Re: Subduction disproven!!!
- From: Jim Willemin <jimwillemin@xxxxxxxxxxx>
- Date: Wed, 11 Jan 2006 15:11:15 -0600
"Sterling L. DeRamus" <sderamus@xxxxxxxxxxx> wrote in
news:hh0xf.678$be5.659@xxxxxxxx:
> Well, at least according to one creationist and a couple of others on
> various other internet boards:
>
> http://www.creationscience.com/onlinebook/TechnicalNotes10.html
>
> "Because the inequality cannot be satisfied, a pushing force will not
> cause subduction. Remember, we made the very generous assumption that
> F=0. In other words, the blunt end of a plate 30-60 miles thick, and
> hundreds of miles wide, experiences no resistance as it is pushed
> through the Earth's rock crust.
>
> Some believe that a pulling force causes subduction. They say, for
> example: "at a given depth, the subducting plate is colder, and
> therefore denser, than the mantle. The plate sinks through the mantle,
> like a dense rock falling through mud. As it falls, it pulls the rest
> of the plate."
>
> This proposal overlooks the fact that the tensile strength of rock is
> much less than its compressive strength. If the pushing force,
> described above, cannot cause subduction, a pulling force certainly
> will not. Therefore, subduction will not occur. Therefore the
> Evilutionist Atheists are wrong and going to hell!!!!!"
>
> Well, OK, I added the last sentence. But it makes it more fun.
>
> So what's the group's take? How full of scat are these "equations."
> I'm not a geologist.
>
> SLD
>
>
I am a geologist, though out of practice.
Well, as equations go at first glance they seem fine - the mathematical
description of friction between rigid bodies is very well known. What is
in error here (egregious error) is the assumption that motion is
controlled by sliding friction between rigid bodies. When you get to the
depth where the descending slab achieves the geometry indicated in
Brown's sketch the 'sliding' boundaries are viscous shear zones.
Frictional mechanics do obtain in the (much colder) interior of the
downgoing slab - that is why there are deep earthquakes in subduction
zones); however, that is the interior of the slab, where everything is
going down. The actual plate boundary at that depth is pretty weak (due
in part to aqueous-rich fluids and melts given off by metamorphic
reactions as rocks that were happy at the surface re-equilibrate to
conditions at depth).
A further gross error is the assumption that the rigid lithospheric plate
has an upper and lower boundary that are both in contact with a passive
substrate. The lower boundary of the descending slab in Brown's sketch
doesn't exist in a mechanical sense; that is part of the downgoing limb
of the convection cell and the plate is really just the skin on top of
that viscous rock mass. The upper boundary is, again, a viscous shear
zone just chock full of fluids (mostly water vapor and carbon dioxide at
very high temperature and pressure - and 'chock full' is, of course, a
relative term...).
Basically, it is the sort of GIGO number crunching that one would expect
of an engineer who is used to making lots and lots of simplifying
assumptions and has no clue about the actual physical conditions and the
more appropriate mechanical model those conditions dictate. If one's
mechanical model says that what one observes is not possible, prudence
would suggest scrapping the mechanical model as a poor abstraction of the
phenomenon in question.
--
Jim
"Value nothing but truth, compassion, and love"
.
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