METEORITE OR METEORWRONG?
fusion crust
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.
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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. |
| 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.
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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 (H5 chondrite) 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. Photo by Randy Korotev.
Not all fusion crusts are smooth.
This is a freshly fallen
stone from a minor meteorite shower that occurred in India
on May 22, 2012. Note the regmaglypts and the light-colorered
interior on the left where the fusion crust has chipped off.
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 The
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. (Photo by Randy Korotev. Thanks to Karl Aston
for showing us the stones.)
 This is
one of the many Park
Forest (L5 chondrite) stones that fell in Chicago on March
26, 2003. This one went through a roof and broke into four
pieces. (Thanks to Karl Aston for showing us the stones.)
Many Park Forest stones have a patchy fusion crust. The meteorite
is a breccia, with light gray clasts in a dark matrix. Thanks
to Carrie Seniw for the photos.
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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. Photo
by Randy Korotev. Thanks to Karl Aston for showing us the stones.
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| Two views of Northwest Africa 7496
(polymict eucrite). This meteorite is very fresh; the fusion
crust is still shiny. On the left, clasts are evident as protrusions.
(I think this is a bit unusual.) Click on image for enlargement.
Again, the interior is lighter colored than the fusion crust.
Click on image for enlargement. Photos by Randy Korotev. More
photos here. |
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This is a beautiful photo of a cracked
fusion crust and many small regmaglypts on an unnamed meteorite (probably
an ordinary chondrite) found in the Sahara Desert. Thanks to Habib for
the photo.
Millbillillie
is a rare monomict eucrite that fell in Australia in 1960. This is one
of many stones. The fusion on Millbillillie is redder than that seen
on ordinary chondrite. Photo by Randy Korotev. Thanks to Karl Aston
for showing us the stone.
Iron Meteorites and Pallasites
Experts disagree on whether metal-rich meteorites like irons
and pallasites have a fusion crust. In my opinion, they do not, but
they do have a "patina."
Sikhote-Alin (IIAB) is a good example
of an iron meteorite with a patina. It fell in 1947, so it is not weathered.
Photo by Randy Korotev.
The Bur-Abor
(IIIAB) iron meteorite from Kenya also has a nice patina.
The Sterley
(Texas) pallasite (PMG). Like many other iron and pallasite meteorites,
it was found by a farmer plowing a field, so it has a weathered exterior
and no obvious fusion crust. The cube is 1 cm in dimensions. Photo by
Laurence Garvie.
The Conception
Junction (Missouri) pallasite (PMG-an) was also found in a farm
field. The exterior is highly weathered, but the sawn area shows bright
metal with imbedded grains of olivine. Photo by Randy Korotev.
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