These symbols show the locations of active volcanoes throughout the world. Most volcanoes occur near the leading edges of moving plates – near the subduction zone. Here’s how some volcanoes are created. The plate on the left is carrying an ocean basin. The plate on the right is carrying a continent. They are colliding. The plate carrying the ocean basin, because it is heavier, dives beneath the plate carrying the continent. As the leading edge of one plate is consumed beneath the other, some of the magma finds its way up between cracks and crevices in the continental crust.When the magma reaches the surface, the earth bulges and pressures build. The result? The eruption of Mount St. Helens in 1980 was the direct result of plate subduction. Mount St. Helens is only one of many volcanoes in the Pacific Northwest. As this plate continues to subduct or dive beneath the plate next to it, more eruptions will undoubtedly occur...with devastation perhaps far greater than that experienced at Mount St. Helens.