Geology Reference
In-Depth Information
Ridge
Buoyant uplift
Ridge
Heat
Heat
Buoyant uplift
(b)
(a)
Buoyant
upwelling
Buoyant
upwelling
Th
erma
l bou
ndar y
laye
r
Heat generation
Heat
Therm
al bound
ary laye
r
Heat generation
Figure 8.6. Sketches of the relationship between heat flow from depth, buoyant
upwellings and topography. (a) The situation inferred in Chapters 6 and 7, in which
the only heat from depth is heat from the core plus a small amount generated by
radioactivity in the thin D
layer. (b) The situation proposed by Kellogg
et al.
[108] in which a thick, deep layer contains about half of the Earth's radioactive
heat sources. This layer should generate strong upwellings and major topography.
After Davies [118]. Copyright by the American Geophysical Union.
that mantle layering existed until relatively recently (perhaps only through the
Phanerozoic), but a breakdown of layering ought to have produced dramatic events
at the Earth's surface, as we will see in Chapter 9, and there is no evidence for such
an event.
The evidence from seismic tomography [90] persuaded most geochemists by
about 1997 that there is no barrier to flow at 660 km depth. However, it wasn't
long (1999) before a deeper interface was proposed, around 2000 km depth [108],
supported by geophysical evidence [117] that is circumstantial at best. The second
strong argument against layering also applies to this proposal. The argument is
that any such interface should generate strong mantle plumes, and there is no
evidence for such plumes in the surface topography of the Earth. The argument is
summarised in Figure 8.6.
The picture developed so far in this topic is of a strong plate mode of con-
vection and a secondary plume mode (Figure 8.6(a)). About 7-10 TW of heat is
carried from the deep mantle by plumes. Most of this heat comes from the core,
though perhaps 1 TW might be generated in the thin D
layer, as we will see in
Chapter 10. A heat budget of this model is depicted in Figure 8.7(a) [118]. By the
time the plumes reach the shallow mantle, they account for only about 3.5 TW, much
less than the
30 TW carried by plates. The topography generated by plumes, the
hotspot swells, is correspondingly secondary compared with the mid-ocean ridge
system, as we have seen.
∼
