David Iain Greig wrote:

What are you telling me this for all about stress and strain, when
it's convection we're discussing?

Wow, you really don't understand basic rheology at *all*.

Have you *ever* studied transport phenomena (heat/mass/momentum transfer)?
From your statements, the answer is 'no'.

Have you ever studied geology? Could it be the answer is 'yes'? At
school maybe, ..like me.. Then perhaps You can ('coz I can't) come
up with an answer how the lithospheric plate with the floating crust on
it, pushes the mantle plate down to make it subduct and drive
convection - instead of the mantle plate just sliding under and not
bothering about driving anything - just there at any rate... Do you
think that could be the reason there are no subduction zones in the
Atlantic? ..the mantle is just sliding under, ..say the Americas, say,
...South America, ..and going along until it meets the Nazca Plate and
THEN bending down. Both of them. One of them bending sort of
normally, and the other at an acute angle. And both of them bending
creates this updraft that lifts the Andes? (There, do Plate
Tectonics a favour and Falsify that one.)