Fusion Crust
Any material that comes from space and finds its way to
the surface of our planet must show the
effects
of heating that are consistent with a passage through the Earth's atmosphere.
Friction during this process generates large amounts of heat and any exterior
portion of
the meteorite must show the results of this heating.
Although the Frass Meteorite represents a new kind of meteorite (newly made
volcanic material from a planetary body), it clearly shows the effects of being
heated and shaped by high velocity air flow. Vesicle edges have been
melted, "fattened," and elongated in the direction of air flow.
I will admit that the fusion crust on the Frass Meteorite
is different than most other meteorites. However, everything about the
exterior of the meteorite is consistent with it
passage through our atmosphere. In the picture to the left, you can
clearly see the blackened area in the middle and top right. Since the
meteorite is made of such thin walls and vesicles,
those parts that "stick out" the most are the ones most melted. If one
looks carefully, one can find many places where material that is the most
exposed has been completely melted, while
material just a millimeter or two deeper into the rock appears almost unmelted.
In many of these places, the sand is
partially or totally melted to the exterior of the meteorite, indicating that it
was hot enough for sand to stick to it. The picture on the right clearly shows massive melting at
this part of the meteorite.
It appears the rock
had a single orientation as it "flew" through our
atmosphere, so there is more melting on the bottom and
leading edges, while the top and trailing edges show much less damage.
This is very similar to the heating patterns shown on the space shuttle as it comes through the Earth's atmosphere
after leaving orbit to return to Earth. To the left we can see "black"
melted spots over the entire surface of the meteorite. Especially notice
the streaks that clearly show at the far left and near the top left. This
was caused when the meteorite came through our atmosphere, the thin walls melted
and the air currents shaped the rock. The parting plane of the meteorite
is also clearly visible. This feature probably occurred when the rock
separated from the main lava flow, much like pillow lava on Earth where
lava under the ocean cools rapidly and thus makes round shapes with a parting
plane.
If you examine the rock carefully, you can see where even
the sandy material is involved in the fusion crust. Whatever was on the
exterior of the rock was melted. Since the melting occurred
after the sand was deposited in the meteorite, it clearly shows that the
external melting was the last event that happened in the life of the rock.
The picture to the right clearly shows the fusion crust of the Frass Meteorite.
For those that like black melted fusion crust, the top right portion of the
picture demonstrates melting that is the most similar to traditional meteorites.
But the interesting thing about this picture is the sand that was in the
vesicles before the meteorite came through our atmosphere. The sand in the
upper right is melted into the vesicles, while the sand in the vesicles on the
left side show much less evidence of heat. The difference between these
two
spots
in relation to the exterior of the meteorite is probably less than 1/2 inch.
This meteorite was so able to handle the heat of entry into our atmosphere, that
these small dimensions make the difference between what is melted and what is
not. This is due, in part to the structure of the meteorite which has lots
of thin walls separating small air chambers. It is also due to the high
level of silicon in and on the meteorite, protecting inner vesicles from the
fierce heat of entry into our atmosphere.
For comparison, look at the picture to the right of one of
the core samples taken from the
interior of the meteorite. All the edges are very rough and sharp and
delicate. Most of the inner walls
of the meteorite can be broken with one's finger nail and every
time I sit the rock on a table, many little particles break off because they are
so delicate. I can tell if a piece of the meteorite was on the inside or
outside very quickly and easily by looking for the melting that represents the
fusion crust. The
last
picture on the left shows the material left over after my last microscope
session with the meteorite. This is how much material broke and/or fell
out of the meteorite during this three hour process. Fusion crust, core
samples, and sand samples are now available for study.