Re: What is this convection cell nonsense

On Aug 11, 12:24 pm, oriel36 <geraldkelle...@xxxxxxxxx> wrote:
On Aug 10, 6:29 pm, Darwin123 <drosen0...@xxxxxxxxx> wrote:

Be my guest,stick with your silly convection cells for crustal motion
but do not forget that it is useless for explaining why the shape of
the planet is not a perfect sphere.
The earth is oblate because it acts as a fluid. The convection
cells are not necessary to explain the oblateness, but they are
themselves a consequence of the fluid-like behavior of rock on large
scales of distance and time.
For convenience, I will use the reference frame of a rotating
earth. One can compensate for the noninertial aspect of the frame by
introducing a centrifugal force. I can explain it just as accurately
in an inertial frame, but I think this gets to the point faster.
There are three forces acting on a fluid element on the surface
of the earth. Two of those forces are gravity and elasticity. At the
poles, these are the only two forces. The balance bulls the surface
close to the earth. At the equation, due to the rotation of the earth
seen in an inertial frame, there is centrifugal force. The centrifugal
force pushes the earth outward, so it bulges at the equator. The three
forces balance out.
The elasticity turns out to be a fluid pressure, with few shear
forces. This is easily seen from merely a fluid statics analysis.
Because the surface of the earth approximately forms a geodesic with
respect to the sum of gravitational and centrifugal forces. The
effective gravitational acceleration of the earth is the same at the
poles as at the equator. Here, effective includes both gravitational
and centrifugal force. It wouldn't do that unless the rock really was
acting as a fluid on a long time scale.
The small deviations caused by temperature differences couldn't
change the geodesic part of the problem. Hence convection is
irrelevant to the average shape of the earth itself. Treating the
earth as an axially symmetric body is no problem. This is merely a
consequence of the fact that the fluid in the earth is nearly static.
Once you allow a small amount of fluid motion into your model,
you open up the possibility of nonaxial geometry. Differential
rotation the way you describe it is a fluid motion that is forced to
have axial symmetry. There is no law in fluid mechanics that requires
the motion of a fluid to have an axial symmetry. It is this axial
symmetry that YOU propose that is ad hoc, not the concept convective
currents. Once you have a temperature gradient in a fluid under the
action of gravity, convective currents must form.
Thus, it is really your model that seems to have the ad hoc axial
symmetry. Projecting the fallacy of axial symmetry on the plate
tectonics model is deceptive. No geologist or physicist ever said that
plate tectonic theory is inherently axial, let alone spherical. You
have been attacking a straw man model.

.

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