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 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-ocean ridges and sinking currents under the continents. 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 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 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 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 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 sinking part of the plate, which is denser than the 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 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 Japan. These seas have a typical oceanic floor, except that the floor is overlaid by several kilometers of sediment. Their floors have probably been sinking for long periods. It seems possible that a sinking current of cooled mantle material 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 because, in addition to the enclosed seas that are developing at present behind island arcs, there are a number of 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 following, the deepest sediments would be found in the
(A) Indian Ocean
(B) Black Sea
(D) South Atlantic
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
3. 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
4. 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
5. 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.
6. 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
7. 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
The fossil remains of the first flying vertebrates, the pterosaurs, have intrigued paleontologists for more than two centuries. How such large creatures, which weighed in some cases as much as a piloted hang-glider and had wingspans from 8 to 12 meters, solved the problems of powered flight, and exactly what these creatures were—reptiles or birds—are among the questions scientists have puzzled over.
Perhaps the least controversial assertion about the pterosaurs is that they were reptiles. Their skulls, pelvises, and hind feet are reptilian. The anatomy of their wings suggests that they did not evolve into the class of birds. In pterosaurs a greatly elongated fourth finger of each forelimb supported a wing-like membrane. The other fingers were short and reptilian, with sharp claws. In birds the second finger is the principal strut of the wing, which consists primarily of feathers. If the pterosaurs walked on all fours, the three short fingers may have been employed for grasping. When a pterosaur walked or remained stationary, the fourth finger, and with it the wing, could only turn upward in an extended inverted V-shape along each side of the animal’s body.
The pterosaurs resembled both birds and bats in their overall structure and proportions. This is not surprising because the design of any flying vertebrate is subject to aerodynamic constraints. Both the pterosaurs and the birds have hollow bones, a feature that represents a savings in weight. In the birds, however, these bones are reinforced more massively by internal struts.
Although scales typically cover reptiles, the pterosaurs probably had hairy coats. T. H. Huxley reasoned that flying vertebrates must have been warm-blooded because flying implies a high rate of metabolism, which in turn implies a high internal temperature. Huxley speculated that a coat of hair would insulate against loss of body heat and might streamline the body to reduce drag in flight. The recent discovery of a pterosaur specimen covered in long, dense, and relatively thick hairlike fossil material was the first clear evidence that his reasoning was correct.
Efforts to explain how the pterosaurs became airborne have led to suggestions that they launched themselves by jumping from cliffs, by dropping from trees, or even by rising into light winds from the crests of waves. Each hypothesis has its difficulties. The first wrongly assumes that the pterosaurs’ hind feet resembled a bat’s and could serve as hooks by which the animal could hang in preparation for flight. The second hypothesis seems unlikely because large pterosaurs could not have landed in trees without damaging their wings. The third calls for high waves to channel updrafts. The wind that made such waves however, might have been too strong for the pterosaurs to control their flight once airborne.
1. It can be inferred from the passage that scientists now generally agree that the
(A) enormous wingspan of the pterosaurs enabled them to fly great distances
(B) structure of the skeleton of the pterosaurs suggests a close evolutionary relationship to bats
(C) fossil remains of the pterosaurs reveal how they solved the problem of powered flight
(D) pterosaurs were reptiles
(E) pterosaurs walked on all fours
2. The author views the idea that the pterosaurs became airborne by rising into light winds created by waves as
3. According to the passage, the skeleton of a pterosaur can be distinguished from that of a bird by the
(A) size of its wingspan
(B) presence of hollow spaces in its bones
(C) anatomic origin of its wing strut
(D) presence of hooklike projections on its hind feet
(E) location of the shoulder joint joining the wing to its body
4. The ideas attributed to T. H. Huxley in the passage suggest that he would most likely agree with which of the following statements?
(A) An animal’s brain size has little bearing on its ability to master complex behaviors.
(B) An animal’s appearance is often influenced by environmental requirements and physical capabilities.
(C) Animals within a given family group are unlikely to change their appearance dramatically over a period of time.
(D) The origin of flight in vertebrates was an accidental development rather than the outcome of specialization or adaptation.
(E) The pterosaurs should be classified as birds, not reptiles.
5. It can be inferred from the passage that which of the following is characteristic of the pterosaurs?
(A) They were unable to fold their wings when not in use.
(B) They hung upside down from branches as bats do before flight.
(C) They flew in order to capture prey.
(D) They were an early stage in the evolution of the birds.
(E) They lived primarily in a forest-like habitat.
6. Which of the following best describes the organization of the last paragraph of the passage?
(A) New evidence is introduced to support a traditional point of view.
(B) Three explanations for a phenomenon are presented, and each is disputed by means of specific information.
(C) Three hypotheses are outlined, and evidence supporting each is given.
(D) Recent discoveries are described, and their implications for future study are projected.
(E) A summary of the material in the preceding paragraphs is presented, and conclusions are drawn.
7. It can be inferred from the passage that some scientists believe that pterosaurs
(A) lived near large bodies of water
(B) had sharp teeth for tearing food
(C) were attacked and eaten by larger reptiles
(D) had longer tails than many birds
(E) consumed twice their weight daily to maintain their body temperature
*Note:- The answers of above RC passages