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.