Most people thought Mount St. Helens was a beautiful, peaceful mountain and not a dangerous volcano. Volcanoes occur because the Earth's crust is broken into plates that resemble a jigsaw puzzle. There are 16 major plates. These rigid plates float on a softer layer of rock in the Earth's mantle. As the plates move about they push together or pull apart.
Most volcanoes occur near the edges of plates.
When plates push together, one plate slides beneath the other. This is a subduction zone. When the plunging plate gets deep enough inside the mantle, some of the rock on the overlying plate melts and forms magma that can move upward and erupt at the Earth's surface. At rift zones, plates are moving apart and magma comes to the surface and erupts. Volcanoes grow because of repeated eruptions.