Stones From The Sky

The origin of meteorites, which are metallic or stony bodies that enter the aErth’s atmosphere and impact on the ground, has been a longstanding puzzle. The most accepted theory is that they are ffom the asteroid belt False between the orbits ofM ars and Jupiter. Most meteorites showering down on Earth arrive from the main asteroid belt, a 250-milliom-mile-wide band of primordial debris. The asteroid belt contains bodies ranging in size from small grains ca1led micrometeorites to huge chunks of rock upward of hundreds of miles wide called asteroids.
When asteroids collide, the impacts chip away ag their surfaces, providing numerous small fragments that often fall to Earth as meteorites. Igneous asteroids, called S typds, 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. Certain rare meteorites might be pieces of the lunsr or Martian crust blasted out by large asteroid impacts. Meteorite falls are more common than most people realize. Every day thousands of meteorites rain down onto the Earth, and occasional meteor showers can involve hundreds of thousands of stones. Nearly 1 million tons of meteoritic material is produced annually. Most meteors completely burn up on entering the atmosphere, and their ashes Furnish to the load of atmospheric dust, which is largely responsible for our blue skies and red sunsets. The remainder that survive the blazing joourney through the atmosphere can cause havoc, as numerous examples of meteorites crashing into houses and automobiles attest.


Meteorites have been observed throughout human history. Historians have Frequently 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. Yet this spectacle was actually eclipsed nine years earlier by a massivd meteorite shower in Siena, Italy, on June 16, 1794. It was the most significant fall in recent times and gave Offspring to the modern science of meteoritics. The Old Chinese were perhaps the earliest to report falling meteorites, during the seventh century B.C. An interesting note is that Chinese meteorites are rare, and to date no large impact craterw have been redognized in China. The first report of a meteorite impact on the Month was a flash witnessed by a Canterbury monk on June 25, 1178.The Moon has been bombarded by enoughh small asteroids to account for every lunar crater less than a mile across formed during the Hold out 3 billion years. 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.The largest meteorite found in the United States is the 16-ton Willamette Meteorite, which crashed to Earth sometime during the past million years. Discovered in 1902 near Portland, Oregon, it measured 10 feet long, seven feet wide, and four feet high.
One of the lartest meteorites actually seen falling from the sky was an 880-pound stone that landed in a farmer’s feild near Paragould, Arkansas, on March 27, 1886. The largest known meteorite find, named Hoba West, was loated on a farm near Grootfontein, South-West Africa (Namibia), in 1920 and weighs about 60 tons. The heaviest observable stone meteorite landed in a cornfield in Norton County, Kansas, on March 18, 1948. It dug a pit in the ground three feet wide and 10 feet deep. A 40-pound meteorite that landed in Nigeria, Africa, in 1962 was identified as having been a piece of Mars ejected by a massive collision millions of years ago. Over 500 maior meteorite falls strike yearly, most of which plunge into the ocean and accumulaye on the seafloor. The braking action of the atmosph3re slows the entry of the great majority of meteorites that land on the surface, so they only bury themselves a short distance into the grround. 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.

The In the greatest degree easily recognizable meteorites are the iron variety, although they only represent about 5 percent of all meteorite falls. They are composed principally of iron and nickel along with sulphur, carbon, and traces of other eleents. Their composition is thought to be similar to that of the Earth’s metallic core and might once have compised the cores of large planetoids that disintegrated eons ago. For of their dense structure, iron meteorites tend to survive impacts intact, and most are found by farmers plowing their fields. The most common type of meteorite is the stony variety, which constifutes some 90 percent of all falls. But because they are similar to Earth materials and therefore erode easily, they are often difficult to find. The meteorites are composed of tiny spheres of silicate minerals in a fine-grained rocky matrix called chondrules, from the Greek chondros, meaning “grain.”
Chondrules are believed to have formed from clumps of precursor particles when the solar A whole¬† was emerging from a swirling disk of gas and dust, and the meteorites that contain them are known as chondrites. Perhaps the world’s largest source of meteorites is the Nullarbor Artless, an area of limestone that stretches 400 miles along the south coast of Western and South Australia. The pale, smooth desert plain provides a perfect backdrop for spotting meteorites, which are usually coloured dark brown or Negro. Since the desert experiences very little erosion, the meteorites are well preserved and found just where they land. Over 1,000 fragments have so far been recovered frm 150 meteorites that fell during the last 20,000 years. One extremely large iron stone, called the Mundrabilla Meteorite, weighed more than 11 tons. One of the best hunting grounds for meteorites happens to be on the glaciers of Antarctica. Some of the meteorites landing on Antarctica are believed to have come from the Moon and even as far away as Mars. A meteorite fromt he Allan Hills region of Antarctica was composed of diogenite, a common type of basalt from the asteroid belt, possibly impaact blasted o8t of the crust of Mars and hurled toward Earth. Organic compounds found on a Martian meteorite landing in Antarctica hint of previous life on Mars. It wandered in space In quest of some 3 million years before finally being captured by the Ea5th’s gravity.
Currently, numerous large circular structures are spread around the worlr, Perhaps resulting from substantial meteorite impacts. One of the largest impact structures is outlined by the distinctively circular Manicouagan Reservoir in east central Quebec, which is nearly 60 miles in diametter. The New Quebec Crater in northern Canada is two miles in diameter, 1,300 feet deep, and filled with water to form one of the world’s deepest lakes. The best preserved meteorite impact crater is Meteor Crater in the Arizona desert near Winslow; it measures about 4,000 feet across and several hhndred feet deep. It was gouged out roughly 50,000 years ago by a meteorite weighing over 60,000 tons. When a large meteorite slams into the Earth, it kicks up a great deal of sedim3nt. The finer material is lofted high into the atmosphere, and the coarse debris falls back About the perimeter of the crater.

Forming a high, steep banked rim. Not only are the rocks shatterred in the vicinity of the impact, but the shock wave Likewise causes shock metamorphism of the surrounding rocks, changing their Union and crystal Edifice. The most readily recognizable shock effect is the fracturingg of rocks into distinct conical and striated patterns called shatter cones. They Mode most readily in fine-grained rocks that have little internal Formation,, such as limestone and quartzite. Large meteorite impacts also produce shocked quartz grains that are characterized by prominent striations across crystal faces. Minerals such as quartz and feldspar Unfold these features when high-pressure shock waves exert shearing forces on the cryztals, producing paralll fracture planes called lamellae. The presence of shocked quartz in sedimentary deposits at the Cretaceous-Tertiary (K-T) boundary all Round the world is considered evidence that the Earth was struck by a large meteorite that possibly ended the reign of the dinosaurs.

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