Predicament: You're 1,500 feet below the crater rim of Mount St. Helens, the still-active Cascades Range peak that famously blew its top in 1980. Suddenly, the ground begins to shake, and you look up as a huge cloud of ash and steam billows skyward. This mountain is definitely alive. But will you be, 10 minutes from now?
Lifeline: Descend as quickly as possible while looking for shelter. Most eruptions blast thousands of small rocks into the air. If they start landing nearby, crouch behind a boulder and protect yourself by placing your pack over your head and neck. Ash isn't toxic, but it will irritate your eyes and lungs and reduce visibility. Cover your mouth with a dry bandana, and use a headlamp to see and signal.
Avoid low-lying areas (where asphyxiating gases such as carbon dioxide and sulfur dioxide can settle), and any place with a rotten-egg odor. While lava streams are extremely rare, mudflows from melting snow and ice are more common and can reach 30 mph. As you descend, avoid gullies or streams; these can channel runoff. Most minor eruptions, or "burps," last a half-hour or less and are usually preceded by a swarm of small earthquakes.