Mount Adams, one of the largest volcanoes in the Cascade Range, dominates the Mount Adams volcanic field in Skamania, Yakima, Klickitat, and Lewis counties and the Yakima Indian Reservation of south-central Washington. The nearby Indian Heaven and Simcoe Mountains volcanic fields lie west and southeast, respectively, of the 500 square mile Adams field.
Even though Mount Adams has been less active during the past few thousand years than neighboring Mount St. Helens, Ranier, and Hood, it assuredly will erupt again. Future eruptions will probably occur more frequently from vents on the summit and upper flanks of Mount Adams than from vents scattered in the volcanic fields beyond. Large landslides and lahars that need not be related to eruptions probably pose the most destructive, far-reaching hazard of Mount Adams.
Mount Adams is second in eruptive volume only to Mount Shasta, and it far surpasses its loftier neighbor Mount Rainier (which is perched on a pedestal of Miocene granodiorite). There have been no recorded eruptions of Mount Adams, and, of the 11 Holocene vents, none is known certainly to have erupted products younger than 3,500 years. Where exposed in glacial headwalls, the 4-square-kilometer rotten core is a persistent source of avalanches and debris flows; the longest traveled 40 kilometers after breaking loose approximately 5,000 years ago, creating the southwest notch and shelf for the perched White Salmon Glacier.
The most serious tephra hazards in the region are due to the proximity of Mount St. Helens, the most prolific producer of tephra in the Cascades during the past few thousand years. Even though Mount Adams is a meager tephra producer, the region around Mount Adams 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.
During many volcanic eruptions, fragments of lava or rock are blasted into the air by explosions or carried upward by a convecting column of hot gases. These fragments fall back to earth on and downwind from their source vent to form a pyroclastic-fall or "ash" deposit. Pyroclastic-fall deposits, referred to as tephra, consist of combinations of pumice, scoria, dense-rock material, and crystals, that range in size from ash (less than 2 millimeters) through lapilli (2-64 millimeters) to blocks (greater than 64 millimeters). Eruptions that produce tephra range from short-lived weak ones that eject debris only a few meters into the air, to cataclysmic explosions that throw debris to heights of several tens of kilometers. Explosive eruptions that produce voluminous tephra deposits also commonly produce pyroclastic flows.
Close to an erupting vent, the main hazards to property posed by eruptions of tephra include high temperatures, burial, and impact of falling fragments; large falling blocks can kill or injure persons who cannot find shelter. Significant property damage can result from the weight of tephra, especially if it is wet, and 20 centimeters or more of tephra may cause structures to collapse. Hot tephra falling near a volcano may set fire to forests and structures. Farther away, the chief danger to life is the effect of ash on the respiratory system. Even 5 centimeters of ash will stop the movement of most vehicles and disrupt transportation, communication, and utility systems. Machinery is especially susceptible to the abrasive and corrosive effects of ash. These effects, together with decreased visibility or darkness during an eruption, may further disrupt normal transportation, communication, and electrical services; they can also result in psychological stresses and panic among people whose lives may not be endangered.