Robert Grumbine wrote:
In article . com,
don findlay wrote:

Klaus wrote:

[snip]

Natural convection is simply heat transfer be circulation in a fluid,
caused by gravity acting on fluids of differing density. There is a heat
source at the bottom and a heat sink at the top. Hot fluid is generally
less dense than cold fluid. The hot fluid rises, and the cold fluid
sinks. These motions usually spontaneously set up currents. This can
easily be oserved in a pot of water being heated on a stove, prior to
boiling. The material of the mantle is hot enough to flow slowly as a
fluid; it can undergo plastic deformation indefinitely.

('viscous', ..it's viscous) (not plastic)

... so continuing my sporadic comments about rheology, we're now
to plastics, elastics, and fluids.

To back up a second, rheology is the relationship between stress
and the strain and/or rate of strain. The divergence of stress produces
forces, which then can drive motion. Strain is the displacement of
material from a reference position. Rate of strain is the speed at
which this occurs.

In a simple fluid, the stress is proportional to the rate of strain.
Fluids have no 'memory' of where the particles started from, so only
detect rates of change.

In an elastic material, the particles do know where they started from,
so stress is proportional to the strain itself.

An ideal plastic (elastic-plastic) behaves like an elastic material
as long as the stresses are small enough (stay below the yield stress
of the material). Above that yield stress, they behave like viscous
fluids -- stress becomes proportional to the rate of strain.

In the case at hand, mantle material can be examined as a plastic
medium. For low stresses, it is an elastic solid. For higher stresses,
it is a viscous fluid. People concerned with portions of the earth
which are always on one side of the yield stress talk about that behavior,
hence a viscous lower mantle and elastic crust/lithosphere.

Since stress, strain, and rate of strain are all tensors, the possible
relations get quite involved. Gets even more involved when, as for
glacial ice, stress is related to nonlinear functions of the rate of
strain. Even worse when, as for sea ice, stress responds to both
strain and rate of strain.

What are you telling me this for all about stress and strain, when
it's convection we're discussing? Klaus there was busily refuting what
nasa was saying, and you feel a discourse on stress and strain is a
necessary diversion? Why? I'm saying nasa's position is ridiculous,
and earns me one strike for falsifying plate Tectonics.
http://groups.google.com.au/group/sc...f6ec783?hl=en&
Do you agree?