At least five lahars and a debris avalanche, originating at Mount Adams, moved into the White Salmon River drainage basin during the past 12,000 years. The deposits produced by these lahars range in volume from 4 to 66 million cubic meters and they moved as much as 60 kilometers down the valley. About 6,000 years ago, the largest of the lahars inundated about 15 square kilometers of the lowland near Trout Lake and dammed a tributary stream to form Trout Lake. About 200 years ago, another lahar filled valleys to depths as much as 50 meters, and produced run-ups of as much as 30 meters on objects in its path, but left only thin veneers on valley sides and floors. Three smaller lahars and the debris avalanche of 1921 extend between 5 and 15 kilometers from Mount Adams.
The two largest lahars and the debris avalanche began as slope failures of hydrothermally altered rock from the cirque that forms the accumulation zone of the White Salmon and Avalanche Glaciers. Rocks in the headwall of the cirque, as well as in the deposits of the avalanche and lahars, contain the following distinctive secondary minerals: alunite, kaolinite, smectite, and opal. The hydrothermal alteration that formed these minerals weakened the volcanic structure, thereby setting the stage for slope failure. Formation of the secondary minerals, especially smectite and kaolinite, reduced permeability and increased porosity, thereby increasing the capability of the rocks to retain moisture. The slide masses that formed the two lahars apparently contained enough moisture to move as wet flows.
Exposures of altered andesite on the southwest, east, and north sides of Mount Adams indicate that hydrothermal alteration has variably affected at least 4.3 square kilometers of the central cone. Alteration in rocks that form the summit plateau is most severe, and alteration of rocks that form the flanks extends at least 1,000 meters below the summit; this indicates a volume of 1.5 to 3 billion cubic meters of altered rock near the summit of the volcano. This altered rock is a potential source of lahars and debris avalanches as large or larger than any of postglacial age at Mount Adams.
If future events at Mount Adams are similar in scale and type to those of postglacial time, the hazard from lahars or debris avalanches greatly exceeds that from lava flows and pyroclastic flows. Lahars, especially those resulting from slope failures of large volume, are dangerous because of their ability to move great distances downstream at speeds of tens of kilometers per hour. Beyond the slopes of the volcano, lahars are most dangerous along valley floors. If future magmatic eruptions are like those of the past, they will probably occur at vents on the flanks of the volcano and produce cinder cones and lava flows. Lava flows would probably extend no more than about 15 kilometers from their sources. Fallout of cinders from cinder cones could extend from several kilometers to tens of kilometers downwind. Eruptions producing pyroclastic flows, if they occur, are likely to produce tephra fallout for several tens of kilometers downwind. Because Mount Adams is situated in wilderness surrounded by National Forest and an Indian Reservation, flank eruptions are unlikely to endanger nearby communities. An eruption at or near the summit of Mount Adams, however, could rapidly melt snow and ice, causing floods and lahars to sweep far down valleys.
If Mount Adams becomes active, State and local officials need to collaborate with scientists in planning how to cope with an eruption. Such a plan might include restricting public access to potentially hazardous areas, establishing evacuation plans for areas likely to be threatened, and installing lahar- and flood-warning devices in endangered valleys. A network of seismic, geodetic, and visual monitoring needs to be established as quickly as possible in order to determine the possible nature and extent of hazards.
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