As the rising magma nears the Earth's surface, pressure decreases, which causes the gases in the magma to expand. Once magma is erupted, it is called lava . Where Volcanoes Occur Volcanic eruptions occur only in certain places and do not occur randomly. These plates are rigid, but they float on the hotter, softer layer in the Earth's mantle. As the plates move about, they spread apart, collide, or slide past each other.
Volcanoes occur most frequently at plate boundaries.
Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots . 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.