According to the theory of plate tectonics, the lithosphere (earth’s relatively hard and solid outer layer consisting of the crust and part of the under-Line lying mantle) is divided into a few dozen plates (5) that vary in size and shape; in general, these plates move in relation to one another. They move away from one another at a mid-ocean ridge, a long chain of sub-oceanic mountains that forms a boundary between plates.At a mid-ocean (10) ridge, new lithospheric material in the form of hot magma pushes up from the earth’s interior. The injection of this new lithospheric material from below causes the phenomenon known as sea-floor spreading. (15) Given that the earth is not expanding in size to any appreciable degree, how can "new" litho- sphere be created at a mid-ocean ridge For new lithosphere to come into being in one region, an equal amount of lithospheric material must ,be (20) destroyed somewhere else. This destruction takes place at a boundary between plates called a sub- duction zone.At a subduction zone, one plate is pushed down under another into the red-hot mantle, where over a span of millions of years it (25) is absorbed into the mantle. In the early 1960’s, well before scientists had formulated the theory of plate tectonics, Princeton University professor Harry H. Hess proposed the concept of sea-floor spreading. Hess’s original (30) hypothesis described the creation and spread of ocean floor by means of the upwelling and cool- ing of magma from the earth’s interior. Hess, however, did not mention rigid lithospheric plates. The subsequent discovery that the oceanic (35) crust contains evidence of periodic reversals of the earth’s magnetic field helped confirm Hess’s hypothesis.According to the explanation formu- lated by Princeton’s F. J. Vine andD、H. Matthews, whenever magma wells up under a (40) mid-ocean ridge, the ferromagnetic minerals with- in the magma become magnetized in the direction of the geomagnetic fielD、As the magma cools and hardens into rock, the direction and the polarity of the geometric field are recorded in the magnetized (45) volcanic rock. Thus, when reversals of the earth’s magnetic field occur, as they do at intervals of from 10,000 to around a million years, they pro- duce a series of magnetic stripes paralleling the axis of the rift. Thus, the oceanic crust is like a (50) magnetic tape recording, but instead of preserving sounds or visual images, it preserves the history of earth’s geomagnetic fielD、The boundaries between stripes reflect reversals of the magnetic field; these reversals can be dated independently. (55) Given this information, geologists can deduce the rate of sea-floor spreading from the width of the stripes. (Geologists, however, have yet to solve the mystery of exactly how the earth’s magnetic field comes to reverse itself periodically.) According to the passage, a mid-ocean ridge differs from a subduction zone in thatA．it marks the boundary line between neighboring plates B．only the former is located on the ocean floor C．it is a site for the emergence of new litho- spheric material D．the former periodically disrupts the earth’s geomagnetic field E、it is involved with lithospheric destruction rather than lithospheric creation
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