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 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.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
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 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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