The theory of plate tectonics describes the motions of the lithophere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle.The lighosphere is diveded 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 below.As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at teh base of the lithosphere.

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 lighospheric plates.The hypothesis was substantiated soon afterward by tehdiscovery 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 direction of the field are preserved in the magma become magcetized in the direction of the geomagnetic field.When the magma cools and solidfies,the direction and the polarity of the field are preserved in the magnetized volcanic rock.Reversals of the field give rise to a series of magnetic stripes running parallel to the anix o the rift.The oceanic crust thus srves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently;the width of the stripes indicates the rate of the sea-floor spreading.