Volcanism is the eruption of magma to the surface of a planet. Molten rock wells up through a vent in the planet's mantle, spewing lava, gases and volcanic material into the surrounding area. Over time, this material hardens and accumulates, creating cone-shaped volcanoes and other structures, such as craters.
In most cases, volcanic activity occurs around the edges of tectonic plates. When two plates collide, one is typically forced under its neighbor, and during the process, it may open vents into the planet's mantle. In other areas, plates may pull apart, creating a larger void to allow magma to flow upward. In rare cases, a tectonic plate may form a weak point somewhere away from an edge, creating a hot spot capable of spawning volcanoes. The Hawaiian Islands are the result of one of these hot spots that created a chain of islands in the Pacific stretching all the way from Kilauea to Midway.
Volcanism can be a relatively calm event, with magma bubbling up through an unobstructed vent and flowing out over the surface. In other cases, a blocked vent or particularly viscous magma can cause a buildup of pressure, creating an explosive eruption like the one that occurred at Mount St. Helens in 1980.