Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along spreading ridges . Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land. Known as subduction zone volcanoes, they occur along the edges of continents where one plate dives, or subducts, beneath a second plate. Some of the magma rises and erupts as volcanoes. How explosive an eruption is depends on the magma's chemical composition and gas content, which in turn affect the magma's stickiness, or viscosity .
All magma contains gases that escape as the magma travels to the Earth's surface.
As a result, lava flows instead of exploding during an eruption. In an explosive eruption, the sudden expansion of gases blasts magma into airborne fragments called tephra , which can range in size from fine particles of ash to giant boulders. After the initial explosive phase of the eruption, however, quieter lava flows can follow. Three Types of Volcanoes Repeated volcanic eruptions build volcanic mountains of three basic types, or shapes, depending on the nature of the materials deposited by the eruption. Lava flow upon lava flow slowly builds a broad, gently sloping volcanic shape that resembles a warrior's shield.