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Plate Tectonics and Sea-floor Spreading

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The theory of plat。e 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 lithospher。e(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 anot。her. 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 dive。s under the edge of another and is reincorporat。ed into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcani。sm, 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 d。estruction 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. Revers。als 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 geomagnet


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