GRE考试

【单选题】ReadingComprehension
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, lithospheric material at the site of a subduction zoneA．rises and is polarized
B．sinks and is reincorporated
C．slides and is injected
D．spreads and is absorbed
E、diverges and is consumed

网考网参考答案：B
网考网解析：
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