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www.meteorite-recon.com
Meteorite Fusion Crust
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An illustrated introduction. Text & photos by Svend Buhl
Fusion crust on meteorites has been a subject of interest since rocks falling from the sky have
been known to mankind. A description of a meteor stone’s exterior as “dusky black
in color, rough, uneven with some edges projecting” was given around the year 275
AD by the Nubian orator Arnobius the Elder.
Similar descriptions were provided among others by the Greek
historian Herodian (170 – 240 AD) of the black, probably
oriented, meteor stone of Elagabalus. By attributing the heavenly messengers’
sooty appearance to
their fiery descent onto earth, the ancients interpreted the black coating
of meteoritic stones correctly.
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Coarse fusion crust on an Allende CV3 chondrite |
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The first detailed and comparatative studies on fusion
crust, which were later joined by the works of Chladni (1819),
Tschermak (1885) and Brezina, we owe to Scherer and Schreibers (1809).
Since then, every major researcher in the field has addressed the subject at
least marginally.
Buchwald delivered a comprehensive study on fusion crust of
iron meteorites (1975). Ramdohr (1967) and, more recently, Genge and Grady (1999)
untertook detailed microscopic and micro probe supported research. Still,
misconceptions on fusion crust are fairly common in the popular media
and on the Internet, which is reason for us to attempt a brief illustrated
introduction into fusion crust and its common morphology.
Formation
Fusion crust, or fusion rind, is a thin melted surface layer of thermally transformed
components of a meteorite.
On stone meteorites it is mainly composed of olivine, glass, wuestite and other iron oxides of the
magnetite series and rarely exceeds a thickness of 1 mm. On
iron meteorites it is almost completely composed of magnetite
and even finer, regularly less than ¼ mm.
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Fine textured fall fresh fusion crust on a Chergach H5 chondrite
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Fusion crust forms when meteoroids enter the gas atmosphere
of our planet at speeds between 15 and 70 kilometers per second.
This is fast enough to cross the North American continent from east
to west in 4 to 5 minutes. One can imagine the enormous front wave of
compressed air that is produced by these cosmic missiles.
At an altitude of about 70 km, where the air
is less than 1 percent as dense as it is at sea level,
the atmosphere begins to slow the meteoroid down. At this stage,
the meteoroid has already compressed a wave of air in front of it.
The deeper the space rock penetrates
into the increasingly dense atmosphere, the more the
wave of air in its path is compressed and increases in temperature.
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Fusion crust less than 0.3 mm thin framing the cut section of the LL6 chondrite NWA 5882 (W0-1)
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Single regmaglypt carved from the surface of a Noktat Addagmar LL5 chondrite |
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Eventually the hot air causes the exterior of the meteoroid
to melt. At temperatures of around 2,700°F, asteroidal lithologies
put up little resistance. Once the melt becomes fluid, it is instantly
carried away in the gas and plasma jet stream and new material is
melted underneath. In this process of ablation, depending on entry
angle, entry velocity and mineral composition, the meteoroid loses
up to ninety percent of its original mass. Contrary to widespread
belief, the inner portion of the meteoroid is not affected by the
temperature of the ablation process since the heat is instantly
conducted with the fluid melt in the gas stream.
Ablation Process
At this stage of hot ablation, the meteoroid’s original shape
is being remodeled. Due to their irregular shape the majority of
meteoroids spin or tumble uncontrolled during their flight. In this
case, all surfaces are ablated more or less uniformly. Unless the
meteoroid is ablated to an aerodynamic shape and develops a stable flight,
only inhomogenities in its mineral composition cause local differences
in the degree of ablation. Components with a relatively lower melting point,
such as troilite, tend to melt much more quickly than Fe-poor olivine, for
example. Ca-rich lithologies melt even faster. Once the ablative process
reaches aggregates with a lower melting point, small pits and dimples
form which, in turn, cause micro turbulences that carve out these
dents even further. The resulting grooves are called regmaglypts.
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Large regmaglypts on the L5 chondrite Dhofar 1511
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Solidification
Once Earth’s atmosphere has slowed down our meteoroid
to a point where no melting occurs, it enters the dark
and cold stage of its flight. Now the moment has come where
fusion crust is formed. When the last melt cools, a thin, often glassy
and dull coating solidifies: the fusion crust. Thus, fusion
crust is a snapshot of a particular moment of a meteroid’s morphological
formation frozen in time.
Fusion Structures
In some cases, fusion crusts develop fine lines of solidified melt.
These are named flow lines and usually point to the side of the
meteorite representing the surface pointing away from the direction
of flight at the moment the crust cooled. Sometimes a meteorite’s
surface shows tear-shaped splash droplets which also point in this
direction. Contrary to the flow lines, these are formed by material
which has been stripped off the surface and is subsequently caught
again by the spinning meteorite.
On other spots, particularly on the
rear, on flanks and on portions protected from the airflow, melted
material accumulates and forms local rims, or lips, of thicker crust.
These are found mostly along the edges of surfaces pointing away
from the direction of flight. These rims may frame the complete
trailing surface of a meteorite and
may be called roll over rims. They formed as the semi-liquid melt
rolled over the edge and solidified in the lee of the gas stream.
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Splash marks and lip forming on the trailing flank of a Noktat Addagmar LL5 chondrite
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If liquid material is trapped and blocked to escape with the gas stream
it forms thick ridges or fills cavities. Under certain conditions, melted material
accumulates on the trailing side of meteoroids with stable flight attitudes and forms
several layers on top of each other. In extreme cases, these mega crusts
can reach a thickness of 1 cm and more. Fragments recovered from the main mass
of the Tamdakht meteorite fall (Buhl, et al. 2009) show evidence for this rare phenomenon.
Characteristic flanges as known from the australite tektites have
not yet been observed on meteorites. According to Ramdohr (1967) this
is due to the relatively
low viscosity of basic meteoritic melts
compared to the high silica melts of the australites.
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