Geology Reference
In-Depth Information
(a)
(b)
CORE
MANTLE
Radius
Figure 7.3. Schematic temperature profiles through the Earth. Heat released during
the formation of the Earth and the segregation of the metallic core would heat both
the core and the mantle (mantle profile (a), dashed). Heat would be lost from the
cold surface, and the mantle would cool as a result (profile (b), solid). A thermal
boundary layer at the base of the mantle would be formed by heat conducting into
it from the core, and this would be a likely place for plumes to be generated.
would form a fluid-dynamically plausible source of our inferred column. Thus we
arrive at the essence of Morgan's plume hypothesis.
It is possible to argue that the column has a different composition, but there
are difficulties that require
ad hoc
assumptions to overcome. It is not obvious
how a compositional source might be continuously renewed, and if it cannot then
persistence of the hotspot requires a source of large volume. A problem with this
is that a buoyant volume of material will tend to rise towards the surface, and
the larger the volume the faster it rises, as we will see a little later. Thus a large
buoyant volume is not likely to generate a narrow, persistent compositional plume
that could generate hotspot tracks, but rather to rise as a volume and generate a
larger, relatively brief eruption, more like the flood basalt eruptions we will also
discuss a little later.
The preference for a thermal rather than a compositional column is reinforced
by the expectation that the core is very likely to be hotter than the mantle. The
gravitational aggregation of the Earth from infalling bodies releases enough heat
to vaporise much of the Earth, and even the separation of iron core material from
silicate mantle material could heat the Earth by around 2000
◦
C. Thus the Earth
is expected to have formed with a hot interior. It would then begin to cool from
the surface downwards, which means the mantle must cool significantly before
much heat will conduct from the core. The sequence is illustrated schematically in
Figure 7.3. Heat conducting into the base of the cooler mantle from the hot core
would then form a hot thermal boundary layer, and such a boundary layer would
