Astroblemes are the scars left on Earth's surface by the high velocity impact of large objects from outer space. Such colliding bodies are usually meteorites, but some may have been comet heads or asteroids. Few of these impacts are obvious today because our active earth tends to erode meteorite craters over short periods of geologic time. The term astrobleme was coined in 1961 by Robert S. Dietz from two Greek roots meaning "star wound." Most geologists were not convinced until the 1930s that a mysterious handful of huge circular depressions on the earth were caused by meteorites. The most studied astrobleme during that time was Barringer Crater, a meteor crater in northern Arizona, measuring 0.7 mi (1.2 km) across and 590 ft (180 m) deep. It is now thought to have been blown out about 25,000 years ago by a nickel-iron meteorite about the size of a large house traveling at 9 mi (15 km) per second. Over the years aerial photography and satellite imagery have revealed many other astroblemes. About 100 around the world have been confirmed by various geological methods. A number have diameters 10–60 times larger than that of the Barringer Crater and are hundreds of millions of years old. The largest astrobleme is South Africa's Vredefort Ring, whose diameter spans 24 mi (40 km).
Exploding or collapsing volcanoes can make roughly circular craters, so it is not easy to interpret such features unless there are a lot of meteorite fragments present. However, because only meteorites collide with the earth at terrific speeds, geologists also have the option of searching for the effects of tremendous pressure applied in an instant of time at potential astrobleme sites. Important clues along this line are a large body of shattered rock (impact breccia) radiating downward from a central focus, similar small-scale "shatter cones," very high pressure forms of the mineral silica not found anywhere else in the earth's crust (coesite and stishovite), finely cracked, "shocked" quartz particles, and bits of impact-melted silicate rock that cool into tiny balls of glass called "tektites."
See also Comets.
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