The water that creates lahars can come from melting snow and ice, intense rainfall, or the breakout of a summit crater lake. Large lahars are a potential hazard to many communities downstream from glacier-clad volcanoes, such as Mount Rainier in Washington. Volcanic ash is actually fine, glassy rock fragments and can affect people and equipment hundreds of miles away from the cone of the volcano. Volcanic ash can contaminate water supplies, cause electrical storms, disrupt the operation of all machinery, and collapse roofs. Volcanic ash poses an ever-increasing threat to aviation safety as transportation expands throughout the Pacific rim.
Airborne ash can diminish visibility, damage flight control systems, and cause jet engines to fail.
Volcanoes usually give warning that they will erupt. USGS scientists have developed a forecasting system to alert public officials and the general public to the fact that a volcano may erupt. Learn about volcano risk in your community. Contact your local emergency management office, American Red Cross chapter, or state geological surveys or departments of natural resources. Ask about the type of volcano hazards that could affect your area.