A volcano is a vent in the earth's crust through which magma, rock fragments, gases, and ash are ejected from the earth's interior. Over time, accumulation of these erupted products on the earth's surface creates a volcanic mountain.

Washington State has five major volcanoes in the Cascade Range – Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens and Mount Adams.

Volcanoes can lie dormant for centuries between eruptions, and the risk posed by volcanic activity is not always apparent. When Cascades volcanoes do erupt, highspeed avalanches of hot ash and rock called pyroclastic flows, lava flows, and landslides can devastate areas 10 or more miles away, while huge mudflows of volcanic ash and debris called lahars can inundate valleys more than 50 miles downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to the engines of jet aircraft hundreds or thousands of miles away.

The eruption of Mount St. Helens in 1980 virtually paralyzed Eastern Washington. It is anticipated that another eruption of a Cascade volcano will occur.