There is no controversy that asteroid impacts shaped all the solid worlds in the solar system; a quick glance at our own Moon should shake any lingering doubt. The surface has over a million craters more than a kilometer wide!

On Earth, though, that’s not so obvious. There are only about 200 confirmed impact craters (not including some new ones recently found in Wyoming/Nebraska), and there’s a deficit of small ones. That’s due to erosion, of course: our planet erases nearly all evidence of a crater in a geologically short time.

Despite this, geologists expect to see some craters dating back to the time when continents were first forming around 4 billion years ago. Yet the oldest known crater is 2.2 billion years old. Where are the older ones?

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They’re tough to find, because, again, erosion, plus there’s not a lot of solid above-water surface of the Earth dating back that far. A lot of what we do have is in western Australia, which has some of the oldest rocks known. And now a team of scientists thinks they have found a crater there dating back a staggering 3.5 billion years. The data look good, which means they’ve pushed back the record by well over a billion years [link to journal paper].

The crater itself has been, unsurprisingly, heavily altered since it formed. But the scientists found a region loaded with shatter cones, which is a smoking gun. These formations are conical in shape and usually striated (streaked), and are thought to form when the immense shock wave of an impact passes through bedrock. Nothing else can generate the pressures needed to form them, so they are the only macroscopic way we have of knowing an impact occurred (besides the crater structure itself, and craters can form in various ways).

There is lava and other kinds of rock above the shatter cones that can be dated to 3.47 billion years ago, so the impact is at least that old.

The site of the impact is in northwestern Australia, in a region called the East Pilbara Terrane, which is a craton, a pristine bed of rock that is a kind of a nucleus of continental plates today. They’re usually found in plate interiors, and are incredibly old. In that East Pilbara Terrane craton is a geologic dome called the North Pole Dome; these can form in many ways, but given its location the scientists think it could be the central peak of the impact crater.

These peaks form in large craters when the shock wave from the impact moves through the rock with so much energy the rock acts like a liquid. The crater forms, and the rock wave rebounds and moves back toward the impact point. The rock then splashes upward, like the “bloop” rebound of water when you throw a rock in a pond. The rock in this case becomes solid again after it splashes up, leaving a mountain behind. A great example is the utterly spectacular Tycho crater on the Moon which has a 2-km-high central peak.

One thing I learned from reading the paper on this impact is that there’s some idea that large asteroid impacts billions of years ago could punch through Earth’s crust and create the mantle plumes seen in various places, like the ones that formed Hawai’i and the Canary Islands. Very cool! They may also have triggered plate subduction as well as the formation of the primary basaltic crust we live on today. If that’s the case, we owe our existence to ancient large impacts.

If you want more info, the scientists who found the crater did an interview that’s at Scientific American. It’s fascinating.

I love this kind of stuff. Geology is a secondary scientific love of mine, so I get jazzed when it collides — in this case literally — with astronomy. It’s another reminder that we are literally and fundamentally a part of the Universe.

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