Small meteorites, which are more common than larger ones, generally cause little damage and do not produce significant craters. For example, the 10 centimeter-long Claxton, Georgia, meteorite, one of the few meteorites to cross paths with civilization, only dented a mailbox before burying itself in the ground 28 centimeters (3). Such low-mass meteorites are slowed in the Earth's atmosphere and thus their impact velocity is limited to that produced by gravitational forces.
In contrast, large meteorites are not significantly impeded by the atmosphere and may strike the ground with greater velocities (tens of kilometers per second). An excellent example of a crater produced by this type of impact is the geologically young (50,000 year old) Meteor Crater in northern Arizona. This crater is 1.25 kilometers (4,100 feet) across, 170 meters (560 feet) deep (that of an approximately 60 story building), and has a circumference of nearly 5 kilometers (over 3 miles). Numerous fragments of Canyon Diablo, the iron meteorite responsible for making the crater, have been found in the area. These fragments range in size from minute particles to pieces weighing over 1000 pounds; the total mass of material found so far is over 30 tons (2).
When the Canyon Diablo iron meteorite struck the plateau region it produced a huge explosion. Within a fraction of a second, large portions of the meteorite and plateau sediments were vaporized. Small, surviving fragments of the meteorite were mixed with shocked sediments and ejected from an expanding crater. Some of the sediment was ejected as fine-grained particulate debris and deposited for some distance around the crater. Large blocks were also ejected, particularly near the rim of the crater, where they were simply overturned. Within a few minutes the vast hole in the plateau was completely excavated, the rocky ejecta had landed, and the cloud of vapor that had risen above the crater was already dissipating.
In the case of large impacts, where a significant portion of the targets are melted, molten droplets may also be ejected from the craters. Depending on several physical characteristics, most notably how much glass and water they contain, these droplets are called tektites, krystites, or more generically, impact melt spherules. There are several very large tektite strewn fields on Earth, some spread out over several continents. In many cases, the craters associated with the strewn fields have been masked by later geologic events, and thus the tektites are the only surviving evidence of these ancient impacts.
Because previously collected meteorites appear to have come from asteroids, most impacts are usually associated with colliding asteroids. However, impacting comets are also likely to affect Earth. Some scientists believe the Tunguska event in Siberia in 1908 was the result of a colliding comet. People in the area reported seeing a meteor, hearing a deafening roar, feeling an air blast (from shock waves produced in the atmosphere), and subsequently seeing a huge fire where the meteoroid was believed to have hit the ground. Closer inspection of the area confirmed there had been a forest fire, but in addition, investigators found the trees in the area had been knocked down within a 32 kilometer (20 mile) radius, and that the trunks of the trees all pointed to a single area as if there had been a huge centrally-located blast. No meteoritic debris was found however; consequently, a weak asteroid is suspected to have exploded in the atmosphere just before striking the ground.
Even larger events than Tunguska have occurred in Earth's history. For example, 210 million years ago a large impact produced the Manicouagan Crater in Canada which is 70 kilometers in diameter. To date, there are roughly 120 known impact structures on Earth; many other landforms are suspected to be impact craters, and many more are assumed to lie hidden in the oceans since water masks 70 percent of the Earth's crust.
In addition, there is evidence of a much larger impact occurring 65 million years ago, at the end of the Cretaceous Period, when the dinosaurs and many other types of plants and animals became extinct. Ejecta from this impact has been found around the entire globe, indicating the impact event was larger than any other known on Earth. It is believed such a large impact ignited wildfires, lowered temperatures for a short period, then created a greenhouse effect and produced acid rain. University of Arizona scientists found that the impact occurred on the Yucatán Peninsula of Mexico where it produced an approximately 180 kilometer-diameter impact crater. They named the crater Chicxulub, which is a Maya word that loosely means tail of the devil.
These studies, in addition to our spacecraft missions to other planetary bodies, indicate that impacts are the dominant geologic process in the solar system. Because of the correlation between some extinctions and large impacts, we are also beginning to suspect that impacts may have affected the biologic, as well as geologic, evolution of the Earth. Furthermore, because large impacts may have created conditions similar to the hazards of our modern industrial society, such as a greenhouse effect and acid rain, studies of these events should help us assess the environmental consequences of our age.
(3) Povenmire (1985) Meteoritics, v. 20, pp. 541-544.
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