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Ash and Tephra Fall Hazards

Map showing one-year probability of accumulation of 1 centimeter
Map showing one-year probability of accumulation of 1 centimeter (0.4 inch) or more of tephra from eruptions of volcanoes in the Cascade Range. (Credit: Nathensen, Manny. Public domain.)

Eruptions from Mount Jefferson, a nearby monogenetic vent, or a more distant volcano has the potential to impact the local communities, especially those downwind of the tephra plume. Since the last ice age, the thickest tephra fall in the Mount Jefferson area (about 10 – 15 cm or 4 – 6 in) occurred from the eruption of ancient Mount Mazama, which created Crater Lake about 7,700 years ago. Between about 35 and 100 ka, a large explosive eruption from Mount Jefferson dumped nearly 2 meters (6 feet) of ash in areas within 20 km (12 mi) of the volcano. During explosive eruptions, large particles would fall back to the ground within a few kilometers of the vent, and finer particles (ash) would travel to higher altitudes and then be carried tens to hundreds of kilometers away where it could disrupt air traffic. It is likely that a large eruption would produce a high plumeand eruption cloud that could collapse and form pyroclastic flows that would travel down valleys on the volcano's flanks. Smaller eruptions from the monogenetic vents may produce deposits up to several meters (about 10 ft) thick within 2 km (1.2 mi) of the vent, but seldom exceed 10 cm (4 in) at distances 10 km (6 mi) away from vents.