Re: Keeping the Earth's plates oiled


"Jim Willemin" <jim***willemin@hot***mail.com> wrote in message
news:Xns998C8CC69473jimwillemingmailcom@xxxxxxxxxxxxxxxxx
anthonyjames.kkl@xxxxxxxxx wrote in news:1187111016.151378.214050
@x40g2000prg.googlegroups.com:

Earth's surface is a very active place; its plates are forever
jiggling around, rearranging themselves into new configurations.
Continents collide and mountains arise, oceans slide beneath
continents and volcanoes spew.

http://theanalystmagazine.com/pr/n302.html

As far as we know Earth's restless surface is unique to the planets in
our solar system. So what is it that keeps Earth's plates oiled and on
the move? Answers this ...


This is speculation, but I'd guess that a significant part of it is the
continual introduction of fluids (particularly water) into the upper
mantle via subduction zones. It is well known that very small amounts
of aqueous fluids in hot, stressed rocks act as a flux, both reducing
melting points and, very significantly, reducing the effective viscosity
of the solid rock as it deforms plastically. Although much of this
aqueous fluid is removed near the subduction zone (where it probably
abets the subduction-related magmatism), I don't think it would take a
whole lot injected into the upper mantle to maintain significant
weakness in the asthenosphere. There are ways to test this (I think),
involving looking at the stable oxygen isotopes of volcanic gasses, but
I am not familiar with the literature in that area. One observation
consistent with this hypothesis is that Venus, which clearly has
sufficient internal heat to produce large amounts of magma at (probably)
long intervals, and which also lacks liquid water on the surface, seems
to lack the characteristic morphologic signatures of terrestrial plate
tectonics (i.e. ridges and trenches). Anyhow, that is my hypothesis,
which is mine, although others may have thought of it too.

In a more general sense, the 'oil' of plate tectonics is heat flow - the
flow of the internal heat of the earth from the core of the planet to
the surface, where it radiates into space. Why this heat flow manifests
as plate tectonics on Earth and as something entirely different on Venus
is a good question.


I'll go along with what you said.

There is also one other major feature of the Earth that for some reason is
seldom mentioned in this context and that is its Moon. A disproportionately
massive traveling mate. During the latter part of the Earth's surface's
'molten' history and subsequent cooling the Moon was a lot closer and moving
rather quickly.

This had to have had enormous tidal effects on a plastic surface. It is easy
to visualize large 'plates' of cooled crust rising and falling with
interesting tectonic effects occuring at the edges. I doubt it has that much
effect on tectonic events today - but could surely be a factor into what
might have "jump-started" the process. <g>

-ralph


.

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