[meteorite-list] Re: Researcher Says... Tektite Events
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:43 2004
Hi, Steve, Darryl, and List,
While the terrestrial impact theory is the current orthodoxy on tekti=
te origin, there is no single "impact" theory. There is absolutely no agr=
eement as to what the production mechanism is. Everyone supporting "impac=
t" puts forward differing (and contradictory) mechanisms.
The majority of impactists say surface jetting is the source of tekti=
tes, 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 l=
arge number of geochemists point to surface deposits as the only possible=
source for a tektite composition. The question is, why doesn't this happ=
en with all (or most) impacts?
Wasson's "atmospheric cratering event" proposes that there are no crat=
ers created in tektite producing events, which is curious when you consid=
er 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, o=
ne 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 i=
s impossible to produce tektites by jetting. He proposes that they form f=
rom deep rock below the crater on rebound decompression. But the associat=
ed craters show no evidence of any deeper excavation than non-tektite cra=
ters, 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 t=
hree 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 pro=
duce a model tailored to avoid any contact of target material with the ma=
terial of the impactor. This is necessary because tektite material is pre=
tty much free of any "fingerprints" of an impactor. Let's face it; it's h=
ard to impact something without touching it! It's a really obvious proble=
m for the impact theories.
Surface jetting theories have yet another problem. A little simple ge=
ometry shows that a surficial jet would have to escape by the time the im=
pactor has penetrated about its own radius into the crust (that's the poi=
nt when it vaporizes). To do so, a test particle of a forming jet would h=
ave 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 veloci=
ties near Earth's escape velocity when they hit, the jet would have to gr=
eatly exceed escape velocity. How would that produce tektites?
Digging into the literature of tektite-from-impact theory, I keep loo=
king for three little words, the three little words that if they could be=
explained away would quiet my skepticism about impact theories: Rayleigh=
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 flo=
w of air. Increase the wind speed very slightly and the flag begins to wa=
ve back and forth; those are Rayleigh Taylor waves. Increase the wind a l=
ittle bit more and the flag waves faster and faster. In a 25 mph wind, th=
e 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 fabr=
ic starts to shred itself because the propagation velocity of the Rayleig=
h Taylor waves has exceeded the speed of sound by the time they reach the=
Note that driving force (wind) is very moderate, but the Rayleigh Tay=
lor waves increase in intensity in a violently explosive way. No increase=
in velocity, pressure, density, or temperature can suppress Rayleigh Tay=
lor instability, which is why it is the chief difficulty in designing a g=
ood 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 instabili=
ty 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=
=2E Going the route of the Wasson and Melosh variants only makes the prob=
lem worse. If the target material is vaporized, so is the impactor materi=
al, and segregating a gas phase is a lot harder (really, more impossible,=
if you can say that) than segregating a liquid phase. And Rayleigh Taylo=
r 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 imp=
act 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 sili=
ca impactor answers most of the difficult questions, but nobody seems to =
believe in one, probably due to the complete absence of any smaller examp=
les of this composition (no silica meteorites that we know of).
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 evidenc=
e.) Faster impactors? (No evidence.) Cometary impactors (No evidence.) On=
e unique surface composition? (Conflicting evidence.)
Why are there no detectable characteristics of tektite associated cra=
ters that distinguish them from non-tektite producing craters?
Why does it take a huge crater like Chesapeake to produce the North A=
merican tektites, when the rather puny Botsumtwi crater blasts tektites a=
ll over the Altantic? (Note: an Ivory Coast composition tektite was recov=
ered off the NE Australian coast; this is literally halfway around the pl=
anet, 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 rever=
sals in the polarity of the Earth's magnetic field? (We don't even know w=
hy the field reverses, as far as that goes.) Another "coincidence"?
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 theor=
ies had implications that were testable, if the theories didn't exclude e=
ach other, they'd be a lot more convincing.
I carry no brief for lunar origin nor any other of the 30-odd other t=
heories. 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. D=
r. John Wasson has done, and is as I understand it, doing work on tectite=
s. His research, as he explained it to me, indicated a terrestrial impac=
t origin for these objects. A "atmospheric cratering event" such as what=
occured at Tunguska, but of a much greater magnitude, would generate eno=
ugh heat and the conditions to create them. Such an explosion would spla=
sh the atmosphere back so that the vacuum of space would reach the ground=
(even though no land crater was created). The enourmous heat pulse woul=
d have been such that the sands and rocks on the ground would be vaporize=
d and then recondensed in that vacuum. All water found in earth material=
thus vaporized would be released, and not become part of the recondensin=
g 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 ed=
ges would be expelled over and above the onrushing air before it came bac=
k into the void created by the initial explosion. Such events, if this c=
ase scenario is true, will not produce any large and visible crater on th=
e earth's surface. I am no expert on tectites, but this is one of the bes=
t theories to explain their origin that I have heard thus far.
Received on Mon 26 Mar 2001 04:12:09 AM PST