, but the basic structure still exists a few hundred meters underground.

Interestingly, there are other craters dotting the Midwest that are about the same age, as well as others in Sweden and Estonia. It’s not clear if they are from the same impactor, which may have actually been a loosely bound group of small asteroids. But we’ve seen such things before, like when the comet Shoemaker Levy 9 broke up before impacting Jupiter in dozens of separate events.

That’s little more than speculation at this point, but what we know for sure is that something hit Iowa eons ago, carving out a crater the size of a town. And it joins the 175 or so confirmed impacts on Earth

I almost feel sorry for impact geologists, since there are so few craters for them to study in person, and they’re all old. But then I remember how those craters got formed… and I rather hope that they don’t get any fresh local ones to study any time soon.

Post script: I said the evidence for impact is obvious by looking at the Moon. However, it hasn’t always been clear those were from impacts; volcanoes were suspects for a long time. It wasn’t until the mid-20th century that impacts were indicated as their origin.