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Figure 6.2. Convection beginning in a 'normal' fluid, i.e. one whose viscosity is
constant. In this example there are two thermal boundary layers, a cool one at the
top and a hot one at the bottom (where heat is conducting into the fluid). As the
cool thermal boundary layer thickens, it becomes unstable and forms a 'drip' that
sinks into the interior of the fluid. The lower thermal boundary layer also becomes
unstable and forms a rising blob at the left. The initial instabilities then trigger a
chain reaction in which each thermal boundary layer forms a succession of drips
that sink or rise through the fluid, until a set of convection cells is formed.
In the mantle, on the other hand, the lithosphere is so strong that it can prevent
the formation of a drip. It is possible this could inhibit convection, or at least
prevent the lithospere from deforming at all. This is the situation with Mars and
the Moon, whose lithospheres have not deformed since early in their history. Their
lithospheres are undeformed and unbroken by major faults, and there is no plate
tectonics operating on either body. They are sometimes also called 'one-plate'
planets - you can think of the unbroken lithosphere as being a single plate that
embraces the entire planet.
Evidently on Earth there are stresses large enough to overcome the strength of
the lithosphere and to break it into pieces. It is evidently only at plate margins that
the lithosphere is weak enough to allow parts of it to sink. The result is what we
call a subduction zone (sketched in Figure 5.2). It is also only at plate margins that
upwelling mantle reaches the surface, at a mid-ocean ridge or 'spreading centre'.
Occasionally a plate may break and form new plate margins, either subducting or
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