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 belows. 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 geomagnetic field. when the magma cools and solidifies,