Impact craters are known from every continent including Antarctica. Several hundred impact craters have been mapped and described in detail by geologists, and some patterns about the morphology, shape, and size of the craters have emerged from these studies. The most obvious variations in crater style and size are related to the size of the impacting meteorite, but other variations depend on the nature of the bedrock or cover, the angle and speed of the impact, and what the impactor was—rock or ice. Impacts are known from all ages and are preserved at various states of erosion and burial, allowing study of the many different levels of cratering and a better understanding of the types of structures and rocks produced during impacts.
The collision of meteorites with Earth produces impact craters, which are generally circular, bowl-shaped depressions. There are more than 200 known impact structures on Earth, although processes of weathering, erosion, volcanism, and tectonics have undoubtedly erased many thousands more. The Moon and other planets show much greater densities of impact craters, and since Earth has a greater gravi
tational pull than the Moon, it should have been hit by many more impacts than the Moon.
Meteorite impact craters have a variety of forms but are of two basic types. Simple craters are circular, bowl-shaped craters with overturned rocks around their edges, and are generally fewer than three miles (5 km) in diameter. They are thought to have been produced by impact with objects of fewer than 100 feet (30 m) in diameter. Examples of simple craters include the Barringer Meteor Crater in Arizona and Roter Kamm in Namibia. Complex craters are larger, generally greater than two miles (3 km) in diameter. They have an uplifted peak in the center of the crater and a series of concentric rings around the excavated core of the crater. Examples of complex craters include Manicougan, Clearwater Lakes, and Sudbury in Canada; Chicxulub in Mexico; and Gosses Bluff in Australia.
The style of impact crater depends on the size of the impacting meteor, the speed at which it strikes the surface, and, to a lesser extent, the underlying geology and angle at which the meteor strikes Earth. Most meteorites hit Earth with a velocity between 2.5 and 25 miles per second (4-40 km/sec), releasing tremendous energy when they hit. Meteor Crater in Arizona was produced about 50,000 years ago by a meteorite approximately 100 feet (30 m) in diameter that hit the Arizona desert, releasing the equivalent of 4 megatons (3.6 megatonnes) of TNT. The meteorite body and a large section of the ground at the site were suddenly melted by shock waves from the impact, which released about twice as much energy as the eruption of Mount Saint Helens. Most impacts generate so much heat and shock pressure that the entire meteorite and a large amount of the rock it hits are melted and vaporized. Temperatures may exceed thousands of degrees within a fraction of a second as pressures increase a million times atmospheric pressure during passage of the shock wave. These conditions cause the rock at the site of the impact to accelerate downward and outward, and then the ground rebounds and tons of material are shot outward and upward into the atmosphere.
Impact cratering is a complex process. When the meteorite strikes, it explodes, vaporizes, and sends shock waves through the underlying rock, compressing the rock and crushing it into breccia, and ejecting material (conveniently known as ejecta) up into the atmosphere, from where it falls out as an ejecta blanket around the impact crater. Large impact events may melt the underlying rock forming an impact melt and may crystallize distinctive minerals that form only at exceedingly high pressures.
After the initial stages of the impact crater-forming process, the rocks surrounding the excavated crater slide and fall into the deep hole, enlarging the diameter of the crater, typically making it much wider than it is deep. Many of the rocks that slide into the crater are brecciated or otherwise affected by the passage of the shock wave, and may preserve these effects as brecciated rocks, high-pressure mineral phases, shatter cones, or other deformation features.
Impact cratering was probably a much more important process in the early history of Earth than it is at present. The flux of meteorites from most parts of the solar system was much greater in early times, and it is likely that impacts totally disrupted the surface in the early Precambrian. At present the meteorite flux is about a hundred tons (91 tonnes) per day (somewhere between 107-109 kg/yr), but most of this material burns up as it enters the atmosphere. Meteorites that are about one-tenth of an inch to several feet (mm-m) in diameter produce a flash of light (a shooting star) as they burn up in the atmosphere, and the remains fall to Earth as a tiny glassy sphere of rock. Smaller particles, known as cosmic dust, escape the effects of friction and slowly fall to Earth as a slow rain of extraterrestrial dust.
Meteorites must be greater than 3 feet (1 m) in diameter to make it through the atmosphere without burning up from friction. The Earth's surface is currently hit by about one small meteorite per year. Larger-impact events occur much less frequently, with meteorites 300 feet (90 m) in diameter hitting once every 10,000 years, 3,000 feet (900 m) in diameter hitting Earth once every million years, and six miles (10 km) in diameter hitting every 100 million years. Meteorites of only several hundred feet (hundreds of m) in diameter could create craters about one mile (1-2 km) in diameter, or if they hit in the ocean, they would generate tsunamis, more than 15 feet (5 m) tall over wide regions. The statistics of meteorite impact show that the larger events are the least frequent.
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