As hot mantle rock rises from beneath the thinner oceanic crust at the mid-ocean ridges, two tectonic plates diverge. Volcanism and the formation of new oceanic crust are brought about by the adiabatic expansion and partial melting of the rising mantle rock caused by the decrease in pressure in the rock.
The majority of volcanic activity on Earth occurs underwater, forming new seafloor because divergent plate boundaries are at the bottom of the oceans. This kind of volcanic activity can be seen in the form of black smokers, which are also called deep sea vents. Iceland and other volcanic islands are formed above sea level where the mid-oceanic ridge is elevated.
Subduction zones are locations where oceanic and continental plates, typically, collide. A deep ocean trench is created just offshore as the oceanic plate subducts (dives beneath the continental plate). Water released by the subducting plate lowers the melting temperature of the overlying mantle wedge, resulting in the formation of magma in a process known as flux melting.
Due to its high silica content, this magma frequently fails to reach the surface and instead cools and solidifies at depth. However, once it reaches the surface, a volcano is formed. As a result, volcanic arcs, or chains of volcanoes, surround subduction zones. Examples include the Cascade Volcanoes in the Pacific Ring of Fire, the Japanese Archipelago, and the eastern Indonesian islands.