No very satisfactory account of the mechanism
that caused the formation of the ocean basins has
yet been given. The traditional view supposes
that the upper mantle of the earth behaves as a
(5) liquid when it is subjected to small forces for
long periods and that differences in temperature
under oceans and continents are sufficient to
produce convection in the mantle of the earth
with rising convection currents under the mid-
(10) ocean ridges and sinking currents under the con-
tinents. Theoretically, this convection would
carry the continental plates along as though they
were on a conveyor belt and would provide the
forces needed to produce the split that occurs
(15) along the ridge. This view may be correct: it has
the advantage that the currents are driven by
temperature differences that themselves depend
on the position of the continents. Such a back-
coupling, in which the position of the moving
(20) plate has an impact on the forces that move it,
could produce complicated and varying motions.
On the other hand, the theory is implausible
because convection does not normally occur
along lines. and it certainly does not occur along
(25) lines broken by frequent offsets or changes in
direction, as the ridge is. Also it is difficult to see
how the theory applies to the plate between the
Mid-Atlantic Ridge and the ridge in the Indian
Ocean. This plate is growing on both sides, and
(30) since there is no intermediate trench, the two
ridges must be moving apart. It would be odd if
the rising convection currents kept exact pace
with them. An alternative theory is that the sink-
ing part of the plate, which is denser than the
(35) hotter surrounding mantle, pulls the rest of the
plate after it. Again it is difficult to see how this
applies to the ridge in the South Atlantic, where
neither the African nor the American plate has a
sinking part.
(40) Another possibility is that the sinking plate
cools the neighboring mantle and produces convection
currents that move the plates. This last
theory is attractive because it gives some hope of
explaining the enclosed seas, such as the Sea of
(45) Japan. These seas have a typical oceanic floor,
except that the floor is overlaid by several kilo-
meters of sediment. Their floors have probably
been sinking for long periods. It seems possible
that a sinking current of cooled mantle material
(50) on the upper side of the plate might be the cause
of such deep basins. The enclosed seas are an
important feature of the earth’s surface, and
seriously require explanation in because, addition
to the enclosed seas that are developing at
present behind island arcs, there are a number of
(55) older ones of possibly similar origin, such as the
Gulf of Mexico, the Black Sea, and perhaps the
North Sea.