[meteorite-list] shock help?

From: mafer_at_domafer.com <mafer_at_meteoritecentral.com>
Date: Thu Apr 22 10:06:18 2004
Message-ID: <001301c295c4$20d12020$6401a8c0_at_vs.shawcable.net>

Hi Tom and list

Well, your sure digging correctly. Actually, terrestrial rock can show
minute "fractures" and crack from a variety of things such as, but not
limited to: earthquake, volcanic eruption and weathering. Now whats
interesting about weathering is that it take a wide variety of forms. Quite
often, "shocked" quartz grains are found in volcanicly derived "green" sand
layers and are a result of the eruption itself. The simplest forms we see
are the wind scoured rocks like sandstone or water worn pebbles (which is
actually a tumbling grinding effect and not so much to do with water doing
the grinding away, but the tumbling on rocks against themselves). Less
realized are the freeze-thaw effects which can most easily be seen on
exposed plutonic rock such as seen in Yosemite in Ca. or Stone Mountain in
Ga. which causes the rock to break apart and almost looks like orange peels
till it slides down into a talus slope of rubble. Freeze-thaw also causes
most of the falling rock around the continent for which the dot puts up the
"watch for falling rock" signs. And basicly, what happens here is that
moisture gets into minute crevices of the rock and when it freezes, it
expands, then it thaws in the spring and is repeted till the chunk of rock
gives way to gravity and heads toward the road. Now, about these minute
crevices. They can be from a few things, one of which is acid rain. Not
something only invented by 20th century man, it has been around as long as
rain has and happens every time there is any volcanic activity which places
acids into the atmosphere. This acid rain accounts for a lot of long term
weathering and will attack the carbonates first which is often the cementing
agent in sedimentary rock. It also attacks the silica cements in many types
of rock, just not seds, but metamorphic and volcanic rock too. Water too can
disolve as its a polar solvent. So, to finally answer your initial question,
fractures are found in terrestrial rock all the time and in order to
determine if its from shock or weathering, one needs to look at the frains
of the rock itself, not the cracks, for a shock fracture of any kind, be it
terrestrial or extra-terrestrial, will not just go around the grains
(crystals ), but will go through them if thats the easiest path to relieving
the stress of shock.

Mark



----- Original Message -----
From: Tom / james Knudson <peregrineflier_at_hotmail.com>
To: <meteorite-list_at_meteoritecentral.com>
Sent: Tuesday, November 26, 2002 6:55 PM
Subject: [meteorite-list] shock help?


> Hello List, pardon me for picking your brains! I have a meteorite hopefull
> that if you look at it with a loop at an angle in good light you can see
> that the stuff that makes up the mass of the rock has small cracks ( only
> visible with a loop) all through it. Every thing looks like it has been
> shattered. Now then, Is this shock? Do or can terrestrial rocks have these
> tiny cracks all through them?
>
>
> Thanks, Tom
> The proudest member of the I.M.C.A. #6168
>
>
>
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Received on Tue 26 Nov 2002 10:21:50 PM PST