Geoscience Reference
In-Depth Information
Fig. 6.2
Geoid undulations (to degree 180) referred to a
hydrostatic shape,
flattening of 1/299.638
[called
the
non-hydrostatic flattening of the geoid
].
Contour interval is 5 m (after Rapp, 1981).
associated with the slab is pulling down the sur-
face. A thinner-than-average crust or a colder or
denser shallow mantle could also depress the
seafloor.
Cooling and thermal contraction of the
oceanic lithosphere cause a depression of the
seafloor with age and a decrease in the geoid
height. Cooling of the lithosphere causes the
geoid height to decrease uniformly with increas-
ing age, symmetrically away from the ridge crest.
The change is typically 5--10 m over distances
of 1000--2000 km. The elevation and geoid offset
across fracture zones is due to the age differences
of the crust and lithosphere. The long-wavelength
topographic highs in the oceans generally corre-
late with positive geoid anomalies, giving about
6--9 meters of geoid per kilometer of relief.
There is a good correlation between inter-
mediate-wavelength geoid anomalies and seismic
velocities in the upper mantle; slow regions are
geoid highs and vice versa. Subduction zones are
slow in the shallow mantle, presumably due to
the hot, partially molten mantle wedge under
back-arc basins.
In subduction regions the total geoid anomaly
is the sum of the positive effect of the dense
sinker and the negative effects caused by bound-
ary deformations. For a layer of uniform vis-
cosity, the net dynamic geoid anomaly caused
by a dense sinker is negative; the effects from
the deformed boundaries overwhelm the effect
expansion, will cause the elevation of the surface
to increase (
ρ
=
positive) and gives a positive
geoid anomaly because the center of mass is
closer to the Earth's surface. The mass deficiency
of the anomalous material is more than canceled
out by the excess elevation.
All major subduction zones are characterized
either by geoid highs (Tonga and Java through
Japan, Central and South America) or by local
maxima (Kuriles through Aleutians). The long-
wavelength part of the geoid is about that expec-
ted for the excess mass of the cold slab. The
shorter wavelength geoid anomalies, however,
are less, indicating that the excess mass is not
simply rigidly supported. There is an excellent
correlation between the geoid and slabs; this can
be explained if the
viscosity of the mantle
increases with depth by about a factor
of 30
. The high viscosity of the mantle at the
lower end of the slab partially supports the excess
load. Phase boundaries and chemical boundaries
may also be involved. The deep trenches rep-
resent a mass deficiency, and this effect alone
would give a geoid low. The ocean floor in back-
arc basins is often deeper than equivalent-age
normal ocean, suggesting that the mass excess
