Since the 1950s, the hypothesis of continental drift (originally proposed by the meteorologist Alfred Wegener in 1915) has been refined into the theory of "plate tectonics," first elaborated by Canadian geologist J. Tuzo Wilson. This theory holds that the lithosphere (the rigid outer layer of the earth) consists of about seven major and numerous minor plates in which the continents are embedded. These plates move over an underlayer of semiplastic rock, the asthenosphere. This constant shifting of plates results in faulting and the uplifting of the earth's crust into folded mountain chains. The precise cause of plate movement is unknown, though it is thought to be related to convective heat currents in the upper portion of the earth's mantle.

Plate tectonics, while more widely credited than its more rudimentary forerunner , has yet to achieve universal acceptance. Although the positions of earthquake lines and mountain chains support the theory, V.V. Beloussov in the Soviet Union and Sir Harold Jeffreys in Britain are just two of the many scientists who have raised serious objections to it. As yet, however, no competing hypothesis has shown enough explanatory power to seriously challenge it.

1. The author would most probably describe the theory of plate tectonics as:

A. a proven geological fact
B. a hypothesis that fails to explain relevant data
C. a useful theory supported by strong evidence
D. a fruitful, though inexact, hypothesis
E. a scientific conjecture by an imaginative layman


2. Which of the following statements is (are) consistent with the theory of plate tectonics, as described in the passage?

I. The continents rest on a rigid layer called the asthenosphere.
II. The cause of plate movement has not yet been fully explained.
III. Mountain chains may result from collisions of the moving plates of the earth's outer layer.

A. I only
B. II only
C. III only
D. I and II only
E. II and III only


3. The "more rudimentary forerunner" of plate tectonics mentioned in the second paragraph is the theory

A. originally propounded by J. Tuzo Wilson
B. supported by Beloussov and Jeffreys
C. of heat flow in the upper mantle
D. of mountain chain formation
E. proposed by Alfred Wegener


4. The author's primary purpose is to:

A. debunk the theory of plate tectonics
B. advocate the theory of continental drift over that of plate tectonics
C. explain in detail the challenges to the theory of plate tectonics
D. offer new evidence that explains the precise cause of plate movement
E. describe the theory of plate tectonics and its acceptance in the scientific community