Geologists long insisted Meteor Crater was volcanic, not a cosmic hit
In 1903 a mining engineer named Daniel Barringer was sure the huge pit he owned in Arizona had been punched by something from space. Most geologists thought a volcanic steam blast was more likely. It took more than half a century, and a young scientist named Gene Shoemaker, to settle the argument.
An impact crater forms when a smaller object slams into a solid body at enormous speed. Unlike volcanic craters, impact craters usually have raised rims and floors sunk below the surrounding ground. They dominate the landscapes of the Moon, Mercury and many moons and asteroids, but on geologically lively worlds such as Earth, Venus and Io, erosion, volcanism and tectonics erase or bury them. Ancient surfaces on the Moon, Mercury and Mars preserve evidence of a fierce early rain of impacts, roughly 3.9 billion years back, across the inner planets.
Acceptance was slow. Grove Karl Gilbert proposed in 1893 that the Moon's craters came from asteroid strikes, and Ralph Baldwin argued the same in 1949. Walter Bucher studied American sites in the 1920s and blamed volcanic explosions, until John Boon and Claude Albritton reinterpreted them as impacts in 1936. Around 1960 Shoemaker, working on Meteor Crater for his Princeton doctorate, revived the idea that lunar craters formed in seconds, not over ages. Searches by Carlyle Beals in Canada and Wolf von Engelhardt in Germany had tentatively identified more than 50 impact sites by 1970, and the Apollo landings then under way lent support.
The physics resembles an explosion more than a collision. Objects from space strike Earth at a minimum of about 11 kilometres per second, and potentially around 72. A shock wave compresses both projectile and target to extreme density; it can turn ordinary quartz into rare high-pressure forms such as coesite. The compressed zone then decompresses violently, melting the impactor, vaporising most of it in larger strikes, and hurling rock outward. Contact and shock last only a few tenths of a second; digging out the crater takes longer.
About 190 craters have been confirmed on Earth, from a few tens of metres to about 300 kilometres across. The youngest include the Sikhote-Alin craters in Russia, whose formation people witnessed in 1947. On average Earth suffers one to three impacts every million years big enough to leave a 20-kilometre crater.
Source: Impact crater
