Astroblemes

Astroblemes are the scars left on Earth’s surface by the high velocity impacts of large objects from outer space. Such colliding bodies are usually meteorites, but some may have been comet heads or asteroids. Few of these circular or oval impact craters (also called impact basins) are obvious today because the active Earth (with erosion by wind and water, lava flows, sand, and other means) tends to erode meteorite craters over short periods of geologic time. Astroblemes can also refer to impact craters on other planets, moons (such as Earth’s moon), and other celestial bodies that are larger than the colliding body.

Daniel Moreau Barringer (1860–1929) is considered the first scientist to have identified an astrobleme. He identified a meteor crater in northern Arizona, measuring 0.7 mi (1.2 km) across and 590 ft (180 m) deep around 1906. It was eventually named The Barringer Meteorite Crater after him. Based on the 1920s studies from German-American geologist Walter H. Bucher (1889–1965), American geologists John D. Boon and Claude C. Albritton, Jr. stated that the perceived results of volcanic eruptions were in actuality the result of astroblemes. American geophysicist and oceanographer Robert Sinclair Dietz (1914–1995) coined the term astrobleme in 1961 from two Greek roots meaning star wound. Barringer Crater was the most studied astrobleme during that time, and most geologists were not convinced that meteorites caused a mysterious handful of huge circular depressions on Earth. In the 1960s, several researchers, including Eugene M. Shoemaker Carlyle S. Beals, and Wolf von Engelhardt, performed more detailed studies of astroblemes. However, their studies, along with other scientists’ studies, remained unverified and questioned by the scientific community until the late 1960s and early 1970s when the Apollo moon landings proved the accuracy of their work. Barringer Crater 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. Over

150 astroblemes around the world have been confirmed by various geological methods. A number have diameters 10 to 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 many meteorite fragments present. However, because only meteorites collide with 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 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.

Astroblemes have also been discovered on other planets in the solar system. Some of them include Petrarch crater on Mercury, Hellas Basin on Mars, and South Pole-Aitken basin on the Moon.

See also Comets.