convection in a two dimensional layer with physical
parameters resembling a simplified asthenosphere.
His calculations were for a free upper boundary
uncomplicated by a lithospheric like slab. He found
in all cases positive gravity anomalies over the
rising limbs.
The lithosphere will undoubtedly introduce com
plications but as it is relatively thin over the ridges
the generalization of this conclusion appears in
order. A more serious question of generalization
is that the half wavelengths of the anomalies that
he finds tends to be smaller than that found in the
positive anomalies over the ridges as indicated by
figures 2, 3, 6, 7. and further work is required in
refining the upper mantle models and in refining
the gravity field.
Kaula also explains the negative anomalies over
the basins in terms of flow if we assume that as the
flow moves away from the up-currents below the
ridges, it cools and contracts in volume and is
possibly associated with a settling out of denser
materials. This would lead to a lowering of the
lithosphere and the depressions so formed would
be filled with the low density sediments giving the
typical deep ocean basin structure.
The mechanisms just described can only be called
approximate and to find a more definite model
some concrete calculations are required of the orders
of magnitudes of the forces responsible for, and
ngt 72
being created by, such a flow pattern and for the
observed gravity anomalies. Such calculations are
complex in that we really know nothing that is
quite definite about the physical parameters in
volved and about the way the lithosphere distorts
the picture of what is happening deeper down. We
also have to look to other geophysical data to
supply the necessary evidence, particularly to seis
mology and heat flow measurements, to be able to
construct reliable models so that we can strip back
the lithosphere to see which parts of the gravity
field can be used to interpret the flow in the astheno
sphere. Unfortunately the available heat flow and
seismic data is very sparse when considered on a
global scale and major improvements in the data
collection will be required.
Future improvements in the determination of the
earth's gravity field
In the first part of this paper we have discussed the
methods used to determine the earth's gravity field
as well as presented some recent results. In the
second part we have discussed some possible inter
pretations of these results. In particular, we have
stressed the importance of the correlations found
between the gravity and the structure of the earth's
upper mantle. The accuracy of the determination of
gravity field has been estimated at about 9 mgal yet
some of the features on which we have based our
Fig. 7. Global pattern of areas of tectonic activity. The solid lines are areas of compression (trenches or island arcs, and
mountain belts) and the double lines indicate areas of extension (ocean ridges and rift valleys). The shaded areas indicate
the major deep ocean basins.
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