It's remarkably hard to find evidence of impact craters on the Earth.
If you're thinking, "What's the difficulty? Just look for a big hole in the ground," you're probably thinking of one of two things -- either craters on the Moon, or Barringer Crater near Winslow, Arizona. The craters on the Moon stick around pretty much indefinitely because the airless, waterless surface experiences virtually no erosion; as far as Barringer, the impact that caused it only happened around fifty thousand years ago, which is the blink of an eye, geologically speaking. (Plus, it's in the high desert, with little vegetation to hide underneath.)
With older impact craters, the forces of erosion eat away at the telltale signs -- the raised, oval or circular ridges, especially. The oldest craters have been destroyed by subsequent tectonic shifts and faults, and (for ones in oceanic plates) because the damaged strata themselves were subducted and melted.
One massive impact crater that was only detected in 1983 -- despite the fact that tens of thousands of people live more or less right on top of it -- is the one left by the Chesapeake Bay Impact Event, which occurred during the Eocene Epoch, on the order of 35.5 million years ago. At that point, the impact site, on the southern tip of the Delmarva Peninsula, was coastal tropical rainforest; the global temperature was still dropping following the massive Paleocene-Eocene Thermal Maximum, but was still a good two degrees Celsius warmer than today. The mass of the impactor isn't known for certain -- it was completely vaporized -- but it's estimated to have been about three kilometers across and traveling at eighteen kilometers per second, and punched a hole eight kilometers deep into the crystalline basement rock, blasting the sediments on top to smithereens and creating a crater over eighty kilometers across. Because at least part of the impact was in the shallow ocean, it also created a massive tsunami that travelled inland as far as the foothills of the Blue Ridge Mountains.
Since the impact, it refilled -- first with unconsolidated, unsorted sediments, essentially broken up pieces of the rock that was blown out from the collision, then with eroded material as the whole place gradually settled down. Part of it was refilled with seawater. The only way it was discovered was the presence of an anomalous "fault" that turned out to be the edge of the crater wall, followed by the analysis of some rock cores that showed a huge, thick layer of jumbled junk that geologists figured out was the debris formed as the crater walls slumped inward. It also explained the North American Tektite Field, an enormous splatter field of what amounts to cooled droplets of melted rock.
But visiting the area today, you don't see much that would tell you that only thirty-five million years ago, the place got slammed by an enormous chunk of rock from outer space.
Even the much larger Chicxulub Impact Crater, near the Yucatán Peninsula, took a lot of work to identify. It's just shy of twice as old as the Chesapeake Bay site (about 66 million years), and is almost entirely underwater and filled with oceanic sediments. Today, the impact site that ended the 180-million-year hegemony of the dinosaurs is only visible to sensitive gravitometers and magnetometers.
Which makes the discovery of an impact crater 3.47 billion years old, in East Pilbarra, Western Australia, even more astonishing.
A paper in Nature Communications this week, authored by Christopher Kirkland of Curtin University et al., shows convincing evidence of an impact crater over a hundred kilometers wide near the northwestern coast of Australia. The center of the crater shows regions of shocked crystalline rock, along with layers of breccia (the same sort of jumble of debris found at the Chesapeake Bay site). Further stratigraphic work has confirmed that this was, indeed, the site of a "massive hypervelocity impact." This makes it the only Archaean-age crater known to have survived.
The authors write:
Despite the high modeled frequency of bolide impacts in the early Archaean, the rarity of verified impact craters of Archaean age suggests that: (a) the impact flux was much less than predicted by lunar data; (b) the evidence has been eradicated, or (c) that we have failed to recognise them. On a young Earth covered in primitive (mafic–ultramafic) crust, identifying shatter cones or impact breccias may represent the best chance of finding other large Archaean impact structures. However, these highly fractured rocks will be the first to undergo (presumably intense) weathering and erosion. Notwithstanding their fragility, we believe many more Archaean craters await discovery.
Myself, I think it's astonishing that they've found even one. For any traces to have survived for nearly three and a half billion years is staggering. At that point, life was only getting started; the first known microbes appeared 3.7 billion years ago, and when the impact occurred, it would still be another half a billion years before the first certain multicellular life. So unlike the Chesapeake Bay and Chicxulub Impacts, which were (respectively) regionally and globally devastating to life, the East Pilbarra collision probably didn't make much of... um... an impact.
But it definitely stirred things up, created an enormous crater and rain of debris, and would have been a dramatic thing to witness. From a safe distance. The fact that even today, 3.47 billion years later, geologists can detect the hole it left behind, indicates that it was one hell of a punch.
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