The theory of plate tectonics describes the motions of thelithosphere, the comparatively rigid outer layer of the Earth thatincludes all the crust
and part of the underlying mantle.The lithosphere is divided into a few dozen plates of various sizes andshapes, in general the plates are in motion with respect to one another. Amid-ocean ridge is a boundary between plates where new lithosphericmaterial is injected from belows. As the plates diverge from a mid-oceanridge they slide on a more yielding
layer at the base of the lithosphere.Since thesize of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material isconsumed elsewhere. The site of this destruction is another kind of plateboundary: a subduction zone. There one plate dives under the edge of anotherand is reincorporated into the mantle. Both kinds of plate boundary areassociated with fault systems, earthquakes and volcanism, but thekinds of geologic activity observed at the two boundaries are quitedifferent.
The idea of sea-floor spreading actually preceded thetheory of plate tectonics. In its original version, in the early 1960s, itdescribed the creation and destruction of the ocean floor, but it did notspecify rigid lithospheric plates. The hypothesis was substantiated soonafterward by the discovery that periodic reversals of the Earths magneticfield are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma becomemagnetized in the direction of the geomagnetic field. When the magma coolsand solidifies, the direction and the polarity of the field are preserved inthe magnetized volcanic rock. Reversals of the field give rise to a seriesof magnetic
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