Craters, meteor forming

Another indication of vulcanism is the lack of meteoritic craters that one would expect to find on the surface of any terrestrial planet. Such craters are formed when meteors collide with a planet s surface. They are clearly visible on Mars and Mercury although less clearly visible on Earth, craters are still evident in some areas of the planet s surface. On Venus, however, such craters are largely invisible because (as scientists believe) they have been filled in and otherwise covered up by constant lava flows from the planet s many volcanoes. 

Defects in Coesite and Stishovite. The high-pressure phase of Si02, coesite and stishovite were synthesized and irradiated by y-rays to check the ESR spectra and to find ones formed by impacts at meteor craters. 

Dense forms of Si02, called coesite and stishovite, were first made under drastic conditions (250-1300°C at 35-120 katm), but they were subsequently identified in meteor craters where the impact conditions were presumably similar stishovite has the rutile structure. Both are chemically more inert than normal Si02 to which they revert on heating. 

A few lakes result from meteors or volcanic activily. Gases at high pressure under crests of volcanic lava can explode, forming basins that collect water. Volcarric basins rp to 1 mi (1.6 km) in diameter are called craters, and... 

These events liave sporadically happened even in our history. The Meteor Crater of Arizona is tliought to liave been formed between 20,000 and -30,000 years ago [58], Explosion of Tunguska meteor in 1908 and collision of Comet Shoemaker-Levy 9 against Jupiter in 1994 are the representative great events in the history. 

In the last ten years, at least a dozen polymorphs of pure Si02 have been reported [6], Stishovite, another form of silica obtained at high temperatures and pressures, has, rather than a tetrahedral-based geometry, a rutile (Ti02) structure in which each Si atom is bonded to six O atoms and each O atom bridges three Si atoms [6], Stishovite (found in Meteor Crater, Arizona) is more dense and chemically more inert than normal silica but reverts to amorphous silica upon heating. 

Coesite was discovered by Coes, in 1953 (59). It is made from amorphous silica in the same temperature range as for keatite, but at 10 times the pressure and with weakly acidic catalysts such as boric acid or ammonium chloride (59). It was found in nature in I960 at Meteor Crater, Arizona, apparently formed under the high temperature and pressure conditions of the impact. 

Earth-Moon Differences. Scientists believe that the moon s surface has a large number of craters formed by the impact of meteorites. In contrast, there are relatively few meteorite craters on the Earth, even though, based simply on its size, the Earth is likely to have been hit by as many or even more meteorites than the Moon. This notable difference is attributed to the Earth s atmosphere, which bums up incoming meteorites, particularly small ones (the Moon does not have an atmosphere). Larger meteorites can pass through the Earth s atmosphere, but their impact craters may have been filled in or washed away over millions of years. Only the more recent ones, such as Meteor Crater in northern Arizona, with a diameter of 4,000 feet and a depth of 600 feet, remain easily recognizable. 

A meteor strikes the Earth and forms a conical crater. The cone is shaped such that at a depth X to the bottom of the crater, the diameter of the crater is 4x. Thus the volume when filled to a depth x is... 

Lonsdaleite is a crystalline form of carbon, and is named after the English crystallographer K.Y. Lonsdale (1903-71). It is polymorphous with the other hexagonal forms of crystalline carbon, chaoite and graphite (qq.v.), and the cubic form, diamond. Lonsdaleite was first described by Frondel and Marvin in 1967 from the Canyon Diablo meteorite (Meteor Crater, Arizona,... 

One impact crater that remains intact is Arizona s Meteor Crater, found 20 km west of Winslow, Arizona. This crater, measuring 200 m deep and 1.2 km in diameter, was formed about 50,000 years ago by a meteorite 45 m in diameter. The impact of this relatively small object had an energy roughly equivalent to 20 millions tons of TNT—about the same as a hydrogen bomb. However, most impact craters are not so obvious. 

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