Volcanoes on Plate boundaries
There are three main types of plate boundaries: divergent, convergent, and transform. Divergent boundaries occur where plates are moving apart. The volcanoes that form along such boundaries are generally nonexplosive and have new basaltic magma filling the widening separation or feeding lava flows. Even though most of the earth's volcanism occurs along divergent boundaries, the eruptions often occur unobserved because divergent boundaries are covered by the oceans, except those in Iceland and East Africa. Convergent boundaries separate plates that are moving toward each other. Most of the world's above-sea volcanoes are located along such boundaries, which are also called subduction zones. Although composite volcanoes near subduction zones produce only about 15 percent of global volcanism, they account for more than 80 percent of documented historical eruptions, mostly explosive. Transform boundaries are areas where one plate is grinding horizontally past another. These boundaries are often zones of frequent earthquakes, but they are not volcanically active.
Midplate Volcanoes
Some volcanoes are located thousands of kilometers from any active plate boundary. These midplate volcanoes sometimes form long, well-defined volcanic chains. Scientists believe they are formed by magma from a partly melting plate overriding a stationary heat source, or hot spot, in the mantle. The best example of such midplate hot-spot volcanism is the Hawaiian Ridge-Emperor Sea Mounts chain within the Pacific plate. Hot-spot volcanism within oceanic plates typically is nonexplosive and constructs basaltic shield volcanoes such as Mauna Loa and Kilauea in Hawaii. Hot-spot volcanism within the continental regions can be either explosive or nonexplosive. Explosive volcanism forms calderas and ash-flow plateaus or plains. Nonexplosive volcanism forms plateaus composed of basaltic lava flows such as the Columbia Plateau.