[meteorite-list] Largest Crater in the Sahara Desert and LDG

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat Mar 4 03:53:39 2006
Message-ID: <007001c63f69$1d6bfc50$812de146_at_ATARIENGINE>

Hi, Everybody,

    Tektites again, oh, boy!

    The chief reason we don't call LDG tektites is that
workers in the field don't, a question of nomenclature,
but whether it should be changed is another matter.

    LDG was described as early as 1850 by Fresnel.
There is a history of about 100 publications up to
almost a decade ago characterizing them as fulgurites
or a precipitate from a silica-rich sea (silly notion often
applied to tektites; tektites have no OH ions, silly, silly).

    Let's put some numbers on this pig.

    First, LDG is very glassy, with a silicon dioxide content
above 98%, greatly exceeding the usual tektite high glassy
content, even that of moldavites (up to 85%). It's a glass
as pure as many human-made glasses. And some people
have suggested that this factor alone excludes it as a tektite.

    Water Content: LDG 0.055% to 0.166%
    Water Content: Moldavite 0.010% to 0.011%
    (Similar in high silica content, but less variable)
    Water Content: Muong Nong 0.009% to 0.030%
    (Newer, wetter tektite, but a lot drier than LDG)
    Water Content: Bediasite 0.011% to 0.030%
    (Same age roughly, but much, much drier than LDG)
    Water Content: Rio Cuarto Impact Glass 0.115% to 0.129%
    (LDG is wetter than an obviously wet impact glass.)
    Water Content: Nuclear Bomb glasses, as low as 0.007%
    (This rules out those nasty Aliens who built the pyramids
    and went on to star in the STARGATE movie...)
    So, LDG is not anywhere near dry enough to qualify
as a tektite when compared with other similarly aged
tektites (bediasites) or indeed any tektites at all.

    Fluorine/Boron Ratio: Crustal rocks have 5 or 10 times
more fluorine than boron. Tektites should have a ratio of 1.0,
indicating that they were heated to temperatures high enough
to drive off most of the fluorine and leave the two halogens
at identical levels (however low the absolute amount), and indeed
tektites have values that "float" around 1.0 (like 0.8 to 1.2).
The tested LDG F/B ratio is 1.0. Yes, these bad puppies got
really, really hot. They contain baddeleyite derived from zircon
(temperatures well above 1800 C). On this measure, they
could be perfect tektites, but why so wet, pritheee,
why so wet?

    LDG shows a positive excess of Cesium in its series of
rare earth elements, just like the Nubia Sandstone, which is
why that has always been suspected as the source rock. Some
folk have pointed to the very nice match in the platinum group
elements in LDG to that of chondrites as an indicator of the
impactor. The dark inclusions in LDG show higher concentrations
of Fe, Ni, Cr, Ir, Co than the source rock, just like a meteorite.
Or it could just be an odd rock component on the surface. The
variations are quite small and based on hugely vague generalizations
of what "meteorite" composition is. And...

    The whole question of finding traces of impactor in tektites
in one of those still nastily disputed points. Usually, the statement
of "impactor traces" means "elemental or isotopic excesses when
compared to what we think is the source rock," but that means
you are making a circular assumption, which is just substituting one
controversy for another.

    Why doesn't this help? It does nothing to settle the
matter of whether the tektite is derived from the source
rock (terrestrial) or a unique impactor. How do you distinguish
between extra-terrestrial enrichment of the source rock and
an extra-terrestrial impactor of unknown (unique and with high
silica content) origin? If there are craters, there are ALWAYS
impactors, but that isn't the issue.

    As far as actual unaltered remnants of an impactor, the
supposed "iron spherules" (melted but semi-intact) that are
reputed to be found in a very few tektites, there is only one,
yes, just one such result in the many, many tens of thousands
of tektites destructively examined. That is, in itself, a very
suspicious result. For all practical purposes, there are no
direct traces of impactor in tektites. Even the most ardent
impactists do not claim it. Even the compositional argument
is really feeble; most impact theorists dismiss the compositional
biases. (Sorry, Norm.)

    Nor would there be. If you accept the notion that impact
has caused the desiccation of tektites by dissociation of
H2-0 and the loss of (nearly) all H ions, you have temperatures
in excess of 34,000 C., probably up to (Jay Melosh says)
50,000 C. Tell me again how that iron spherule survived?
There would be no surviving "meteoritic grains" or "iron
spherules" at those temperatures! And indeed, there are
no "meteoritic grains" in either tektites or LDG, while
impact glasses are rife with them.

    Lastly, the strewn field. The experts say that no tektite
is found "in situ." I find many different estimates of the
size of the strewn field, ranging from 50x80 km to 150x50 km
in the literature. As for "well-defined," well... What is presently
exposed can be "defined," but that is not a definition of "the"
strewn field. Does anybody believe that the "strewn field" of
bediasites is 5 mi. by 140 mi.? No, that's just the exposure
edge of the strata from which the bediasites are eroding.
Likewise, the Georgia tektites are only found where that strata
is exposed. "The" strewn field must have covered the entire
Eastern United States (and more)! Don't forget the lone
Martha's Vineyard tektite (the Vineyardite?), also part of
the North American strewn field and chemically identical
to the other NA tektites.

    First, the percentage of easily recovered LDG (surface finds)
that has been worked by man is very high. Paleolithic man
picked LDG up, used it, transported it, and then discarded it,
for about 20,000 years. Subsurface chunks weighing 60 pounds
have been found (LDGuong Nongs?).

    The Sahara has only been a desert lately. That's a geologist's
"lately," of course, since the end of the latest glaciation, when it
started to dry up. It's a brand spanking new desert. Before that,
for a long time, it was wet and wooded, rich with game, laced
with hundreds of rivers, swamps and deltas. Even in its last
days, it was a tallgrass prairie until about 3000 years ago,
with many rivers. In the East Sahara, there are Roman grain
farms occupied in 400 AD that are now buried that lie 160
miles into the present Sand Sea.

    This description of the Sahara is very hard to picture if you
go to the present location where the LDG roams, with its
1 mm of rain per year, 58 degree C. temperatures and those
500-foot-high dune fields, but the traces of the former paradise
are everywhere in the Sahara and have been duly charted
by dedicated European geologists who had access to the
Sahara while all of Northern Africa was colonial.

    So, the strewn field has been "churned" by man for
20,000 years, and then blasted and buried by strata-
eating sand storms for 10,000 years, making its footprint
less than perfectly clear. This is true of all tektite strewn
fields. There were elaborate theories about the sectorial
distribution of various types of australites... until other types
were found where they "shouldn't" have been. Ivorites
were only found in a narrow strip on the coast... until they
were found far out in the Atlantic, making the strewn
field 1500 miles wide!. One lone ivorite was recovered in
the SOUTH PACIFIC, off Australia, for godssake!
And, oh, yeah, some australites in Central America...
(Mystery of the antipodal tektites.)

    The BU press release does not give the location
of the Kebira Crater, but the strewn field is centered
on 25.5 degrees North and 25.5 degrees East. Guess
it's time to search the Internet.

    Going to this Terraserver view of the Kebira Crater:
<http://www.terraserver.com/imagery/image_gx.asp?cpx=25.53024396&cpy=24.67255158&res=375&provider_id=350&t=pan&dat=&OL=Off>
You will see the crater at the center of the left edge of the
image, and moving your cursor toward the center of the
upper edge of the image will locate the center of the strewn
field, about 100 km NNE of the crater. (Cursor lat-long
is shown in the dialog box.) Those 500-foot-high parallel
dunes are clearly visible even in the satellite view! Also
visible if you zoom in on the crater are the skeletal rivers,
braided deltas, and wetlands of the Good Old Days.
At impact time (28.5 mya), this area may well have been
very shallowly submerged (like the Fayum). The crater.
is centered on 24.98 degrees North and 24.68 degrees
East. Anyway, this puts the strewn field outside the crater,
a Pro-Tektite Point..

    So, is LDG a tektite? Me, I say, too wet. Would I
buy Semi-Tektite? A Near-Tektite? An Odd Tektite?
 No, I think it is only because the source rock was a
pure sandstone composed almost entirely of silica
that we ended up with a nice transparent and contaminant-
free impact glass that reminds us so much of a yellow
moldavite (found at similar distances from their crater),
that we are misled into regarding LDG as a potential
tektite.

    To me, the interesting point is that the LDG should
be a tektite. IF the naive form of the impact theory is
true, all the conditions have been met. The crater's
big enough (ten times bigger than the ivorites' crater).
They're found at a suitable distance outside the crater,
as far away as moldavites. All the factors required to
produce an unquestioned tektite are in place. They
have a superb match to the source rock, and that
source rock is exactly what impact theorists have
always said it should be, sedimentary. The LDG's
SHOULD BE perfect casebook tektites.

    So, why are they so damn wet? Why are there NO
indications of any aerodynamic modeling features or
traces thereof? (The similar moldavites have fantastic
aerodynamic features.) And why would a "classic"
impact-theory impact fail to produce these two
hallmarks? (Something seriously wrong with the
theory, that's my guess...) This is actually a more
general point: there are lots and lots of impact
craters but very few tektite producing ones; why?


Sterling K. Webb
-------------------------------------------------
----- Original Message -----
From: <MexicoDoug_at_aol.com>
To: <nlehrman_at_nvbell.net>; <bernd.pauli@paulinet.de>;
<Meteorite-list_at_meteoritecentral.com>
Sent: Friday, March 03, 2006 7:15 PM
Subject: Re: [meteorite-list] Largest Crater in the Sahara Desert and LDG


> Hola Norm, so it seems we actually agree on most of the points, including
> the
> most important one: the subjectivity of the definition. You are just
> wanting
> to be more liberal...and me more stoodgy...I wasn't dodging the layered
> tektite issue when I said not to bring it up (which you unfortunately
> did:)).
> Clearly layered tektites are closer to impact glasses in the continuum and
> I was
> just trying to cleanly conceptualize. The definition of 'tektite' is a
> human
> classification which like most, depends on a clear understanding of a
> concept,
> not a recipe. The Muong Nong glasses (vs. tektites) as many experts also
> call
> them deserve a category by themselves so if you want to point to experts
> calling them tektites as support for calling the LDG's also tektites, all
> I can
> say is we are pushing the concept even further. You do mention the
> meteoritic
> content of Indochinites (=Australasian tektites). Yes a small component
> of
> iron has been detected, but this is very rare, and no where near the
> content in
> LDG which can approach a 0.5%.
>
> You didn't mention that the partial pressure of the air in the bubbles of
> the
> Indochinites corresponds to the upper atmosphere, and that in LDG I am
> assuming it corresponds to the surface. This shouldn't be a surprise as
> the water
> should not be linearly independent - thus they ought to track similarly.
>
> Good point on the desert weathering, but is there a real strewn field
> defined
> for LDG's, as we find with other conceptually true-to-form tektites
> (pun:))?
> If any evidence could be found, your argument would be more solid, as a of
> evidence isn't any proof of anything. Try checking nobel gas ratios and I
> bet
> the tektite concept will be even further away...
>
> Where I must really agree with you and put all grammatical gymnastics and
> opinions aside, is where you make the best point of the whole discussion,
> imho.
> That maybe our definition of tektites whatever that concept may be is
> based on
> faulty ideas. With liberty taken, that maybe it will change as we learn
> more. Yes, I buy that, I believe that is a distinct possibility. Things
> were so
> much simpler when we all agreed they were blasted from the Moon and the
> aerodynamic shapes and low water content actually meant something more to
> the
> experts of that time. Gor the time being, I be conservative on the
> definitions for
> the distinctions mentioned. Show me one aerodynamically shaped LDG
> besides
> one sculpted by a Neanderthal, and I'll recommend you for a Harvey award
> which
> would be quite fitting:), and definitely a nobel prize in the meteoritical
> community...for the moment we think there is a crater now, well, we
> already called
> them impact glasses, and now we have all these years of human transport
> mucking it up for these highly prized special glasses.
>
> Perhaps little Norm and little Doug in the 100th century will follow in
> our
> footsteps. Norm will say, Doug, look at all the chondrites in the USA,
> and
> there are none in the Sahara. Looks like the major strewn field is into
> North
> America and then a minor one into Europe. And Doug will say, I don't
> know, they
> weren't witnessed falls.... Jokes aside, the concepts are pretty
> clear ---
> high energy, less meteoritic content, water content too low for earth's
> surface
> under all available explanations, aerodynamic shapes, minimal nobel gas
> concentration typical of higher atmosphere, upper atmosphere
> pressures(=low)...where does LDG have a positive? A crater in the same
> environment///I'll sit this
> one out on the fence...but note it duly with curiosity and opportunity...
>
> Saludos, Doug
>
>
> Norm L. wrote:
>
> <<Doug,
>
> Good points all, but if you want to raise the
> water/purity issue, you can't dodge the Muong Nong
> issue. (The best answer is that they shouldn't be
> called tektites, BUT, they ARE so called by all
> authorities).
>
> With LDG, it can be reasonably argued that
> flight-related morphology has been erased by
> ventifaction. In the area where this stuff is found,
> it is literally reasonable that ALL of the material
> has seen the wind and its entrained sand. LDG is
> pretty fine, clean glass, albeit with a higher water
> content. (So, here again, people have dodged the issue
> by calling them Muong Nongs---)
>
> As for inclusion of impactor material in LDG, you've
> got to remember that iron spherules are found in
> Australasian tektites. Good chance that's impactor
> condensate.
>
> I truly have no argument with the water content
> criterion. That's probably the best definitional
> parameter we have. But it makes me a bit nervous to
> turn the whole matter over to such a narrow
> definition. Are we positive, given all that we don't
> know about tektites, that there can't be any wet ones?
> Should we now start calling Pyrex another variety of
> tektite? Clearly, we are including some
> process-related factors (even if just inferred) in our
> definition.
>
> It is very much like the planet issue. I keep
> thinking that there have been a lot of grade-school
> kids that got marked down on tests for answering the
> question: "How many planets are in our solar system?"
> wrong according to the erroneous wisdom of a given
> time. How many tektite-producing impacts have there
> been? I get weary of qualifying my answers with,
> "Well, depending on whether or not you count LDG----"
>
> Cheers,
> Norm
> http://tektitesource.com>>
>
>
> --- MexicoDoug_at_aol.com wrote:
>
> > Norm L. writes:
> >
> > << Where is the dividing line between impactite and
> > tektite? I'd like to hear what others may
> > understand,
> > but my impression is that it fundamentally hinges
> > on
> > distance the glassy material is ejected from the
> > crater. Material found only in and immediately
> > around
> > the source crater is impactite. Stuff blasted tens
> > to
> > hundreds of km or more crosses the definitional
> > boundary into "tektites".
> >
> > If this is the criterion, LDG was already home free
> > >>
> >
> > Hola Norm, yet again here's another one of those
> > awkward definitions that
> > when overyly analyzed starts falling apart. I think
> > the distance criterion is
> > not THE criterion, but rather a tektite differs from
> > an impact glass in that the
> > tektite has actually been exposed to general
> > conditions of enough kinetic and
> > thermal energy to create a greater melt uniformity
> > where the original
> > impactor has transmitted that energy "cleanly", and
> > in such a great quantity that the
> > energy is also enough to propel tektites into the
> > upper atmosphere and have
> > them re-enter ablating like meteorites.
> >
> > These are a bunch of hand-waving concepts, but as we
> > know, it seems the one
> > factor that really distinguishes "tektites" is the
> > low water content. LDG's
> > have at least 5 times the typical water content of
> > the cleaner tektites, and
> > they contain inclusions including those of the
> > impactor, and aerodynamic shapes
> > are not really known I believe.
> >
> > In fact the water content of LDG's at the low end of
> > 5 times the amount of
> > the cleaner tektites actually goes practically as
> > high as obsidian. They don't
> > usually look very aerodynamic and they have
> > meteorites inside them. They
> > deserve some distinction, they are dirty glass. Now
> > all of this about water
> > content might be just an academic distinction,
> > except for one exception. One of
> > the greatest mysteries of tektites is derived from
> > the mystery of exactly what
> > physical laws were twisted to get that low water
> > content and this more than
> > anything else is the criterion as much as the
> > mystery. Plus they are generally
> > clean (OK, they have smalled fused cuartz. etc., but
> > there there tends to be a
> > bimodal distribution between clean tektites and
> > impact glasses as far as
> > inclusions = so far you have clean ones and dirty
> > ones) Please don't bring up
> > layered tektites I don't want the definition system
> > to fail even more...
> >
> > But practically speaking, you would have to be right
> > that there is a
> > continuum, just like in the definition of a planet,
> > etc., the world tends towards
> > complexity just when you get it all figured
> > out...and soon we will come to know of
> > the impektite that bridges tektites, water and all,
> > with LDGs and other
> > impact glasses. Better yet how about just saying
> > they are all impact glasses -
> > which they are no matter who starts talking about
> > flying - and that tektites just
> > had a higher energy/diffusion/flux melt event which
> > is witnessed in the
> > record by water content...If cats could only talk
> > they could tell us how long we
> > have erred on visible light as they see into the
> > near UV, don't they? What's
> > the use of going at it with a cat over the
> > definition of "visible light"?:)
> >
> > My 2 centavos...Doug
> > >>
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Received on Sat 04 Mar 2006 03:53:30 AM PST