What process causes volcanoes to form along the mid-ocean ridge?

The mid-ocean ridge is a continuous underwater range of mountains which virtually encircle the globe. These mountains are formed along the boundaries of tectonic plates where new ocean floor is created when tectonic plates move apart.

Tectonic plates are moved apart by convection in the upper mantle. As the tectonic plates move apart, molten rock ascends, right through to the sea floor, producing huge volcanic eruptions of lava, called basalt. The enormous force of the lava eruption further aggravates the divergence of the tectonic plates.

Tectonic plates spread apart at varying rates. Plates with slow spreading rates of about 2 to 5 cm per year develop deep rift valleys along their crest but fast spreading ones with rates of about 6 to 16 cm per year develop a smooth volcanic summit with only a crack along their crests.