The solid base beneath the oceans consists of oceanic crustal rock, averaging 5 km thick, overlain with sediment. Smaller seas occur in basins in continental crust. Moving outwards from a continent there is usually a continental shelf that slopes gently to the continental slope, where the gradient increases. Sediment sliding down the continental slope accumulates on the continental rise, sloping gently to the abyssal plain. A volcanic mountainous region, the mid-ocean ridge, runs down the centre of every ocean, where two tectonic plates are moving apart and magmatic material is rising to fill the gap. There is often a rift along the edge of the ridge, and there are trenches where plates converge and one is subducting beneath the other.