At 02:26 PM 2001/06/07 +0100, Craig Storey wrote:
>...
>... surely if the rotational forces are
>strong enough to influence plate tectonics globally then why would a small
>fragment such as India move northwards away from the equator unless other
>stronger forces were operating...?
India is at the northwest corner of the Australindian Plate, most of which
is actually moving northwards _towards_ the equator. In reference to either
"mantle hotspots" (lePichon 1968 JGR 73 3661) or the Antarctic Plate
(Knopoff & Leeds 1972 Nature 237, 93-95), the Australindian Plate is also
moving slowly westwards. If we assume uniform asthenospheric drag on all
plates and no net torque due to plate motion, we get a reference frame in
which the Australindian plate would have an easterly component of motion,
but the Antarctic Plate would be _rotating_ west-to-east despite
westerly-directed friction against not only the subjacent asthenoshere but
also most of the adjoining plates, and without any help from subducting
slabs! In my opinion this geotectonic no-no provides one of the strongest
arguments in support of "westerly drift of the lithosphere". Ricard et al
(op cit) have shown that westerly net drift about an axis in the southern
Indian Ocean can be achieved with no net torque provided that
asthenospheric drag on the continental parts of plates is about 7 times
higher than on the oceanic parts. But obviously this number could be
adjusted so as to accommodate a westerly net torque due to tidal drag.
It seems likely that forces generated from the Earth's heat flux
("ridge-push", "slab-pull", etc.) do most of the global-tectonic work, but
tidal flexing of the lithosphere could augment (or at least facilitate)
plate motion near the equator, and tidal drag could make it E-W asymmetric.
In the hot-spot reference frame the Nazca and Cocos plates are moving
eastwards in opposition to tidal drag. They are near the equator and
relatively small; perhaps tidal augmentation of plate genesis (Torbett op
cit) at the fastest-spreading part of the East Pacific Rise is helping to
push them eastwards (and thus helping to push the Pacific Plate westwards).
The rate of dissipation of kinetic energy by deceleration of the Earth's
rotation is ~3x10^12 watts - "probably larger than the drag power on the
plates" (Knopoff and Leeds 1972 Nature 237, 93-95). It is only one order of
magnitude smaller than the global heat flux and ~30 times larger than the
estimated rate of energy-release by earthquakes. Unless the Earth is
somehow increasing its moment of inertia by net transfer of mass away from
its axis of rotation, this deceleration can only be due to an external
braking torque (i.e. tidal drag, solar-wind drag etc). Ray et al (1996
Nature 381, 595-597) use data from satellite tracking, radar altimetry, and
atmospheric barometry to calculate that ~97% of tidal dissipation happens
in the hydrosphere. Their number for dissipation in the lithosphere is
83+-45 gigawatts, about the same as the ~10^11 watts released by
earthquakes. But the satellite-tracking data provide only a _global
average_ value for the phase lag of the lunar tide. The neat thing is that
only irreversible processes (e.g. pumping tidewater through narrow channels
in and out of shallow seas, pumping groundwater through porous sediments,
pumping subterranean magma, etc.) contribute to the phase lag of the tides,
and the braking torque would be locally larger-than-average wherever these
processes are occurring.
The braking torque must slowly accelerate the moon into a higher orbit. If
we assume that historically the deceleration of the Earth has varied only
in proportion to the inverse cube of distance to the moon, we find that
distance to the moon was zero in the Middle Proterozoic! Evidently tidal
deceleration has been strongly dependent on long-term changes in geography.
Dugald M Carmichael Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
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