Re: The Thickness of Richard Forrest

On 9 Jun 2006 09:25:52 -0700, Seanpit posted in article
<1149870352.734341.237070@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> ...

OvC wrote:

It also follows that it's impossible to reduce continental surfaces to
sea level by erosion, as Sean keeps screeching should have occurred,
as long as there remains a density difference between continental and
oceanic crust.
</pedantry>

I never said that the continental crust should have been eroded to sea
level. What I said was that the material that is currently above sea
level should have been eroded away by now.

There's a difference? Do tell.

Yes, there is a big difference . . .

The uplift/rebound may
certainly keep going, keeping land above sea level, but the overlying
sediments would be washed off many times over by now. I made this point
very clear several times already. Why this strawman?

Message-ID: <1149640012.517683.119160@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
"The fact is that sedimentary layers that are uplifted above
1,000 meters or so average more than 50 cm/kyr of erosion
inward from their surfaces. The continents themselves are
eroding very fast...So, the time needed to erode away all land
currently above sea level is just under 10 million years."

So, "all land" currently above sea level, which in the real world
happens to include continental marginal belts and interiors containing
crystallines, will be at sea level in 10 m.y.

That's not what I'm saying. I'm not saying that the land will be at
sea level in 10 my.

Oh. Well, it's what you said, or at least implied from your
statement. Perhaps it's just sedimentary rocks that you hate, and
crystallines are OK?

Continued uplift and isostatic rebound will no
doubt keep land above sea level. However, the land that is *currently*
above sea level will most certainly be eroded away completely within 10
to 20 million years from now - to be replaced by material from
underneath.

What about global warming? That would flood even more of the
continents, so that there'd be less of continental volume exposed to
erosion, so the continents could be eroded away in, say, 5-10 m.y. Or
if spreading rates increased; that would mean a larger volume
of the ocean basins occupied by oceanic ridges, and that volume would
displace water onto the continents, meaning even less of everything
continental exposed to erosion, so maybe we could cut erosion time of
everything to, say, 2 to 5 m.y.

I'm beginning to worry about my real estate.

Since the earth has
experienced many 10 m.y. periods, where's the global peneplain since
the beginning of the 10 m.y. period?

I'm also not arguing for a peneplain (the formation of a flat surface).
Differential uplifts and isostatic rebound will no doubt create
continued unevenness. Erosion itself does not work evenly over long
periods of time.

Really? You mean current values are no indication of past
performance?

Australia has had virtually no
mountain-building activity in the past 10 m.y. -- according to your
calculations, why do air-breathing animals still call it home?

I do not believe Australia is 10 my old - not even close. The problem
is with those who believe that such land surfaces have been exposed for
many tens of millions of years or even hundreds of millions of years.
The question is, why are these areas still covered by sedimentary
layers that should have been washed off many times over by now?

Australia isn't over 10 m.y. old? I thought geologists had dated some
Australian rocks, at least some zircons in metamorphosed sediments, to
over 4 b.y. Boy, are they going to be surprised!! I suppose you have
some evidence that you could use to convince thenm of their error?

Discussing the survival of Australian sediments in particular, C. R.
Twidale, as far back as a 1976 issue of the American Journal of
Science, noted the following:

"Even if it is accepted that estimates of the contemporary rate
of degradation of land surfaces are several orders too high (Dole and
Stabler, 1909; Judson and Ritter, 1964; see also Gilluly, 1955; Menard,
1961) to provide an accurate yardstick of erosion in the geological
past there has surely been ample time for the very ancient features
preserved in the present landscape to have been eradicated several
times over. Yet the silcreted land surface of central Australia has
survived perhaps 20 m.y. of weathering and erosion under varied
climatic conditions, as has the laterite surface of the northern areas
of the continent. The laterite surface of the Gulfs region of South
Australia is even more remarkable, for it has persisted through some
200 m.y. of epigene [surface] attack. The forms preserved on the
granite residuals of Eyre Peninsula have likewise withstood long
periods of exposure and yet remain recognizably the landforms that
developed under weathering attack many millions of years ago. . . The
survival of these paleoforms [as Kangaroo Island] is in some degree an
embarrassment to all of the commonly accepted models of landscape
development."

Of course, Twidale believes that these surfaces are truly ancient, yet,
where is the explanation as to how these sediments could have survived
such long ages of erosive pressure that, at *minimum* is orders of
magnitude too high for such features to survive?

That AJS article looks interesting. Have you looked to see if there's
anything published in the last 30 years to see if the models of
landscape development in 1976 might have, you know, evolved?

Similarly, why is Canada east of the Rockies still around?

Exactly! . . .

Why are
the Appalachians still Apping?

Very good question!

Or did you have something else in
mind?

Sedimentary layers of the geologic column haven't actually been
uplifted for tens much less hundreds of millions of years?

Oh. I thought you might have something rational and externally
consistent to offer.

Of course, if you abandoned your habits of inappropriately applying
current maximum erosion rates uniformly into the past, making
blanket statements about which you know nothing, mis-quoting or
outright lying about what a referenced author states, or just plain
making shit up, I think you'd accept 2.4 b.y. sediments and 4 b.y.
metasediments above sea level in spite of continuous erosion, and
complete denudation of all land to sea level in under 10 m.y. be
damned.

The erosion rates I'm referring to are not maximum rates at all. They
are minimum rates. In order to hang onto your notions that these
sediments have survived tens and hundreds of millions of years above
sea level, you'd have to propose that current minimum continental
erosion rates were orders of magnitude less in the past than they are
today. Do you really believe that is possible?

Well, there are actually other ways to confirm their survival for
10 m.y. (and why do you hate sedimentary rocks so much?) One way is
to recognize variable erosion rates through time, and use rates
appropriate for the target location and rock type. You won't do that.
Another is to recognize the veracity of radiometric ages in general,
and in cross-cutting relations where buried erosion surfaces are
exposed in particular. You won't do that either. Another is to
recognize that erosion is only one part of the landscape context, that
to even form those hateful sediments requires lithification or
crystallization of some precursor rocks on some landscape that itself
takes time to develop; weathering, erosion, and transport of the
weathered materials to some depositional site; burial, dewatering,
and lithification; uplift and deformation; and finally exposure.
Needless to say, you won't do that.

Oh, and yes, you do rely on maximum erosion rates, and sometimes bait-
and-switch rates from one area to another. Since you didn't address
the other items I mentioned in that paragraph (your mis-quoting, just
making shit up, etc.), I'll accept the accuracy of those observations.

--
OvC

.

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