Geology Reference
In-Depth Information
5
Convection
Thermal convection is driven by boundary layers. Buoyancy. How to
calculate plate velocities - simple mechanical version. Interpretation
of an oceanic plate as a thermal boundary layer. Estimation of plate
thickness from thermal conduction. Plate velocities - thermo-viscous
version. What is a Rayleigh number? Other useful numbers.
Convection is not familiar to many geologists, so before we get to mantle con-
vection, which has some unusual features, we need to get to know convection in
general. If we start from the right place, then convection becomes understand-
able in straightforward terms. Instead of it being something vague down there that
makes things go around, we can know when to expect convection and what will
control it.
The key to convection is that it is driven by boundary layers. I will focus for the
moment on thermal convection (there is also compositional convection), so thermal
convection is driven by thermal boundary layers. If we can identify a thermal
boundary layer, and say something about how it behaves, we are well on the way
to understanding the convection that it drives.
In the mantle, the relevant thermal boundary layers form at horizontal bound-
aries. Later we will look at why that is. For the moment, we can use Figure 5.1 as the
simplest realisation of what I have just said. There is a hot thermal boundary layer
along the bottom boundary of the fluid layer. It is common in textbooks to depict
thermal convection with two thermal boundary layers, a hot one at the bottom and
a cold one at the top. However, this is only one of a range of possibilities, and it is
not exactly the situation in the mantle. Therefore I have kept the situation as simple
as possible in Figure 5.1 to emphasise that there may be one, or the other, or both
thermal boundary layers.
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