fusion crust

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Stony Meteorites

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 that 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 cover 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.


One of atmospheric entry is that any corners, edges, or protuberances are the first parts to ablate away. 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.


One of the Camel Donga stones from Australia.
(Photo courtesy of Jim Strope)   

These two meteorites (left and above) are from Antarctica.
Both stones are fragments of larger meteorites.
The shiny fusion crust is evident in both.

Some meteoroids break apart as they pass through the atmosphere or when they hit the Earth's surface. Stones from such meteoroids might have sharp edges and corners, but usually one side is still smooth and glassy. The interior of a meteoroid that breaks apart after passing through the atmosphere will not have a fusion crust.


Above and below: On these two meteorites, both ordinary chondrites from the Sahara desert, some of the fusion crust has flaked away. Note that the fusion crust is darker than the underlying material.


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 (see, e.g., Dimmit and Harrisonville).

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.

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.
unnamed ordinary chondrire from Mauritania
When an ordinary chondrite has been on Earth hundreds to thousands of years, the iron metal rusts. The conversion of iron metal to hematite leads to a volume expansion that cracks the rock apart. The fusion crust on this meteorite is cracked, but still shiny. Click on image for enlargement. Photo by 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 meteorites is 7 inches wide. Click on image for enlargement. Photo by Randy Korotev.

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Read about vesicular fusion crusts on lunar meteorites.


Prepared by:

Randy L. Korotev

Department of Earth and Planetary Sciences
Washington University in St. Louis
