At divergent margins, lithospheric plates
move in opposite directions. The plates, which include crust and part of
the upper mantle, ride over the asthenosphere. As hot mantle rock rises to shallow depths,
it begins to melt due to lowered pressure. This forms magma.
Dense plutonic rock is created as the magma is injected into the crust. Small magma chambers
form low in the crust. Less than one quarter of the magma ever reaches the surface to erupt
as pillow lavas. As it moves away from the ridge, the cooling
rock sits progressively lower than the hotter, more bouyant, rock at the ridge. Like two giant
conveyor belts the plates transport newly formed oceanic crust away from the ridge crest
at 3 to 10 centimeters per year.