The t。heory of plate tec。tonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crus。t。 and pa。rt 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 i。s injected from b。elow. 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 essentiall。y constan。t, new l。ithosphere can be。 crea。ted 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 s。ubduction zone. There one plate dives under the edge of ano。ther and is reincorporated in。to the mantle. Both。 kinds of plate boundary are associated with。 fa。ult systems, earthquakes and volcanis。m, but the kinds。 of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actual。ly preceded the t。heory of plate te。ctonics. In its original ve。rsion, 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 sub。stantiated soon afterward by the discovery that periodic reversals of the Earth's magnetic field are recorded in the oceanic crust. As magm。a rises under the mid-ocean r。idge, ferromagnetic minerals in the magma become magnetized in the direction of the magma。 become magnetized in the direction of the g。eomagnetic field. When the magma c。ools and s。olidifies, the di。rection and the polarity。 of the fi。eld are preserved in the magnetized volcanic rock. Reversals o。f 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