Although there has not been an eruption at Mount Jefferson for some time, experience at explosive volcanoes elsewhere suggests that it may be too soon to regard Mount Jefferson as extinct. If Mount Jefferson erupts again, areas close to the eruptive vent will be severely affected, and even areas tens of kilometers downstream along river valleys or hundreds of kilometers downwind may be at risk. Fortunately, the populations at immediate risk in the Mount Jefferson area is small, but far traveled lahars and tephra fall can affect communities at greater distances from the volcano.
Numerous hazards can occur at composite volcanoes like Mount Jefferson including lava flows and domes, pyroclastic flows, ballistics, tephra fall, lahars and debris avalanches. Most are caused by eruption of molten rock (magma), but some, like debris avalanches and lahars can occur without eruptive activity.