Glacier Peak and Mount St. Helens are the only two volcanoes in Washington that have generated large, explosive ash eruptions. Their explosive history results from the type of magma they produce. Dacite, the magma of Mount St. Helens and Glacier Peak, is too viscous to flow easily out of the vent. It must be pressed out, like toothpaste. The pressure of millions of internal gas bubbles then break it apart into tiny fragments known as "ash".
About 13,100 years ago, a few millenia after continental glaciers had retreated from the Pacific Northwest, Glacier Peak began an eruptive sequence that produced nine ash eruptions in six hundred years. The largest expelled more than three times as much ash as the 1980 eruption of Mount St. Helens, and was the second largest in the Cascades in postglacial times.
Since 12,500 years ago, Glacier Peak has produced only a few ash eruptions, all of small volume.
In the past fourteen thousand years, Glacier Peak has generated major eruptions during two episodes; one at 13,100-12,500 years ago, and the other at 6,300-5,900 years ago. At least seven smaller eruptions have occurred in the past 2,000 years.
The total number of eruptions produced by Glacier Peak in the last 14,000 years, both big and small, makes it one of the more active volcanoes in the Cascades. Yet Glacier Peak produces big eruptions relatively infrequently. The probability is therefore fairly low, perhaps one in a few hundred, that we will see a big eruption in our lifetimes.
If a large eruption were to occur, lahars would present the greatest danger. They could inundate river valleys as far downstream as Puget Sound. Voluminous pyroclastic-flow deposits in the headwater of streams around Glacier Peak may be redistributed by streamflow for years or decades, burying developed areas, periodically blocking transportation routes, reducing the capacity of river channels, and increasing heights of floods. The low-lying Skagit flood plain and river delta would be most impacted by large events. Smaller lahars would impact primarily upstream areas, including perhaps the town of Darrington.
Airborne ash would represent the second greatest hazard. Small ash eruptions or ash clouds from pyroclastic flows could disrupt local air traffic and deposit up to a half inch of ash in nearby towns of Chelan and Leavenworth. Ash from eruptions comparable to Glacier Peak's largest could collapse roofs in nearby communities, and stall transportation throughout much of the Pacific Northwest.
Landslides, pyroclastic flows, and lateral blasts (of the type that killed 57 people at Mount St. Helens in 1980) would impact mainly the wilderness area surrounding the mountain. However lahars generated by large landslides or pyroclastic flows could damage downstream communities well outside wilderness areas.
Although the probability of a large eruption occurring in our lifetimes is low, the consequences of such an eruption could be severe. With some low-cost, common sense measures, we can minimize the damage those events would cause. Those measures could include, for example, consideration of potential hazards when siting schools, hospitals, or other important structures in potentially impacted areas.
Earthquakes below Glacier Peak are currently monitored as part of a statewide network of seismic stations. If Glacier Peak were to reawaken, the U.S. Geological Survey would rapidly deploy additional instruments, and would work with federal, state, and local officials to evaluate developments and advise the public.