At 04:10 AM 2001/06/03 EDT, Brian Walton wrote:
>The Earth is rotating eastwards at about 1000 km per hour at the equator.
>Given the ductility of the asthenosphere it doesn't seem unreasonable to
>suggest that lithospheric plates could lag behind, due to inertia, at a rate
>of a few cm per year. As indicated by Doglioni (1993) plates might tend to
>move westwards at different rates depending on their configuration. The
>horizontal stresses involved are not related to lunar tidal drag ...


  Whether or not there may be other ways to induce westerly-directed
shearing stress, that due to tidal drag is strongest on the equatorial part
of the lithosphere and it acts from east to west (with seasonal variance).
Doglioni (1993 JGSL 150, 991-1002) reviews all the geological and
geophysical evidence for net "westward drift of the lithosphere". Making no
mention of tidal drag, he reaches a timid conclusion: "Although it has not
yet been entirely physically demonstrated, there seems to be some control
on this global tectonic pattern by the earth's rotation." But McKenzie
(1968 GJRAS 15, 457-500) showed that in order for the Earth's rotation to
affect the pattern of thermal convection in the mantle, the Taylor number
would have to be larger by ~14 orders of magnitude.
  A 1991 paper coauthored by Doglioni (Ricard et al 1991, JGR B96,
8407-8415) cites Jordan's rubbishing of tidal drag (1974 JGR 79,
2141-2142): "Jordan (1974) has clearly shown that tidal drag is far from
being significant and that this mechanism should be abandoned." (p.8410),
and makes a similar calculation:

"To produce a differential motion of 1.7 cm/yr over an asthenospheric
channel with a thickness of 100 km and a viscosity of 10^19 Pa s, an
equatorial stress of 5x10^4 Pa s must be applied. This low level stress
will produce on the whole lithosphere a net torque of 1x10^25 Pa s. Such a
torque will change the rotation period of the Earth whose moment of inertia
is 8x10^37 kg m^-2 by the totally unrealistic value of 2.5 min every day!
Therefore, to explain the observation, we must understand how a net
rotation can be induced without an associated net torque." (p.8410)

  But there _is_ an associated net torque due to tidal drag (albeit small)
and it acts in the same direction as the observed net rotation. The fallacy
in both Jordan's calculation and Ricard et al's calculation is the
assumption that all parts of the lithosphere move simultaneously at
constant velocity. Historically, the same kind of calculation was used to
"prove" that large-scale low-angle thrust faults are not mechanically
possible.
  A plate that spans 90 degrees of longitude near the equator is ~15 cm
shorter and ~10 cm higher at high tide than it is at low tide six hours
later. A neat paper by a physics student at University of Tennessee
(Torbett MV 1976 Tectonophysics 30, T17-T18) postulates a cyclical process:

"As the lunar and solar tidal waves or bulges progress around this planet,
the predominantly north-south ridge and trench systems are subjected to
periodic tensional and compressive stresses. ...during the relative
tensional phase [at the ridges] the plates separate and mantle material
upwells, cools, and solidifies as new plate material. The release and loss
by conduction of heat from solidifying material, being an irreversible
process, implies that some plate material that forms is not remelted during
the next compressive phase. Hence after one complete cycle there is a net
separation of the plates. ..."

The stress associated with this twice-daily tidal flexing, shortening and
stretching of the plates is ~8 orders of magnitude larger than the "drag"
stress that slow the earth's rotation (Bostrom 1978 Modern Geology
6,171-198). I don't know of any attempt to correlate midoceanic seismicity
and volcanism with the earth tide, but on Kilauea the swarms of small
earthquakes that indicate subterranean flow of magma are strongly
correlated with the earth tide (Rydelek et al 1988 JGR 93, 4401-4411).
Seems to me this cyclical process would enable dissipation of the earth's
rotational energy (i.e. tidal drag) to augment the E-W stress generated by
ridge-push, especially near the equator, and thus help to explain the
striking polar symmetry and E-W assymmetry of the global-tectonic pattern.



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]