Geoscience Reference
In-Depth Information
Africa and adjacent oceans, the Cape Verde--
Canaries--Azores region of the Atlantic, and the
equatorial Pacific. The fast regions are in the
north polar regions, North America, eastern
Indian Ocean and the South Atlantic. Most con-
tinental regions are fast. Near the base of the
lower mantle, the prominent low-velocity regions
are under Africa, Brazil and the south-central
Pacific. The fast regions are Asia, the North
Atlantic, the northern Pacific and Antarctica.
The locations of hotspots do not correlate with
the slower regions at the base of the mantle.
[
global seismic structure maps, scor-
ing hotspots
].
Superplumes?
The so-called Pacific and African
lower man-
tle superplumes
, identified visually in global
tomographic cross-sections, apparently are dense
(Ishii and Tromp, 2004) and must have a chem-
ical origin; they are not thermally buoyant, as
has often been proposed. In the lower part of
the mantle, thermal buoyancy is weak compared
to chemical buoyancy because thermal expan-
sivity decreases with pressure. Purely thermal
upwellings are expected to have low bulk mod-
ulus, low compressional velocity and low den-
sity. This is not the case (Ishii and Tromp, 2004,
Trampert
et al.
, 2004) for the large lower-mantle
features. The large-scale features have the appro-
priate dimensions to be thermal in nature but
resemble more a chemically dense layer at high
pressure, i.e., large-scale marginally-stable domes
with large relief. These domes may have neutral
density, or be slowly rising or sinking. In any
case, they will affect the geoid, the dynamic sur-
face topography, and the relief on other chemi-
cal boundaries. D
would then be a very dense --
probably iron-rich -- layer, and the overlying
'layer', which is called D
, would be a less dense
region trapped between D
and the rest of the
lower mantle. Stratification may have been estab-
lished during accretional melting of the Earth
by downward drainage of dense melts and resid-
ual refractory phases, and iron partitioning into
phases that may include post-perovskite, low-spin
iron-rich oxides and sulfides and intermetallic
compounds. The large low-shear-velocity features
are more appropriately called 'domes,' a geo-
logically descriptive term, than 'megaplumes' or
Fig. 11.12
SH velocities at depths of 490 to 575 km (after
Grand, 1986).
centered near the long-wavelength geoid highs,
which occupy the central Pacific and the North
Atlantic through Africa to the southwestern
Indian Ocean. The slow-shear-velocity regions
were originally thought to represent hotter than
average mantle but the bulk modulus and den-
sity are higher than average, ruling out a thermal
explanation. The range of the long wavelength
velocity variations is much less than in the shal-
low mantle but there are small ultra-low velocity
regions near the core that have 10--20% velocity
reductions.
Slow regions of the lower mantle occur under
the mid Indian Ocean Ridge, the East Pacific
Rise, western North America and South Africa.
Fast regions occur under Australia, China, South
America and northern Pacific. Generally, there is
lack of radial continuity in the lower mantle.
Large-scale convection-like features are not evi-
dent. The fastest regions are Siberia, south Africa,
south of South America and the northeast Pacific.
At
mid-to-lower
mantle
depths
(Figures
11.17
and
11.18),
the
slowest
regions
are
southeast
