This topographic relief map of the Earth shows some of the characteristics of the ocean floor. Oceans cover over 70% of the Earth's surface. The Earth's crust, which is an average 33 km thick, is divided into 15 tectonic plates which "float" on the liquid mantle below. These plates move as much as several centimeters a year. The largest plates are the Antarctic, Pacific, Eurasian, Australian-Indian, and North American plates. Plate boundaries occur where two plates are either receding from one another or are being pushed together. Oceanic ridges are the result of plates moving apart; a gap is left in the crust where molten material rises up from the mantle. More violent are the regions where plates are convergent on the same area. Where oceanic plates and continental plates meet, the oceanic plate is forced to slide under the other plate, creating a subduction zone. When two continental plates are convergent, however, neither plate can be subducted because of the local structure of the crust and mantle. In this case, the pressure on the plates causes them to fold and crack, creating mountain chains and faults. Earthquakes releave the pressure around the faults by allowing the crust to slip up and down.
