Hi, Norm, List,
My apologies. This statement of mine was
a complete and total error. I have no idea where
I got this idea, because it is obviously untrue, and
on some level I knew that.
I offer a fistful of excuses: it was written very late
at night; I was coming down with something nasty
in the head cold department; a cosmic ray flipped
a bit in some brain cell...
Tell the waiter I like my crow well done...
Sterling K. Webb
---------------------------------------------------
----- Original Message -----
From: "Norm Lehrman" <[EMAIL PROTECTED]>
To: "Sterling K. Webb" <[EMAIL PROTECTED]>; "Meteorite List"
<meteorite-list@meteoritecentral.com>
Sent: Monday, February 20, 2006 10:05 PM
Subject: Re: [meteorite-list] THEORIES OF THE ORIGIN OF TEKTITES, Part Two
Sterling,
Thanks for posting this series! One question though:
Item #5: "It would also appear that no one tried
breaking a specimen of each, as the fracture
morphology of each differs."
In what way? I've never tried breaking specimens, but
I've seen plently of broken ones and have never
noticed a difference. As amorphous glass, both
obsidian and tektites have a nice conchoidal fracture.
However, now that you bring it to my attention, I can
imagine a theoretical difference: since most obsidian
does have tiny crystallites, and tektites have
absolutely none, tektites should have a smoother
fracture surface, relatively free of stair-steps.
I'll have to go check as soon as I get this written.
As an interesting aside, various obsidians were
esteemed for varied uses in the stone age. Varieties
packed with incipient crystals flaked more crudely
than more pure glasses, but because the tiny crystals
obstructed the growth of fractures, tools made of such
impure material were tougher. Better coarse, heavy
duty implements could be made of this. More pure
glasses made for perfectly flaked extra sharp
arrowheads, but they were essentially one-use items as
they broke very easily (there being no crystallites to
interfere with fracture growth).
Is this the sort of difference in fracture morphology
to which you refer?
Thanks,
Norm
http://tektitesource.com
--- "Sterling K. Webb" <[EMAIL PROTECTED]>
wrote:
Hi,
Part Two of
THEORIES OF THE ORIGIN OF TEKTITES
Passing through the Colossally Silly Entrance Hall,
we next enter the
extensive and colorful Volcanic Tektite Exhibition.
5. The Terrestrial Volcanic Origin of Tektites:
Mayer, in 1788, published
the first scientific tektite theory; he called
moldavites "glassy lavas."
Charles Darwin, in 1844 (The Voyage of the H.M.S.
Beagle), first described
australite "buttons" and identified them as
obsidian. He wondered a great
deal about their unique shape, but became distracted
by some issue or other
in biology, so the world lost a great tektite
theorist.
The volcanic theory became as predominant in the
19th Century as the Impact
Theory is today. It was endorsed by Wickman, 1893;
van Dijk, 1879; W. D.
Campbell, 1906; La Conte, 1902; and Moore, 1916 (who
said tektites were
identical to "Pele's Tears"); Simpson , 1902,
proposed Australite tektites
came from Krakatoa. Dunn, 1908 and 1912, proposed a
complicated formation of
tektites inside of gas bubbles in fresh lava, a
suggestion further developed
and complicated by Buddhue in 1940, while Dunn then
later (1935) suggested
tektites were formed by rain and snow falling on
molten lava.
The volcanic theories all died when geochemical
analysis advances in the
20th Century, as tektites have a composition that is
quite different from
any terrestrial volcanic rock, and tektites are
easily distinguishable from
obsidian. It should be pointed out, in defense of
Darwin and all the early
geologists, that just from the standpoint of holding
a tektite and obsidian
in your hand and looking at them, they appear to be
materially identical.
Chemical and physical analysis is required to
distinguish them. It would
also appear that no one tried breaking a specimen of
each, as the fracture
morphology of each differs.
However, the last Terrestrial Volcanic Theory was
proposed in 1976! It is:
6. The "Cryptovolcanic" Origin of Tektites: McCall,
1976: To understand this
at all, we need to dig into the strange tribal
relationships of science.
British geologists ("we invented geology, you know")
were firmly wedded
(possibly even welded) to the volcanic origin of
craters, all craters, of
all kinds, on all worlds. An immense amount of
energy and thought had been
invested in lunar volcanic theory in particular, up
through the 1950's.
Those who learned their geology at British
institutions (Australians, New
Zedders, and so forth) were trained in this
tradition. The notion of that
some craters on the Earth or elsewhere might have
been formed by heavy
objects falling out of the sky was regarded as a
crackpot theory put forward
entirely by brash and uninformed colonials of the
American variety who were
well-known to be fond of whizz-bangs ("child-like,
you know"), and the
impact theory was resolutely resisted as errant
nonsense up until the moment
of the Moon landings, when it all unraveled in a
snap.
A "volcanic" explanation was handy; there had always
been craters from which
volcanic characteristics were absent. They were
called by these geologists
"cryptovolcanic," meaning that their volcanic
origins were hidden. This was
a theory built on the absence of evidence as a proof
of the theory, always a
dangerous logical method. Cryptovolcanic craters
were postulated to be the
result of direct venting of very deep, very hot,
high pressure gassy magma
to the surface of the planet in a manner analogous
to kimberlite pipes.
Advances of all kinds, but specifically in the
ability to visualize deep
strata make "cryptovulcanism" a bad historical joke.
McCall, an Australian geologist and a good one, too,
put forward a theory of
the cryptovolcanic origin of tektites in 1976. He
also disbelieved in the
impact origin of terrestrial craters and of
extra-terrestrial craters, lunar
craters, etc. This, in the post-Apollo era!
McCall was neither stupid nor uninformed and he
fought a sharp rear-guard
action, to his credit. He was honest enough to point
out that his own theory
was ruined by its inability to explain how you get
tektites out of the
Earth's atmosphere (to then fall back) without
ablating them up completely!
Leaving the Volcanic Tektite Exhibition Hall, we
enter the spacious
Semi-Extra-Terrestrial Pavilion.
Continued in Part Three...
Sterling K. Webb
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