METEORITE OR METEORWRONG?
fusion crust
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.
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Another effect 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.
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One of the Camel
Donga stones from Australia.
(Photo courtesy of Jim Strope)
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These two meteorites (left and
above) are from Antarctica.
Both stones are fragments of larger meteorites.
The
shiny fusion crust is evident in both. |
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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.
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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.
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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.
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For meteorites found in deserts, wind - and sand
carried by the wind - can erode the fusion crust away after
thousands of years. Most meteorites have at least some fusion
crust, however.
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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.
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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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This is one of many stones of the Gao-Guenie meteorite
that fell in Burkina Fasa (western Africa) in 1960. The stone has a nearly
complete fusion crust. Such stones are always rounded, with no sharp edges
of corners. There is a hint of a regmaglypt on the far right. |
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Ash
Creek (L6 chondrite) meteorite was an observed fall in Texas
on February 15, 2009. The meteor was captured on video,
where it is seen to break apart. Hundreds of small stones have been found
around the town of West,
Here are 11 of them from the collection of Karl Aston. The stone in
the lower left is the most rounded by ablation. It has a complete
fusion
crust
and
some regmaglypts.
Among these stones, it is probably the earliest to have broken off the
main mass of the original meteoroid. Several
other stones have complete or nearly complete fusion crusts, regmaglypts,
and
edges
rounded by ablation. These
stones probably formed lower in the atmosphere. The large stone in the middle
has a smooth, dark fusion crust on the bottom side that we can't see, but on
top
there's
a light fusion crust and only a little ablation. This break must have happened
at even lower altitude, but still high enough that heating occurred.
Finally, some stones have breaks and
chips
that
happened low in the atmosphere or upon hitting the earth. The light-colored
interior
is
visible on these stones. (Thanks to Karl Aston for showing us the stones.)
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| Here are two views of a stone
of the Mifflin (L5 chondrite) meteorite that landed in southwestern
Wisconsin on April 15, 2010. This meteorite also shattered in
the atmosphere, so the stone is rather blocky shaped but it
still has a fusion crust and the edges are rounded. Where the
fusion crust is chipped away, the interior is light-colored.
This is common in freshly fallen chondritic meteorites. (Thanks
to Karl Aston for showing us the stones.) |
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