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Tephra Fall Hazards at Mount Adams

Annual probability of 1 cm or more of tephra accumulation in Washington and Oregon from major Cascade volcanoes; Mount Adams is particularly at risk due to its location downwind of Mount St. Helens. (Public domain.)

In comparison to other volcanoes in the Cascade Range, Mount Adams has had very few explosive eruptions that produced tephra. Magma rising from depth that interacts groundwater or lava flows that encounter ice or surface water can produce explosive eruptions that would distribute tephra across small areas near source. On the basis of its past record, Mount Adams is unlikely to generate large explosive eruptions that distribute ash far downwind rather its explosivity is apt to be mild and ash is likely to be restricted to areas near the volcano. Although Mount Adams is not a prolific producer of tephra, the region around the volcano has the highest probability of tephra fall of anywhere in the western conterminous United States, owing to its location just downwind of Mount St. Helens, the most prolific producer of tephra in the Cascades during the past 4000 years.