Page 414 - Global Tectonics

396 CHAPTER 12

N. America South Africa
(a) (b) (c)
S. America Hawaii

410 discontinuity
660 discontinuity

D discontinuity
D discontinuity Large low-velocity
D anisotropy
D anisotropy structure
Low-velocity scatterer Melt inclusions D discontinuity
Ultralow-velocity Ultralow-velocity zone Ultralow-velocity zone
patches, plume
genesis CMB CMB
Underside-CMB sedimentation
Core–mantle boundary
(CMB) Fluid outer core
CMB depression,
chemical reaction repository
Figure 12.11 Sections through the Earth’s interior beneath regions centered on (a) central America, (b) Hawaii, and (c)
South Africa, illustrating variations in the nature of the D″ layer (reproduced from Garnero, 2004, Science 304, 834–6,
with permission from the AAAS).

lateral and vertical variations within layer D″ may be
caused by variations in chemical composition, mineral- 12.9 THE NATURE
ogic phase changes and/or varying degrees of partial
melting, in addition to temperature differences. Com- OF CONVECTION IN
positional variations may be due to the mixing of
molten iron from the core with the perovskite of the THE MANTLE
mantle to form new high-pressure minerals (Section
2.8.6). It is thought that this is most likely to occur in
the ULVZs where it is facilitated by higher tempera-

tures, partial melting, and low viscosity. The result The evidence for convective flow in the mantle, from
would be a chemically distinct, high-density layer but seismic tomography and studies of the regional eleva-
with a low viscosity. A phase change in perovskite to a tion and subsidence of the Earth’s surface, strongly
denser and strongly anisotropic form is an interesting suggests that there are two main driving forces for this
possibility as some parts of the D″ layer exhibit a convection. The negative buoyancy of cold subducting
marked anisotropy. It is thought that this anisotropy lithosphere would appear to determine the main sites
may be induced by subducted slabs beneath down- of downwelling, and the positive buoyancy of hot, low
wellings and by shear fl ow beneath upwellings. viscosity material originating in the lowermost, D″,
It seems likely that the slabs of subducted lithosphere layer of the mantle determines the upwellings. These
that sink into the lower mantle affect the nature of the two complementary modes of convection in the mantle
D″ layer beneath them, most notably its temperature. have been termed the plate and plume modes, respec-
This in turn will modulate the flow of heat from the tively (Davies, 1999). Both have their origins in thermal

core which will influence convection in the core and the boundary layers: the plate mode in the lithosphere

nature of the Earth’s magnetic field, and determine immediately beneath the Earth’s surface, and the plume

where flow may occur within and above layer D″. mode in the D″ layer of the mantle, immediately above