Why this rock is probably not a meteorite:
1) They have rough exteriors. The smooth coating on the rock on the
upper left is not a fusion
crust but the chill skin or crust that forms where molten lava
is exposed to air. Note that it is flat (top surface of lava flow).
It does not surround the rock, like a fusion crust.
2) They're full of holes. Stony meteorites aren't full of holes.
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What are they?
These rocks are vesicular basalts, which
are very common in the western U.S. and Hawaii. See meteorwrongs 16
and 18. These rocks were found under a tree
in someone's yard. It is quite likely that they had been put there
by someone else for landscaping purposes.
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The finder of these rocks noticed that they
look a lot like lunar meteorite QUE
94281. They do superficially, but not in detail. QUE94 is
a fragment broken from a larger stone, so it has some rough
edges,
like the rocks above. The fusion crust on QUE94 coats only part
of the stone. However, the interior of QUE94 does not
contain gas bubbles (although that's hard to see in the photos)
because it was never molten like the basalts pictured here. Like
many lunar meteorites, QUE94 is a regolith
breccia - lunar soil that was shock compacted into a hard
rock by the impact of a meteoroid on the Moon. Before it was
shock compacted,
the fine-grained material had soaked up a lot of solar wind at
the surface of the Moon. The solar wind is mostly hydrogen and
helium,
both gases. Ions of these gases were implanted into the fine-grained
lunar "soil." The fusion crust of QUE94 and several
other lunar meteorites is vesicular because the solar-wind-implanted
gases
escape when the exterior melts as the meteoroid passes through
Earth's atmosphere at high speed. So, a lunar regolith breccia
is vesicular
on the outside but not vesicular on the inside
- just the opposite of these rocks. (Actually, in the chill skin
of the rock on the upper right, some just-broken gas bubbles were
frozen, leaving depressions and a few holes.) Finally, QUE 94281
is a breccia - a rock made up of bits and pieces of other rocks.
Most of the rock fragments making up QUE94 are, in fact, pieces
of basalt, like the rocks above. However, QUE94 also contains
fragments
of rocks from the lunar highlands. These clasts (anorthosites)
are light-colored; a few can be seen in the photos of our QUE
94281 web page.
One other thing to notice: In many terrestrial basalts (like meteorwrong
number 16), the vesicles are elongated. This
happens in two ways. (1) Cooling lavas are often very viscous, so
even in a ponded magma the gas bubbles get stretched out as they
rise to the surface. (2) If lava is flowing down the side of a volcano,
the gas bubbles also get stretched out from flow in a viscous magma.
The rocks above have stretched and irregularly shaped vesicles probably
from flow. Because of the composition of lunar basalts (high iron
and virtually no water), lunar basaltic magmas were of very low
viscosity - they flowed like water. That's why there are no big
conical volcanoes on the Moon. So, in lunar basalts, vesicles are
mostly round because of the combination of low gravity (if there
is no gravity, bubbles don't rise) and low-viscosity magmas. (See
"vesicles & amygdules"
for some photos of vesicular lunar basalts.) Round vesicles occur
in some terrestrial basalts when the viscosity of the magma is low
and the magma is not moving very fast. |
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