Twisted and folded silicate glasses (up to 50 cm across) concentrated in certain areas across the Atacama Desert near Pica (northern Chile) indicate nearly simultaneous (seconds to minutes) intense airbursts close to Earth’s surface near the end of the Pleistocene. The evidence includes mineral decompositions that require ultrahigh temperatures, dynamic modes of emplacement for the glasses, and entrained meteoritic dust. Thousands of identical meteoritic grains trapped in these glasses show compositions and assemblages that resemble those found exclusively in comets and CI group primitive chondrites. Combined with the broad distribution of the glasses, the Pica glasses provide the first clear evidence for a cometary body (or bodies) exploding at a low altitude. This occurred soon after the arrival of proto-Archaic hunter-gatherers and around the time of rapid climate change in the Southern Hemisphere.
The dry climate is why we even know about this event. Cometary collisions almost never leave a crater; given that comets are mostly made of various kinds of ice, the heat of friction from the atmosphere causes them to evaporate and finally explode, creating an airburst but no solid-object impact. The airburst can be devastating enough, of course. The 1908 Tunguska Event, the largest such occurrence in recorded history, flattened eighty thousand trees in over two thousand square kilometers of Siberian forest, and registered on seismographs all the way around the world in Washington, D.C. If Tunguska had happened over a major city, there wouldn't have been a person left alive or a building left standing in the blast zone.
Like Tunguska, at the time and place of the Atacama airburst, there weren't many people in the danger zone. There was, however, a lot of sand, and the heat from the collision melted it into glass -- indicating temperatures in excess of 1,700 C. In a climate with ordinary amounts of rainfall, the glass would have been degraded and eroded, but here, it rained out of the sky and then has just kind of sat there for the intervening twelve thousand years."It was clear the glass had been thrown around and rolled," study lead author Peter Schultz said, in an interview with Science News. "It was basically kneaded like bread dough."