meteorite is a rock that falls to the Earth,
including particles like fine dust. The source material may be any solid,
including comets, asteroids,
Most meteors burn up in the atmosphere. A typical speed is 15 to 30 kilometers per second, and the air friction at these speeds is enormous. A typical millimeter to centimeter grain burns up in the upper atmosphere (50-100 km in altitude). Boulder-size (1 to 10 meters) may descend further, but the minimum size needed to penetrate the atmosphere is likely 30 to 100 meters, depending upon composition. For meteors larger than perhaps 10 centimeters, the visible glow is mostly due to atmospheric ram pressure, as a shock wave is generated by the meteoroid's passage.
Larger meteors often fragment, splitting into multiple pieces, sometimes repeating the process. This can be easily understood: there is a lot of air blocking the meteor's passage. The minimum amount of air encountered is for a vertical approach. Since air weighs 15 pounds per square inch, a 1 square inch meteor (which might weigh an ounce or two) encounters 15 pounds of atmosphere - perhaps 100-200 times its own weight. Note that a square meter of air weighs 10 metric tons, and even a boulder-size meteor is blocked by a buildup of air several times its mass, which reaches enormous pressure, typically greatly exceeding the structural strength of the meteor which effectively explodes. Imagine a fist-size ball of dried mud, and how well it will survive being struck by a sledge hammer at 15 kilometers per second.
Small pieces of a car-size meteor might reach the surface, if the meteor was a relatively strong rock, and some pieces of it were lucky enough to be protected by other pieces as it blasted through the air. But for the most part, the fragments we see came from much larger meteoroids which broke into smaller pieces before slowing down to terminal velocity and falling to the ground. It takes a really large chunk to leave a crater, as it would have to outmass the air it encounters by a significant ratio to keep from being slowed significantly.
A meteor's surface is heated to thousands of degrees, yet that is a surface effect, and the interior of the meteor will retain the cold of space, even if it has a fusion crust on the outside, formed as the minerals undergo extreme heat, melting, and oxidation. This is simply because of the time it takes for heat to diffuse into the meteorite (especially a rocky one), which is long compared to the few seconds of fiery passage experienced by the meteorite. The heat of ablation might only penetrate a stony meteor to a depth of 1 or 2 millimeters, and might penetrate an iron meteorite to a depth of 1 or 2 centimeters.
Approximately 10,000 tons of meteoric material strikes the Earth each year. Much of that is in the form of micrometeorites - dust - which is small enough that it does not burn up but simply drifts to the surface over a period of months. Try this experiment: take a strong magnet and clean it thoroughly such that no metal filings remain. Then drag it along some dusty window sills and examine it using a loupe or microscope. You are likely to find hundreds or thousands of tiny bits of iron, most of which are likely nickel-iron micrometeorites.
Until the 20th century, over 80% of all known meteorites were nickel-iron meteorites. Today, they comprise only 6%, with the rest being stony meteorites. There are two reasons for this. First, iron meteorites are strong and tend to hold together, and more importantly, they are easily recognized. After all, if you stumble across a rock in a field, the odds are good that it's an ordinary rock, so why would you take a second glance? But if it is magnetic, or extremely heavy, or covered by an obvious layer of thick rust, then it's likely a meteorite. Anyone can find an iron meteorite, nearly anywhere (with a bit of luck), using a metal detector. Several ancient civilizations have even used iron meteorites as a source of metal. But there are only a few places where a stony meteorite would stand out, and these were not recognized until fairly recently. These locations include the surface of glaciers or ice fields far from a mountain, on fields with no other rocks (such as parts of Kansas), desert sand dunes, or on dried up lakebeds or ancient ocean floors far from glacial deposits (places where thick layers of silt would have covered up any local rocks). Some ice fields in Antarctica have yielded thousands of stony meteorites.
Large meteorites, even iron ones, tend to fragment into hundreds or thousands of pieces as they blast through the atmosphere. Often the process is visible as a bright meteor will burst apart into fragments, each of which leaves its own glowing trail, much like fireworks. When the resulting pieces eventually fall to Earth, they result in a strewn field, which is generally a long, oval shaped area containing the fragments. The lightest meteorites fall out near the beginning of the strewn field, and the heaviest at the end.
Note that when a large enough meteorite strikes to create a large crater, the splashed-out materials are not considered meteorites, even if their appearance is similar. For example, tektites are thought to result when a meteorite strikes an area of silicate sands, melting and splashing the liquid silicates into a circularly-shaped strewn field. These materials cool so rapidly that they become glasses, and in some cases their purity is great enough that the resulting tektites are highly valued as jewelry. The best-known of these are called Moldavites, after Moldavia, Czechoslovakia where they are found.
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