Meteoroids* enter the atmosphere at speeds of many miles per second. At those tremendous speeds, the air in the path of the meteorite is severely compressed. When air is compressed rapidly, its temperature increases (like air in a bicycle tire pump). This hot air causes the exterior of stony meteoroids to melt. The melted portion is so hot and fluid that it immediately ablates (sloughs off) and new material is melted underneath. A meteoroid can lose most of its mass as it passes through the atmosphere. When it slows down to the point where no melting occurs, the last melt to form cools to make a thin, glassy coating called a fusion crust. On stony meteorites, fusion crusts are seldom more than 1 or 2 mm thick. Except for some lunar meteorites (less than 1 in 1000 of all meteorites), fusion crusts are not distinctly vesicular – there are no bubbles. Some fusion crusts will show flow features; others may be covered with regmaglypts.
|* Before it enters the atmosphere, it is a meteoroid – a small rock orbiting the sun. The visible light seen as it passes through the atmosphere is a meteor. After the rock lands, it is a meteorite.|
During atmospheric entry any corners, edges, or protuberances are the first parts to ablate away – like putting an ice cube in water. The result is that a meteorite is rounded and aerodynamic in shape.
Unlike many stones found on a beach or in a river, meteorites seldom have symmetrical or spheroidal (oblate, prolate) shapes.
Even though the meteorites in these photos have been on Earth for hundreds or thousands of years, the fusion crusts are still shiny. For meteorites found in temperate environments where it rains more often, however, fusion crusts may not be so shiny and black.
Meteorite fusions crusts consist of glass, but the underlying material is crystalline and sometimes weaker than the crust. As a consequence, the fusion crust sometimes flakes off if a meteorite has been on Earth a long time. Most terrestrial weathering crusts, varnishes, and rinds do not flake like this, so the “flakiness” characteristic is an important characteristic by which to recognize meteorites.