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,
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