When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes. Because there may be hundreds or thousands of years between volcanic eruptions, people may not be aware of a volcano's dangers. When Mount St. Helens in the State of Washington erupted in 1980, it had not erupted for 123 years. 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.