Meteorite, a meteor that reaches the earth's surface. Meteors are naturally occurring objects that enter the earth's atmosphere from space, traveling at high speed. Those larger than about four inches (10 cm) in diameter pass through the atmosphere without being burned up by the intense heat produced by air friction. (Those microscopic in size also survive the passage; they drift to the earth's surface, becoming dustlike micro-meteorites.)

Thousands of meteorites have been discovered, some of them by searching in areas where meteors were seen to fall. Most have been found in Antarctica, lying on ice fields where they have been concentrated over long periods of time by glaciers from surrounding areas.

There are three major types of meteorites.

Stony Meteorites are composed largely of silicon-containing minerals and other rock-forming minerals.

Iron Meteorites are primarily iron mixed with lesser amounts of nickel.

Stony-iron Meteorites are composed of roughly equal amounts of stony materials and metal.

Stony meteorites are the most common type, but they are difficult to recognize because they are often small fragments and look like ordinary stones. Most stony meteorites contain chondrules, very small round mineral grains made up largely of silicon compounds. Many meteorites are composed of materials that scientists believe existed at the time the solar system was formed. A few meteorites are composed of minerals similar to those found on the moon, and a few others, scientists believe, consist of materials somehow ejected from Mars.

The largest known iron meteorite is the Hoba, discovered in Namibia. It is estimated to weigh about 54 metric tons. Its largest measurements are 9 by 4 by 10 feet (2.7 by 1.2 by 3.0 m). The largest known stone meteorite fell in China in 1976; its biggest fragment weighs 2,138 pounds (1,170 kg).

Meteorite Craters

A large meteorite retains most of its initial speed and strikes the earth with enough force to explode, producing a crater. If the meteorite is very large, the explosion is so great that the meteorite is completely destroyed.

The largest crater at which meteorite fragments have been found is Meteor, or Barringer, Crater in northeastern Arizona. It is about 4,000 feet (1,220 m) in diameter and about 600 feet (180 m) deep. Several tons of meteorite fragments have been found in the plain surrounding the crater. The rim stands more than 100 feet (30 m) high. Meteor Crater is approximately 50,000 years old.

In 1908 a glowing object hurtling toward the earth exploded over the Tunguska region of central Siberia. The objectprobably a comet or small asteroidproduced no crater, but the blast felled trees over an area of more than 800 square miles (2,000 km2). At least one crater-producing meteorite has struck the earth in the 20th century. In 1947, more than 100 fragments of a meteorite fell over a relatively small area of eastern Siberia, forming craters of various sizes.

Over time, a crater becomes filled in and its rim erodes. A number of large, ancient meteorite craters are visible today only as large circular lakes or depressions. Some filled-in craters have been identified by the deformation of underground rock. One such crater, more than 100 miles (160 km) wide, was discovered along the northern coast of the Yucatn in 1991. The crater dates from the time dinosaurs disappeared from the earth, and many scientists believe the impact necessary to form such a crater could have produced a global catastrophe that led to the dinosaurs' extinction.