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 liquid
(5)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. Another possibility is that
(40)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, 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.
1. According to the traditional view of the origin of the ocean basins, which of the following is sufficient to move the continental plates?
(A) Increases in sedimentation on ocean floors
(B) Spreading of ocean trenches
(C) Movement of mid-ocean ridges
(D) Sinking of ocean basins
(E) Differences in temperature under oceans and continents
2. It can be inferred from the passage that, of the follo- wing, the deepest sediments would be found in the
(A) Indian Ocean
(B) Black Sea
(C) Mid-Atlantic
(D) South Atlantic
(E) Pacific
3. The author refers to a "conveyor belt " in line 13 in order to
(A) illustrate the effects of convection in the mantle
(B) show how temperature differences depend on the positions of the continents
(C) demonstrate the linear nature of the Mid-Atlantic Ridge
(D) describe the complicated motions made possible by back-coupling
(E) account for the rising currents under certain mid- ocean ridges
4. The author regards the traditional view of the origin of the oceans with
(A) slight apprehension
(B) absolute indifference
(C) indignant anger
(D) complete disbelief
(E) guarded skepticism
5. According to the passage, which of the following are separated by a plate that is growing on both sides?
(A) The Pacific Ocean and the Sea of Japan
(B) The South Atlantic Ridge and the North Sea Ridge
(C) The Gulf of Mexico and the South Atlantic Ridge
(D) The Mid-Atlantic Ridge and the Indian Ocean Ridge
(E) The Black Sea and the Sea of Japan
6. Which of the following, if it could be demonstrated, would most support the traditional view of ocean formation?
(A) Convection usually occurs along lines.
(B) The upper mantle behaves as a dense solid.
(C) Sedimentation occurs at a constant rate.
(D) Sinking plates cool the mantle
(E) Island arcs surround enclosed seas.
7. According to the passage, the floor of the Black Sea can best be compared to a
(A) rapidly moving conveyor belt
(B) slowly settling foundation
(C) rapidly expanding balloon
(D) violently erupting volcano
(E) slowly eroding mountain
8. Which of the following titles would best describe the content of the passage?
(A) A Description of the Oceans of the World
(B) Several Theories of Ocean Basin Formation
(C) The Traditional View of the Oceans
(D) Convection and Ocean Currents
(E) Temperature Differences Among the Oceans of the Worl |
收藏