Geology Reference
In-Depth Information
90% of the total heat loss from the mantle, the balance being lost by conduction
into and through the continents.
The plate-scale flow is thus the dominant means by which heat is lost from the
mantle. The balance of mantle heat loss can be accounted for by conduction through
continental lithosphere. Thus we can conclude that, by the fundamental criterion of
the amount of heat removed from the mantle, the plate-scale flow is the dominant
mode of convection driven by the top thermal boundary layer of the mantle.
In other words it is not necessary to invoke any other mode of convection in order
to explain the heat flowing out of the mantle. So-called small-scale convection used
to be frequently discussed, and one form of this would, for example, be small-scale
drips falling from the lowest, softest part of the lithosphere. We will look at such
alternatives in a later chapter, but already we can conclude that any such additional
mode of convection must not be transporting much heat, because we have explained
the observed heat flow by invoking just the plate mode of convection. Plumes have
also sometimes been invoked as an alternative or additional mode, but plumes are
not driven by the top thermal boundary layer, and their role is not to remove heat
from the mantle but to bring heat into the mantle, as we will see in the next chapter.
Thus plumes may be an additional mode of mantle convection, but they are not an
alternative to the plate mode.
The argument that most of the Earth's heat loss can be accounted for by the plate
mode of convection is strengthened if we look at the geographic distribution of
heat flow, and its close associate, topography. We will now look at these in reverse
order.
6.4 The geography of topography
Chapter 2 summarised the observations that the depth of the sea floor increases in
proportion to the square root of its age (Figure 2.5) and the heat flux through the sea
floor varies inversely with the square root of its age (Figure 2.6). The description
we have developed of the oceanic lithosphere as a thermal boundary layer can be
used to derive the variation of these quantities that is implied by our theory. Both
can be obtained from Eq. (5.13b), which relates the thickness,
d
,ofthethermal
boundary
lay
er to the time,
t
, for which it has been cooling. It can be rewritten here
as
d
=
√
2
κt.
That version was based on a simple approximation, and it is useful
here to use a result from a more rigorous calculation [1]:
4
κt
d
=
π
.
(6.5)
As the oceanic lithosphere cools and thickens, it will undergo thermal contrac-
tion. A column of mantle with unit area in cross-section and extending to depth
