Notice that where the fusion crust is intact, the surface is smooth and shiny. Fusion crust has flaked off portions of the top. Photo credit: Randy Korotev MacAlpine Hills 88108, a 15.4-lb ordinary chondrite (H5), from Antarctica. Photo of an unnamed Northwest Africa meteorite. Fusion crusts frequently have contraction cracks after the glass cools and solidifies.
This photo was sent to me by someone in Morocco. Most meteorites have at least some fusion crust, however.
For meteorites found in deserts, wind – and sand carried by the wind – erode the fusion crust away after thousands of years. Most terrestrial weathering crusts, varnishes, and rinds do not flake like this, so “flakiness” is a characteristic useful for identifying meteorite fusion crusts.
As a consequence, the fusion crust sometimes break away if a meteorite has been on Earth a long time. Meteorite fusions crusts consist of glass, but the underlying material is crystalline and sometimes weaker than the crust. For meteorites found in temperate environments where it rains more often, however, fusion crusts may not be so shiny and black. Photo credit: Randy KorotevĮven though the meteorites in these photos have been on Earth for hundreds or thousands of years, the fusion crusts are still shiny. Note that the fusion crust is darker than the underlying material. Photo credit: Randy Korotev On this ordinary chondrite from the Sahara desert, some of the fusion crust has broken away. The shiny fusion crust is evident in both. Both stones are fragments of larger meteorites. These two meteorites are from Antarctica. The result is that a meteorite is rounded and aerodynamic in shape. During atmospheric entry any corners, edges, or protuberances are the first parts to ablate away – like putting an ice cube in water. Some fusion crusts will show flow features others may be covered with regmaglypts. Except for some lunar meteorites (less than 1 in 1000 of all meteorites), fusion crusts are not distinctly vesicular – there are no obvious gas bubbles. On stony meteorites, fusion crusts are seldom more than 1 or 2 mm thick. 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.
A meteoroid loses most of its mass as it passes through the atmosphere. The melted portion is so hot and fluid that it immediately ablates (sloughs off) and new material is melted underneath. The hot air causes the exterior of stony meteoroids to melt. When air is compressed rapidly, its temperature increases, like air in a bicycle tire pump. At those tremendous speeds, the air in the path of the rock is severely compressed. Meteoroids, i.e., small rocks orbiting the sun, enter Earth’s atmosphere at speeds of many miles per second. It might be a meteorite, but I am not going to suggest to you that it is. If you send me a photo of a rock that does not have a fusion crust, then I am not going to mislead you by saying that it is possibly a meteorite. If a rock does not have a fusion crust and does not contain iron-nickel metal, then there is no reason to suspect that it is a meteorite, regardless of what other meteorite-like features it may have. A fusion crust is the most characteristic feature that distinguishes a meteorite from a plain old Earth rock. Note the lighter colored interior on upper right where the rock has been chipped away. Fresh meteorite fusion crusts are smooth, shiny, glassy, and darker colored than the inside of the meteorite. More photos people have to sent me of things that look like meteorites Stony meteorites One of the Camel Donga stones from Australia.
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