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. There are 16 major plates. As the plates move about, they spread apart, collide, or slide past each other.
Volcanoes occur most frequently at plate boundaries.
U.S.Department of the Interior, U.S.Geological Survey, Teaching Packet. 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 . U.S.Department of the Interior, U.S.Geological Survey, Teaching Packet. Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land.