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.
Volcanoes can lie dormant for centuries between eruptions, and the risk posed by volcanic activity is not always apparent. When Cascades volcanoes do erupt, high speed 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.
It is important to be informed about volcanoes and to know what you should do if one of them erupts. Please use the following websites as informational tools regarding local and worldwide volcanic activity: