Ash and tephra from Mount Shasta
During Holocene time Mount Shasta erupted pumiceous dacite tephra twice about 10,000 years ago. One deposit is more than 0.1 km3 (0.024 mi3) in volume and the other is less than 0.1 km3; both lie mainly east and within about 50 km (31 mi) of the volcano. Lithic ash has been erupted at Mount Shasta many times during the last 10,000 years and the deposits mantle the ground surface within about 25 km (15.5 mi) of the summit.
Based on this recent behavior, it is not likely that Mount Shasta will erupt very catastrophic volumes of tephra and ash in the near future. However, smaller eruptions are possible. If a future eruption of tephra and ash occurs, areas downwind and near the volcano (within about 50 km or 31 mi) will be most affected. The degree of risk from air-fall tephra decreases progressively as the distance from the volcano increases.