Lava flows and pyroclastic flows may affect low areas within about 15-20 km 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 from the summit. Explosive lateral blasts 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 from the volcano.
On the basis of its Holocene behavior, the probability is low that Mount Shasta will erupt large volumes of pumiceous ash in the future. The distribution of Holocene tephra and prevailing wind directions suggest that areas most likely to be affected by tephra are mainly east and within about 50 km of the summit of the volcano. However, the andesitic and dacitic composition of its products suggests that Mount Shasta could erupt considerably larger volumes of tephra in the future. Moreover, Christiansen (1982) has 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 (101 -102) km3. 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 from the vent. The annual probability for such a large event may be no greater than 10^-5, but it is finite.