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 con-
vection 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, addi-
tion 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.
1. According to the traditional view of the origin of the
ocean basins, which of the following is sufficient to