Mount St. Helens in Washington state is the most active volcano in the Cascade Range and it is the most likely of the contiguous U.S. volcanoes to erupt in the future. On the morning of May 18, 1980, a magnitude 5.1 earthquake struck, followed by the largest debris avalanche recorded in history. It resulted in the eruption of Mount St. Helens, leaving the surrounding areas devastated. The Toutle River in Mount St. Helen’s Monument, April 22, 2005, 25 years after the eruption.

An aerial view of Mount St. Helens erupting, May 18, 1980.
Nicholas George/The Chronicle/AP Photo