The cycling of rising, cooling, sinking, heating, and rising again forms convection currents that mix the water shortly before it begins to boil. The pressure on the hot asthenosphere materials decreases as the materials rise towards the Earth's surface. Under lower pressure, the asthenosphere largely melts to produce magmas that erupt as lavas at the spreading ridges. The erupting lavas form new oceanic crust. The addition of new crust at the spreading ridges and the convection currents cause the plates at the spreading ridges to spread apart.
Oceanic crust is destroyed at subduction zones .
If an oceanic plate collides with another oceanic plate, the older, cooler and therefore denser plate will tend to subduct under the younger, hotter and less dense plate. The subducting plate melts and produces magma, which may erupt to form island volcanoes. These volcanoes form island arcs . Other examples of island arcs are Japan, Indonesia and the Philippines. Oceanic plates may also collide with continental plates.