Meteorite fusion crust

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Crust

In English, the word crust is probably most often used to describe the darkened exterior of a loaf of bread. The analogy with a meteorite fusion crust is good in that both crusts are caused by heating, the crusts have a distinct thickness, and both crusts are often shiny and usually darker than the interior bread or rock. The analogy breaks down in that the interior of leavened bread is full of vesicles (gas bubbles) and vesicles are rare in meteorites. Preannotated image credit: Vicky Wasik

Some people who contact me from non-English-speaking countries say “It has a fusion shell.” In English, snails and turtles have shells; meteorites and bread have crusts.

Stony meteorites

One of the Camel Donga stones from Australia. Fresh meteorite fusion crusts are smooth, shiny, glassy, and darker colored than the inside of the meteorite. Note the lighter colored interior on upper right where the rock has been chipped away. Photo credit: Jim Strope.

A fusion crust is the most characteristic feature that distinguishes a meteorite from a plain old Earth rock. 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.

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. It might be a meteorite, but I am not going to suggest to you that it is.

Meteoroids, i.e., small rocks orbiting the sun, enter Earth’s atmosphere at speeds of many miles per second. At those tremendous speeds, the air in the path of the rock is severely compressed. When air is compressed rapidly, its temperature increases, like air in a bicycle tire pump. The 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 loses 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 obvious gas bubbles. Some fusion crusts will show flow features; others may be covered with regmaglypts. 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.

These two meteorites are from Antarctica. Both stones are fragments of larger meteorites. The shiny fusion crust is evident in both. Photo credit: Randy Korotev
On this ordinary chondrite from the Sahara desert, some of the fusion crust has broken away. Note that the fusion crust has a thickness (usually < 1 mm) and is darker than the underlying material. Photo credit: Randy Korotev

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 break away if a meteorite has been on Earth a long time. Most terrestrial weathering crusts, varnishes, and rinds do not flake like this, so “flakiness” is a characteristic useful for identifying meteorite fusion crusts.

For meteorites found in deserts, wind – and sand carried by the wind – erode the fusion crust away after thousands of years. Most meteorites have at least some fusion crust, however. This photo was sent to me by someone in Morocco.
Fusion crusts frequently have contraction cracks after the glass cools and solidifies. Photo of an unnamed Northwest Africa meteorite. Photo credit: Randy Korotev
MacAlpine Hills 88108, a 15.4-lb ordinary chondrite (H5), from Antarctica. The stone is broken on the right side. Several regmaglypts are evident. Fusion crust has flaked off portions of the top. Notice that where the fusion crust is intact, the surface is smooth and shiny. Also, both on this stone and the large Saharan stones above, where the fusion crust is absent the surface texture is rough but still shiny. The shininess is a chemical weathering effect – desert varnish. The white material is chemical alteration (exposure to water vapor) that has occurred since the meteorite was collected in January of 1989. The meteorite is 7 inches wide. Photo credit: Randy Korotev

A fusion crust is the most characteristic feature that distinguishes a meteorite from a plain old Earth rock. 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.

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More photos people have to sent me of things that look like meteorites