Geoscience Reference
In-Depth Information
That it does not could indicate that the rate of descent in the lower mantle was
much reduced, that descent was in some way hindered or that the plate was
deformed (mantle convection is discussed in Section 8.2). The large-scale high-
and low-velocity anomalies at the base of the lower mantle visible in Fig. 8.6 are
also a feature of these body-wave models. These high-velocity regions may mean
that the D
zone is the ultimate destination of subducted plates.
The fine-scale structure of the D
zone is the subject of considerable current
research. There is a localized stratification at the top of the D
zone with increases
of up to 3% in P- and S-wave velocities. These need not be straightforward
increases in velocity but may rather be transition zones up to 50 km thick. Regional
reductions in both P- and S-wave velocities of over 10% have been imaged in a thin
layer (5-40 km vertical extent) immediately above the CMB. This low-velocity
feature, referred to as the ultra-low-velocity zone (ULVZ), is thought to be very
heterogeneous. The seismic phases used to image the ULVZ include (1) precursors
to the short-period reflections PcP and ScP and (2) the longer-period SKS phase
and the later associated phase SPdKS. SPdKS is a phase in which energy is
diffracted as a P-wave (code Pd) along the CMB before continuing through the
outer core as a P-wave. The ULVZ is not a global feature: it has so far been
imaged only beneath the central Pacific, northwestern North America, Iceland
and central Africa; it is absent beneath most of Eurasia, North America, South
America and the south Atlantic. Major velocity reductions of 10% occurring in
such thin localized zones imply major changes in physics and chemistry - partial
melting seems possible.
The processes occurring in the D
boundary layer between mantle and core
are matters of much research and conjecture. Seismology gives glimpses of the
structures, velocities and anisotropy present there. The details of the interaction
of cold, downgoing, subducted plates with the lowermost mantle and the of gen-
eration of plumes are not yet well understood. The extent to which the ULVZ
and any partial melting may be linked to plume location and the role of a chem-
ical boundary layer above the CMB are also far from clear. Investigating links
between hotspot volcanism and past properties of the CMB may seem far-fetched
but may provide information on CMB chemistry and processes.
Anisotropy
The velocity of seismic waves through olivine (which is a major constituent of
the mantle) is greater for waves travelling parallel to the
a
axis of the olivine
crystal than it is for waves travelling perpendicular to the
a
axis. Such depen-
dence of seismic velocity on direction is called velocity
anisotropy
(i.e., the
material is not perfectly isotropic). Anisotropy is not the same as
inhomogene-
ity
,which refers to a localized change in physical parameters within a larger
medium. Any flow in the mantle will tend to align the olivine crystals with their
a
axes parallel to the direction of flow. For this reason, measurement of anisotropy
in the mantle can indicate whether any flow is vertical or horizontal. Plots of
