From: Norm Lehrman <nlehrman_at_meteoritecentral.com>
Date: Mon Feb 20 23:06:00 2006
Message-ID: <20060221040558.67654.qmail_at_web81004.mail.mud.yahoo.com>


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?


--- "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>

> Hi,
> Part Two of
> 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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> Meteorite-list_at_meteoritecentral.com
Received on Mon 20 Feb 2006 11:05:58 PM PST

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