This phenomenon can be explained using the theory of plate tectonics. Nearly 200 million years ago, the gigantic landmass known as Pangaea cracked apart deep inside the earth at the lithosphere layer; the lithosphere then split into several tectonic plates, and the tectonic plates began moving apart. According to plate tectonic theory, the plates move slowly apart, and after 200 million years or so they begin to move back together again, rather like pieces in a geological game of bumper cars. The great majority of the world's earthquakes and active volcanoes occur near the boundaries of these shifting plates.
When two plates collide, one slides beneath the other. The rock that is consequently pushed down toward the earth’s center melts; melted rock rises as magma, forming volcanoes near the collision point. The long arc of the Ring of Fire is the result of the huge Pacific plate sliding under several continental plates. Volcanoes also result from seafloor spreading; where crustal plates are separating, magma pushes up between the plates, creating mid-ocean ridges such as the Mid-Atlantic Ridge.
Around the world a number of regions have been identified as hot spots, areas in the middle of plates where magma rises and breaks through the earth’s crust. Such hot spots include Reunion Island and the island of Hawaii, where periodic eruptions are anticipated. Other areas, where volcanic activity may not occur for several human life spans may deceive folks by the long intervals -in human perspective- between blowups.
Examples of the latter include Mount Saint Helens in the United States, which erupted explosively in 1980 and covered the ground within a 10-mile radius with up to 10 inches of fine ash and pumice, and areas up to 300 miles away with as much as 1/2 inch of ash. Ash also reached an altitude of 80,000 feet in less than 15 minutes. Much of the eastern portion of Washington state was covered with a 6-inch layer of fine ash. Most residents had never experienced a volcanic eruption before and drove their vehicles through it—to their sorrow, as volcanic ash can be extremely destructive of mechanical gears.
