Geoscience Reference
In-Depth Information
Figure 5.11.
Contributions to the
gravity anomaly over a
hot region in the upper
mantle: (a) gravity
anomaly, (b) surface
shape and (c) mantle
temperature (
◦
C). (From
McKenzie (1998).)
anomalies can therefore give insights into the planform of convection in the man-
tle. Comparision of the geoid with bathymetry for the Pacific does not reveal
linear anomalies, indicating that the convection cannot be simple rolls aligned
with the direction of plate motion (e.g., Fig. 8.15). The planform of the convection
seems to be three-dimensional and on a smaller scale than the width of the Pacific
plate.
Figure 5.13 shows the bathymetry and the free-air gravity anomaly north
and south of the Hawaiian island of Oahu. The central 200-km-wide Hawaiian
ridge 'mountain' whose pinnacle is Oahu acts as a load on the Pacific plate
and bends it, resulting in the depression on either side of the mountain. The
bending of the plate and the associated gravity anomaly are confined to distances
within about 200 km of the load. The plates are too thin and too flexible for
local loads to result in any long-wavelength bathymetric or gravity anomalies.
The long-wavelength bulge evident both in the bathymetry and in the gravity is,
however, thought to be the surface expression of a hot upwelling region in the
mantle as discussed previously. Again, the fortunate difference in wavelength
between the elastic deformation of the plate in response to a surface load and
the apparent wavelength of mantle convection allows these two deviations to be
distinguished.