Plates have three kinds of margin. At constructive margins, or spreading centers, new oceanic crust is created by continuous activity along mountain ranges in the middle of ocean basins. Recent groups (largely U.S./French) have used geophysical surveys followed by dives with submersibles to show that processes of ocean-plate construction have differed greatly between the Pacific (fast-spreading at up to 4.3 in., 110 mm, per year) and Atlantic (slow-spreading at about 0.4 in., 10 mm, per year) oceans.
In the Pacific Ocean, hot springs were discovered on the spreading centers. Driven by the interaction of seawater with molten and hot rock below the volcanic zone, the spring waters vent at up to 662° F (350° C), and are commonly laden with particles of metal sulfides. These metallic emissions are responsible for depositing vast volumes of metal-rich material on the ocean floor. The springs are commonly surrounded by communities of animals and other sea creatures such as clams and crabs, some of them previously unknown.
Plates are destroyed along their destructive (or active) margins by a process known as subduction. Plates capped by oceanic crust are thrust below plates capped either by continental crust (as in Japan and the Andean coast of South America), or by oceanic crust (as in the Mariana Islands), along deep depressions known as oceanic trenches. Stresses in the crust arising from subduction generate earthquakes along active margins, and melting about 60 miles (100 km) or more down in a subduction zone generates arcs of volcanoes on the overriding plate.