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Passage 03

      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 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

  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 World

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Passage03 Answers

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