Future eruptions like those of the last 10,000 years will probably produce deposits of ash, lava flows, domes, and pyroclastic flows, and could endanger infrastructure that lie within several tens of kilometers of the volcano.
The record of eruptions over the last 10,000 years suggests that, on average, at least one eruption occurs every 800 to 600 years at Mt Shasta. Future eruptions like those of the last 10,000 years will probably produce deposits of ash, lava flows, domes, and pyroclastic flows, and could endanger infrastructure that lie within several tens of kilometers of the volcano.
Lava flows and pyroclastic flows may affect low areas within about 15-20 km (9 to 13 mi) of the summit of Mount Shasta or any satellite vent that might become active. Lahars could affect valley floors and other low areas as much as several tens of kilometers from Mount Shasta.
Owing to great relief and steep slopes, a portion of the volcano could also fail catastrophically and generate a very large debris avalanche and lahar. Such events could affect any sector around the volcano and could reach more than 50 km (30 mi) from the summit. Explosive lateral blasts, like the May 1980 eruption of Mount St. Helens, could also occur as a result of renewed eruptive activity, or they could be associated with a large debris avalanche; such events could affect broad sectors to a distance of more than 30 km (20 mi) from the volcano.
On the basis of its behavior in the past 10,000 years, Mount Shasta is not likely to erupt large volumes of pumiceous ash in the near future. The distribution of tephra and prevailing wind directions suggest that areas most likely to be affected by tephra are mainly east and within about 50 km (30 mi) of the summit of the volcano.
It has been suggested that because it is a long-lived volcanic center and has erupted only relatively small volumes of magma for several thousand years, Mount Shasta is the most likely Cascade Range volcano to produce an explosive eruption of very large volume. Such an event could produce tephra deposits as extensive and as thick as the Mazama ash and pyroclastic flows that could reach more than 50 km (30 mi) from the vent. The annual probability for such a large event may be no greater than 10-5, but it is finite.
