[meteorite-list] RePost: Researcher says...Tektite events (shortline)
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
(Did it again! Something about replying to Steve turns the wordwrap off
without it showing up until after it's posted. Here's a shortline
Hi, Steve, Darryl, and List,
While the terrestrial impact theory is the current orthodoxy on
tektite origin, there is no single "impact" theory. There is absolutely
no agreement as to what the production mechanism is. Everyone supporting
"impact" puts forward differing (and contradictory) mechanisms.
The majority of impactists say surface jetting is the source of
tektites, even though jetting in theoretical models of impact occurs
from the body of the impactor rather than from the target material. This
would be fine if they proposed silica impactors, but they emphatically
do not. A large number of geochemists point to surface deposits as the
only possible source for a tektite composition. The question is, why
doesn't this happen with all (or most) impacts?
Wasson's "atmospheric cratering event" proposes that there are no
craters created in tektite producing events, which is curious when you
consider the "coincidence" of nearby impact craters of tektite matching
dates, like Botsumtwi, the Ries Kessel, and the Chesapeake Bay
structure. I like most aspects of this explanation except for those
inconvenient craters. (Perhaps these are twin or multiple impacts,
asteroids with satellites, one of which atmospherically craters?)
Another problem is the sheer volume of tektite material. The North
American strewn field is estimated at 0.5 to 13 billion tons of
tektites; that would require some excavation.
Jay Melosh, the chief theorist of impact events in general, says it
is impossible to produce tektites by jetting. He proposes that they form
from deep rock below the crater on rebound decompression. But the
associated craters show no evidence of any deeper excavation than
non-tektite craters, and deep rock sources are compositionally unlikely.
And, again, why doesn't this happen with every crater?
Guy Heinen proposes another kind of jetting of unknown mechanism
that occurs only in glancing, low incidence impact, but nothing about
these three craters supports a low (5 to 10 degrees) angle of incidence
-- they're not ovals, and they don't have one rim wall elevated over the
rest of the crater, and so forth.
All these proposals are hand-crafted fudge mechanisms, created not
to reflect any known characteristic of tektite producing impacts but to
produce a model tailored to avoid any contact of target material with
the material of the impactor. This is necessary because tektite material
is pretty much free of any "fingerprints" of an impactor. Let's face it;
it's hard to impact something without touching it! It's a really obvious
problem for the impact theories.
Surface jetting theories have yet another problem. A little simple
geometry shows that a surficial jet would have to escape by the time the
impactor has penetrated about its own radius into the crust (that's the
point when it vaporizes). To do so, a test particle of a forming jet
would have to travel a distance of about one-quarter of the
circumference of the impactor in that time. This would give it an exit
velocity of more than three times the velocity of the impactor! Since
big impactors have velocities near Earth's escape velocity when they
hit, the jet would have to greatly exceed escape velocity. How would
that produce tektites?
Digging into the literature of tektite-from-impact theory, I keep
looking for three little words, the three little words that if they
could be explained away would quiet my skepticism about impact theories:
Rayleigh Taylor Instability.
What the hell is that? Here's an everyday example. Picture a flag in
a very slow but steady breeze; it stands straight out parallel to the
flow of air. Increase the wind speed very slightly and the flag begins
to wave back and forth; those are Rayleigh Taylor waves. Increase the
wind a little bit more and the flag waves faster and faster. In a 25 mph
wind, the waves become very rapid and chaotic; the flag is fluttering so
fast it's a blur and the fabric begins to snap and pop. At 35 to 40 mph,
the fabric starts to shred itself because the propagation velocity of
the Rayleigh Taylor waves has exceeded the speed of sound by the time
they reach the trailing edge.
Note that driving force (wind) is very moderate, but the Rayleigh
Taylor waves increase in intensity in a violently explosive way. No
increase in velocity, pressure, density, or temperature can suppress
Rayleigh Taylor instability, which is why it is the chief difficulty in
designing a good working boosted fusion device, i.e., the hydrogen bomb.
Just trot up the road to Los Alamos and ask'em.
Take my word for it, there is no way around Rayleigh Taylor
instability in an impact mechanism.
Rayleigh Taylor instability guarantees a thorough mixing of impactor
and target material if they get close enough to interact with one
another. Going the route of the Wasson and Melosh variants only makes
the problem worse. If the target material is vaporized, so is the
impactor material, and segregating a gas phase is a lot harder (really,
more impossible, if you can say that) than segregating a liquid phase.
And Rayleigh Taylor instability applies just the same (even in a plasma
phase), in fact, it gets worse the more energetic the event. In other
words, theories of impact all have this fatal flaw.
There's no problem getting tektites blown out of the atmosphere by
an impact; the problem is forming them in the first place. Actually, a
silica impactor answers most of the difficult questions, but nobody
seems to believe in one, probably due to the complete absence of any
smaller examples of this composition (no silica meteorites that we know
There are plenty of other questions.
Why should only a few impact events produce tektites, out of all the
impact events of the last 40 million years? Bigger impactors? (No
evidence.) Faster impactors? (No evidence.) Cometary impactors (No
evidence.) One unique surface composition? (Conflicting evidence.)
Why are there no detectable characteristics of tektite associated
craters that distinguish them from non-tektite producing craters?
Why does it take a huge crater like Chesapeake to produce the North
American tektites, when the rather puny Botsumtwi crater blasts tektites
all over the Altantic? (Note: an Ivory Coast composition tektite was
recovered off the NE Australian coast; this is literally halfway around
the planet, so maybe I should have said "all over the world.")
Where is the crater for the Australasian tektites?
Why are the "big four" tektite producing events associated with
reversals in the polarity of the Earth's magnetic field? (We don't even
know why the field reverses, as far as that goes.) Another
The "impact solution" just doesn't come together for me. If it were
a coherent set of ideas, if there were a plausible mechanism, if the
theories had implications that were testable, if the theories didn't
exclude each other, they'd be a lot more convincing.
I carry no brief for lunar origin nor any other of the 30-odd other
theories. I just don't know, which leaves me free to hypothesize.
There's nothing wrong with admitting that we just haven't figured it
out yet, you know.
Sterling K. Webb
> Though the idea of tectites being of lunar origin was held by
Nininger, > and others, the notion has with recent evidence fallen into
disfavor. > Dr. John Wasson has done, and is as I understand it, doing
work on > tectites. His research, as he explained it to me, indicated a
> terrestrial impact origin for these objects. A "atmospheric cratering
> event" such as what occured at Tunguska, but of a much greater
magnitude, > would generate enough heat and the conditions to create
them. Such an > explosion would splash the atmosphere back so that the
vacuum of space > would reach the ground (even though no land crater was
created). The > enourmous heat pulse would have been such that the
sands and rocks on the > ground would be vaporized and then recondensed
in that vacuum. All water > found in earth material thus vaporized
would be released, and not become > part of the recondensing melt. The
layered tectites are those that are > closest to ground zero, and they
most likely would not have attained > ejection velocites sufficent
> enough to throw them up into space. Those towards and closer to the >
edges would be expelled over and above the onrushing air before it came
> back into the void created by the initial explosion. Such events, if
> this case scenario is true, will not produce any large and visible
crater > on the earth's surface. I am no expert on tectites, but this is
one of > the best theories to explain their origin that I have heard
Received on Mon 26 Mar 2001 04:34:28 AM PST