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TOEFL Directory > TOEFL writing > 36.Plate Tectonics and Sea-floor Spreading
36.Plate Tectonics and Sea-floor Spreading
The theory of plate tectonics describes the motions of the lithosphere,
the comparatively rigid outer layer of the Earth that includes all the
crust and part of the underlying mantle. The lithosphere(n.[地]岩石圈)is
divided into a few dozen plates of various sizes and shapes, in general
the plates are in motion with respect to one another. A mid-ocean ridge
is a boundary between plates where new lithospheric material is injected
from below. As the plates diverge from a mid-ocean ridge they slide on a
more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can
be created at the mid-ocean ridges only if an equal amount of
lithospheric material is consumed elsewhere. The site of this
destruction is another kind of plate boundary: a subduction zone. There
one plate dives under the edge of another and is reincorporated into the
mantle. Both kinds of plate boundary are associated with fault systems,
earthquakes and volcanism, but the kinds of geologic activity observed
at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate
tectonics. In its original version, in the early 1960's, it described
the creation and destruction of the ocean floor, but it did not specify
rigid lithospheric plates. The hypothesis was substantiated soon
afterward by the discovery that periodic reversals of the Earth's
magnetic field are recorded in the oceanic crust. As magma rises under
the mid-ocean ridge, ferromagnetic minerals in the magma become
magnetized in the direction of the magma become magnetized in the
direction of the geomagnetic field. When the magma cools and solidifies,
the direction and the polarity of the field are preserved in the
magnetized volcanic rock. Reversals of the field give rise to a series
of magnetic stripes running parallel to the axis of the rift. The
oceanic crust thus serves as a magnetic tape recording of the history of
the geomagnetic field that can be dated independently; the width of the
stripes indicates the rate of the sea-floor spreading.
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