Tuesday, May 8, 2012

Stones From Space



"Stones From Space"," The most accepted theory is that they are from the asteroid belt lying between the orbits of Mars and Jupiter.
 The asteroid belt contains bodies ranging in size from small grains called micrometeorites to huge chunks of rock upward of hundreds of miles wide called asteroids.
 Igneous asteroids, called S types, are most frequently found in the inner region of the asteroid belt and are apparently the source of the most common class of meteorites, known as the ordinary chondrites.
 Meteorite falls are more common than most people realize.
 Nearly 1 million tons of meteoritic material is produced annually.
 The remainder that survive the blazing journey through the atmosphere can cause havoc, as numerous examples of meteorites crashing into houses and automobiles attest.
 Historians have often argued that a spectacular meteorite fall of 3,000 stones at l'Aigle in the French province of Normandy in 1803 sparked the early investigation of meteorites.
 It was the most significant fall in recent times and gave birth to the modern science of meteoritics.

 The first report of a meteorite impact on the Moon was a flash witnessed by a Canterbury monk on June 25, 1178.
 The oldest meteorite fall of which material is still preserved in a museum is a 120-pound stone that landed outside Ensisheim in Alsace, France, on November 16, 1492.
 Discovered in 1902 near Portland, Oregon, it measured 10 feet long, seven feet wide, and four feet high.
 The largest known meteorite find, named Hoba West, was located on a farm near Grootfontein, South-West Africa (Namibia), in 1920 and weighs about 60 tons.
 It dug a pit in the ground three feet wide and 10 feet deep.
 Over 500 major meteorite falls strike yearly, most of which plunge into the ocean and accumulate on the seafloor.
 Not all meteorites are hot when they land because the lower atmosphere tends to cool the rocks, which, in some cases, are covered by a thin layer of frost.
 They are composed principally of iron and nickel along with sulphur, carbon, and traces of other elements.
 Because of their dense structure, iron meteorites tend to survive impacts intact, and most are found by farmers plowing their fields.
 But because they are similar to Earth materials and therefore erode easily, they are often difficult to find.
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Chondrules are believed to have formed from clumps of precursor particles when the solar system was emerging from a swirling disk of gas and dust, and the meteorites that contain them are known as chondrites.
 The pale, smooth desert plain provides a perfect backdrop for spotting meteorites, which are usually coloured dark brown or black.
 Over 1,000 fragments have so far been recovered from 150 meteorites that fell during the last 20,000 years.
 One of the best hunting grounds for meteorites happens to be on the glaciers of Antarctica.
 A meteorite from the Allan Hills region of Antarctica was composed of diogenite, a common type of basalt from the asteroid belt, possibly impact blasted out of the crust of Mars and hurled toward Earth.
 It wandered in space for some 3 million years before finally being captured by the Earth's gravity.
 One of the largest impact structures is outlined by the distinctively circular Manicouagan Reservoir in east central Quebec, which is nearly 60 miles in diameter.
 The best preserved meteorite impact crater is Meteor Crater in the Arizona desert near Winslow; it measures about 4,000 feet across and several hundred feet deep.
 When a large meteorite slams into the Earth, it kicks up a great deal of sediment.
 Not only are the rocks shattered in the vicinity of the impact, but the shock wave also causes shock metamorphism of the surrounding rocks, changing their composition and crystal structure.
 They form most readily in fine-grained rocks that have little internal structure, such as limestone and quartzite.
 Minerals such as quartz and feldspar develop these features when high-pressure shock waves exert shearing forces on the crystals, producing parallel fracture planes called lamellae.