The three stages of a volcano are active, dormant and extinct. An active volcano is either erupting or showing signs that it is going to erupt, such as emitting gas. A dormant volcano is not active, but it could become active again, and an extinct volcano is unlikely to erupt again.
Since 1700, more than 260,000 people have been killed by volcanic activity. When magma rises during a volcano's active phase, it can trigger earthquakes and cause the volcano's summit to swell. This can destroy forests and cities and impede the economy for months to years. The USGS Volcano Hazards Program has developed a warning system to notify the public of impending volcano activity.