Geology Reference
In-Depth Information
of a material to be stretched out. Rocks have little ductility in this sense, because
they have very low strength in extension. It is better to describe them as malleable
or, as I have been doing, as undergoing fluid-like deformation.) Thus the major fault
boundaries separating the plates may be literally faults at the surface, but grading
into ductile shear zones and broader deformation zones at increasing depth. Away
from plate boundaries, the strength of the upper brittle part evidently shields the
lower, more deformable parts from undergoing much deformation.
6.2 The role of the lithosphere in mantle convection
The lithosphere is strong because it is cold. Low temperatures (and relatively low
pressures) are required for brittle behaviour to occur. At the temperature of the
interior of the mantle (1300
◦
C or more), the mantle rocks can flow fast enough to
relieve the stresses driving convection, and they are therefore relatively weak.
The lithosphere is cold because heat is conducting to the Earth's surface, which
is cold. In the previous chapter we saw that within 100 Myr, a typical age of old
lithosphere, the mantle cools to a depth of about 100 km. We also saw that the depth
to which the mantle has cooled is proportional to the square root of the time it has
been cooling.
Because the lithosphere is cold, it is negatively buoyant, meaning it is denser
and heavier than the underlying hot mantle. It is the negative buoyancy of the
lithosphere that causes it to sink into the mantle, pulling the surface lithosphere
along behind it and pushing the mantle around as it sinks. In other words, it is the
negative buoyancy of the lithosphere that drives mantle convection.
Putting these things together, we can recognise the lithosphere as the cool thermal
boundary layer at the top of the mantle fluid that drives convection in the mantle.
We saw in the previous chapter that convection is driven by the buoyancy of thermal
boundary layers. In this view, then, the cool thermal boundary layer forms as hot
mantle rises to the surface (refer to Figure 5.2), where it loses heat by conduction
to the cold surface. As the material moves horizontally along at the surface, the
thermal boundary layer thickens, until at some point it sinks back into the mantle
under the action of its own excess weight, or negative buoyancy.
At what point will the lithosphere sink back into the mantle? This question
leads us to a key distinction between mantle convection and more familiar kinds of
convection. In 'normal' convection, the thermal boundary layer becomes unstable
and 'drips' away into the interior of the fluid, as illustrated in Figure 6.2. This
behaviour depends on the fluid in the thermal boundary layer being able to deform,
so it can form a drip. For most familiar fluids the viscosity does not change with
temperature, so the fluid in the cool thermal boundary layer is just as mobile as the
fluid in the interior.
