Meteorites and Tektites
Meteors are particles of space debris that enter the Earth's atmosphere. Those which reach the Earth's surface are called meteorites. It is estimated that as many as 400,000,000 individual meteors enter the atmosphere every day. Most of these are minute dust particles but about 20,000,000 are large enough to be photographed Of these only about 30 reach the Earth's surface. This means that, on average, each square mile of the Earth's surface is struck by a meteorite about once in every 20,000 years.
Meteors enter the atmospheres at very high speeds - between 25,000 and 160,000 miles per hour - but, unless they weigh more than about 10 tons, all of this velocity is lost through atmospheric drag. The speed at which they strike the ground is entirely due to the Earth's gravity. Very large meteors (more than 10 tons) produce enormous impacts not only because of their sheer size but because they retain some of their cosmic velocity.
A meteorite strike large enough to destroy a city or produce a crater like Meteor Crater in Arizona or Tungusta in Siberia occurs about once every thousand years. A strike which would destroy a large urban area, such as New York or Tokyo, occurs every 5,000 years. A strike which would destroy an area the size of a state like Virginia or England occurs about once in 65,000 years. In impact which would destroy a state the size of California or France and have global consequences occurs about once in 250,000 years.
Most meteorites come from the asteroids; some are from comets; a few originate from Mars or the Moon. Compared to Earth rocks, meteorites are generally smooth and featureless but often with depressions or cavities that resemble thumb impression on wet clay. When they first reach the Earth they usually have a black, ash-like crust which slowly weathers to rusty brown and eventually disappears. Most recovered meteorites are between 2 inches and 2 feet in diameter.
About three-quarters of all meteorites are "stony". Stony meteorites resemble basalt in appearance but contain more metal. This can sometimes be seen if the meteorite is chipped. Most of the remaining meteorites are "iron meteorites". These are about 3.5 times as heavy as average Earth rocks. They resemble some iron ores but can be identified by the amount of nickel they contain.
Tektites are silicate-glass particles, usually dark in color, found in several areas called "strewn fields" throughout the world. They are usually several centimeters in size but some layered tektites weighing several kilos have been found. Sub-millimeter sized microtektites have been found in deep-sea sediments.
Tektites are probably formed when a meteorite melts sediments on the earths surface and throws the molten material high into the air. As the molten glass flies through the air, it forms distinctive shapes. Some, which look like flanged buttons, are shaped by the air forcing the molten glass back from the nose to the outer edges where it solidified. Dumbbell-shaped tektites are formed when a rod f molten glass spins end-over-end forcing material to the two ends. If these dumbbells break in two before they solidify, teardrop-shaped tektites are produced.
The largest strewn fields are the Australian, North American, Czech and Ivory Coast. The Australian strew field extends from South China, through the Malay Archipelago to Australia, ending in Tasmania. It contains millions of tektites about 800,000 years old. The North American strewn field contains tens of thousands about 35.4 million years old. The Czech strew field also contains tens of thousands of tektites; these are about 14.6 million years old. The Ivory Coast strewn field is about 1.3 million years old and contains a few hundred tektites.
Tektites found in Haiti and north-eastern Mexico are thought to be debris from the meteorite impact that the caused the Great Extinction of the dinosaurs and other species at the end of the Cretaceous Period, 64.5 million years ago.
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