Meteorite Impact Craters
The smallest asteroids which can be plotted in their orbit around the sun measure about 200 yards in diameter. Many of these have orbits which cross the earth's path, and on rare occasions, one may intercept the earth and survive to penetrate through the atmosphere. These strike the ground at high velocity, creating an impact crater. Most objects smaller than 200 yards across are sheared apart, and the individual fragments either burn up on entry (creating a "shooting star") or are slowed to terminal velocity by aerodynamic drag.
Most of us were surprised when pictures returned by the space probes of the 1970s showed that two earth-like planets, Mercury and Mars, were heavily cratered. Meteorite craters on earth are comparatively rare -- only about 70 have been identified, and these are recognized mainly on the basis of residual information such as shock-induced changes in the surrounding rock, rather than as prominent land forms.
The reason that earth exhibits so few craters compared with other planets of comparable size is that the surface is constantly being reworked. This occurs through both the processes of weathering and mountain-building crustal deformation. While craters several billion years old appear relatively fresh on the surface of the airless and tectonically stable moon, the oldest identifiable craters on earth are only a fraction as old. As an example, the Manicouagan Lakes area in Quebec is a known impact feature dating to about 200 million years ago. The present remnants form a circular chain of lakes, easily identifiable on a map, about 40 miles across. Most of the earth's recognizable impact- structures, such as this one in Canada and nine-mile wide Elgygytgyn Lake in eastern Siberia, occur on continental shields which remain$n structurally stable over long periods.
Recently formed craters are easier to identify. Arizona's famous Meteor Crater (officially known as Barringer Crater) near Flagstaff was formed only about 25,000 years ago. Between 5 and 20 megatons of energy were released when an iron asteroid about 50 yards in diameter, traveling at more than 8 miles per second, struck the earth. The Arizona meteorite survived its trip through the atmosphere even though it was small because it was composed of iron rather than stone.
The Arizona crater is about 0.7 miles across. The impact record of the earth includes such small features only if they are very young, as is this particular crater. Generally, impact structures less than 2 miles in diameter are not recognizable as such.
One such feature that has only recently been identified is in Alaska. This impact crater is presently the site of two-mile wide Sithylemenkat Lake, between Bettles and Tanana. It is believed that the impact occurred about 100,000 years ago. To date, this is the only structure in Alaska which has been verified as being of meteoritic origin. The possibility exists that Savonoski Crater in Katmai National Monument is an impact crater, and there is a small feature on Amak Island which is also suspect. Alaska's vigorous crustal movements and extreme climatic conditions have obliterated any others.
