[meteorite-list] Terrestrialized Meteorite Identification?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 18 May 2007 21:17:58 -0500
Message-ID: <09da01c799bb$ecf6a670$f54de146_at_ATARIENGINE>

Hi, Mike, List

    As a resident of Illinois, right on the 40 degree North
line, I've pondered the fate of the many meteorites that
must have fallen on this nice flat land since the ice went
away 10,000 years ago. Why aren't they just laying out
there on the prairie waiting for a nomad like me to pick
them up?

    The first thing to bear in mind is that all the Midwest
"trio" of relatively flat states, Illinois, Indiana, and Ohio,
were very wet and swampy for most of this time period.
Huge areas were covered with the slowly shrinking remnants
of shallow lakes. Most of the presently flattest areas are
former lake bottoms. They were very shallow lakes (only
a few feet) and often seasonal (dried up every summer),
but meteorites that fall into water are quickly eaten.

    The very earliest settlers avoided the flatter lands that
are today so prized for agriculture. They preferred land
with more relief and better drainage. Subsequent settlers
that were forced to take the flatter spots had to spent
years building drainage systems to dry out the fields
enough to prevent a big percentage of the crops from
rotting in the wet fields. Malaria was rife (it wasn't called
that then; it was the "ague"). The present appearance of
the Midwest is radically different from what it was like
two centuries ago.

    This is more the case as you go to the west and Illinois,
but Ohio was the same in many places. Just look at the
meanders (a flatland drainage feature) of the well-named
Mad River! I suppose we ought to include southern Iowa,
too.

    The second consideration is the high rainfall rate, about
40 inches (65 cm) per year, and the high frequency of the
freeze-thaw cycles in a winter where temperatures oscillate
around the freezing point. Meteorites are poorly consolidated
stones because they have formed in conditions of very low
gravity. As a result, meteorites are highly porous stones,
vastly more so than ordinary terrestrial rocks, and easily
absorb water.

    Ordinary chondrites have measured porosities of 1%
to 20%. The degree of porosity does not depend on its
petrological grade. The one measurement of a carbonaceous
chondrite was about 25%. The result is that they will suck
up water if they are continually wetted. The best mundane
example is an old style Pre-Civil War brick that was fired
at lower temperatures than a modern "industrial" brick.
Take such a brick, weigh it, put it in a bucket of water
for a day, take it out and weigh it again. If the brick is
50% heavier, its porosity is about 15-20%, or very similar
to a nore porous chondrite.

    Of course, the chemical properties of a brick are quite
unlike a meteorite, but their physical and mechanical properties
make an excellent analogy. A typical central Midwestern
winter may involve 10 to 50 freeze-thaw cycles. In each one,
absorbed water expands, widening cracks and flaws and
creating new ones, or if near the surface, spalls a surface
fragment off the body. With each thaw, still more water is
absorbed, and with each freeze, more damage is done than
in the last one.

    I have observed 1820's bricks laid out and exposed to
the weather (too long a story) over a period of decades.
In five years or less of exposure, the entire surface layers
spall away. I've seen a brick lose its entire original surface
(10% of its volume) in two winters. The glassy fusion
crust of a meteorite is more protective, of course, but
fusion crusts are always cracked (from cooling), and water
will penetrate with ease. I give the toughest fusion crust a
maximum of 10 years to be weathered off completely.

    Bigger flaws and cracks are weak points, and large
porous objects like bricks usually fracture into two or three
chunks, thus creating more surface area (which weathers
away faster then a whole stone). Such flaws and cracks
are fairly common in meteorites. The finer the fragments,
the faster they break down further, and ultimately dissolve
into a kind of mud. I've seen old brick evolve from intact
condition to a lens of red mud with a few lumps in it in less
than 10 years.

    While old soft brick is not a perfect analogy to a chondrite,
the chief characteristic that determines the manner and rate of
weathering is the porosity and material strength. Chondrites at
their weakest range from the old-brick-like up to ten times more
durable. I would estimate the chondrite lifetime "on the ground"
from a low of a decade or two up to a century or two for the
really tough ones under present conditions, and much less for
the last few thousands of years when the central Midwest was
swampy.

    This is a real contrast to desert meteorites where one hears
terrestrial ages of 10,000 or 20,000 years being estimated for
many weathered chondrites. These old "dry" meteorites show
alteration almost entirely by abrasion and oxidation; there are
a few old "wet" meteorites, but not from climates with freeze
and thaw cycles.

    I test these assumptions by searching through the NHM
Catalogue of Meteorites (up through the year 2000). Iowa
has 4 chondrite falls and 1 chondrite find. Kansas has 7
chondrite falls (it's 4X bigger) and 115 chondrite finds! Why?
Kansas a) is a lot drier, b) has less heavy vegetative cover,
and c) had Nininger. I think the "dry" part is the single most
important factor; both have a freeze-thaw climate. (The rest
of these states: Illinois, 2 chondrite finds and 3 chondrite falls;
Indiana, 5 chondrite finds (one called "doubtful" and one that's
super fresh, Lafayette) and 4 chondrite falls; Ohio, 2 chondrite
finds and 2 chondrite falls.)

    What's a flat, wet nation with a lot of agricultural land and
no great forests or mountains for meteorites to hide in, as well
as a freeze-thaw climate? Well, how about the Nederland, or
Holland as we used to call it? (Sorry about the flat wet remark,
Piper, but... it's true). The Nederland has 7 chondrite falls and
no chondrite finds. Proportioning the land area to Kansas, I can
only assume that if the Nederland were dusty dry, overgrown
with sunflowers, and had a Nininger, it would have about 25
chondrite finds!


Sterling K. Webb
----------------------------------------------------------------------------
----- Original Message -----
From: "Mike Groetz" <mpg444 at yahoo.com>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Friday, May 18, 2007 12:04 PM
Subject: [meteorite-list] Terrestrialized Meteorite Identification?


   I have been wondering about this for some time.
Here in central Ohio- weather conditions are far from
the best for preservation of meteorites.
   Yet I keep looking in the farm fields when I get
some time.
   This is an awkward question to ask- Is there any
way to identify a former meteorite that has been
terrestrialized? I understand the irons will go to
shale- but how about the stoneys?
   One side of me also questions that no matter what
rock you pick up- ultimately it's compounds have to be
from terrestrialized space material from billions of
years ago.
   Would any of you have any suggestions to
identification of "recent" terrestrialized fall
criteria that could be recognized?

Thank You
Mike Groetz



____________________________________________________________________________________Yahoo!
oneSearch: Finally, mobile search
that gives answers, not web links.
http://mobile.yahoo.com/mobileweb/onesearch?refer=1ONXIC
______________________________________________
Meteorite-list mailing list
Meteorite-list at meteoritecentral.com
http://six.pairlist.net/mailman/listinfo/meteorite-list
Received on Fri 18 May 2007 10:17:58 PM PDT