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Volcanic Hazards at Mount Adams

During much of its history, Mount Adams has displayed a relatively limited range of eruptive styles. Highly explosive eruptions have been rare, and compared to the tens of large explosive eruptions at nearby Mount St. Helens during the past 20,000 years, eruptions of Mount Adams have been meek. Despite their low level of explosivity, eruptions at Mount Adams can be hazardous. The dominant type of eruption at Mount Adams, as well as in the adjacent volcanic fields, produces lava flows. Hazards from lava flows exist downslope of the flow front, especially on steep slopes where the flows can break apart to form pyroclastic flows.

The greatest hazard at Mount Adams, in common with many large stratovolcanoes, is from landslides, debris avalanches, and lahars. At ice-capped Mount Adams, meltwater produced by rapid melting of snow and ice could bulk rapidly into a potentially destructive lahar by incorporation of volcanic rock and soil as it travels downslope. Rainstorms that follow eruptive activity that produces ash or loose-debris deposits can also trigger lahars and larger-than-normal floods.

Debris avalanches and related lahars may have little or no advanced warning and can occur during times of no eruptive activity but are most likely during periods of volcanism or precursory unrest. Onset of precursory earthquakes and deformation increase the probability of debris avalanches, especially large ones that have the potential to spawn massive lahars that can devastate valley floors tens of kilometers from the volcano.