About 13,100 years ago, Glacier Peak generated a sequence of nine tephra eruptions within a period of less than a few hundred years. The largest ejected more than five times as much tephra as the May 18, 1980, eruption of Mount St. Helens and was one of the largest in the Cascade Range since the end of the last ice age.

Some of the tephra from these eruptions fell back onto the volcano and avalanched down its flanks. Much of the rest rose high into the atmosphere and drifted hundreds to thousands of miles downwind. Deposits from these eruptions are more than a foot thick near Chelan, Washington, and an inch thick in western Montana.

Tephra deposit from a Glacier Peak eruption 13,100 years ago, at a site about 10 miles south of Glacier Peak. Overlying this deposit is an ash bed from an eruption at Crater Lake, Oregon, 7,800 years ago. Photo by R.B. Waitt, USGS.

Total thickness of tephra (inches) erupted from Glacier Peak during a series of large eruptions about 13,100 years ago. Light blue indicates approximate area covered by ash during these eruptions.

Since these events, Glacier Peak has produced several tephra eruptions, all of much smaller volume.

Lava Domes Collapsed onto the Volcano’s Flanks

During most of Glacier Peak’s eruptive episodes, lava domes have extruded onto the volcano’s summit or steep flanks. Parts of these domes collapsed repeatedly to produce pyroclastic flows and ash clouds. The remnants of prehistoric lava domes make up Glacier Peak’s main summit as well as its ãfalse summitä known as Disappointment Peak. Pyroclastic-flow deposits cover the valley floors east and west of the volcano. Ridges east of the summit are mantled by deposits from ash clouds.

Glacier Peak from the east, showing the main summit and Disappointment Peak, which are remnants of prehistoric lava domes. Photo by Austin Post, USGS.

A collapsing lava dome generates a pyroclastic flow and ash cloud at Colima Volcano, Mexico, November 22, 1998. Photo by Abel Cortes, (c)1998, University of Colima.

Lahars Inundated River Valleys

Past eruptions have severely affected river valleys that head on Glacier Peak. Pyroclastic flows mixed with melted snow and glacial ice to form rapidly flowing slurries of rock and mud known as lahars.

About 5,900 years ago and 1,800 years ago, dome-building eruptions generated lahars that extended once again to the sea, this time only along the Skagit River. In small eruptions since 1,800 years ago, lahars have extended the entire length of the White Chuck River and part way down the Suiattle.

More than 200 homes and over 120 miles of roads were destroyed by the 1980 lahars at Mount St. Helens. Pictured is a damaged home along the South Fork Toutle River. Photo by Lyn Topinka, USGS.

A few hundred yards upstream from its confluence with the Sauk River, the White Chuck River erodes into lahar deposits produced by prehistoric eruptions of Glacier Peak. Photo by R.B.Waitt,USGS.

Lahars can also be generated by landslides (also called flank collapses) on volcanoes, as has happened repeatedly at Glacier Peak ’s neighbor to the north, Mount Baker. At Mount Baker, lahars from numerous landslides, some without accompanying eruptive activity, have affected valley floors near the volcano. A few much larger landslides during eruptive periods generated lahars that flowed hundreds of feet deep through upper valleys and reached the sea. At Glacier Peak landslide-generated lahars have occurred less frequently than at Mount Baker.

DURING FUTURE ERUPTIONS . . .

Glacier Peak ’s eruptive episodes are typically separated by several hundred to a few thousand years. Thus in any given year, the probability of a new episode beginning is roughly one in a thousand. It is unlikely that we will see an eruption within our lifetimes. If one does take place, its impact would vary dramatically in different geographic areas depending on the size of the eruption, wind direction, and type of hazards produced.

In undeveloped areas near the volcano, the landscape would be severely altered by lava domes, pyroclastic flows, ash clouds, lahars, and associated phenomena.

In river valleys downstream from the volcano, lahars could block transportation routes, destroy highways and bridges, bury houses in mud, cover farmland with debris, choke river channels, and increase the severity of floods for years or decades after the eruptions stop. These effects will be most frequent in the White Chuck and upper Suiattle River valleys. They will be less frequent, but potentially more damaging due to greater population and infrastructure, in the Sauk and Skagit River valleys. Still less likely would be lahars in the Stillaguamish River valley, which would occur only if the Sauk River became choked with enough debris to be diverted west into the Stillaguamish River valley. When a lahar takes place, residents of communities along these rivers should move to high ground as quickly as possible.

In areas downwind from the volcano, even small tephra eruptions could disrupt air and ground transportation and dust towns with ash. Tephra could clog drainage ducts and ventilation filters, short-circuit power transformers, damage machinery and electronic equipment, reduce visibility, exacerbate respiratory ailments, and stall transportation. Large tephra eruptions (comparable to Glacier Peak ’s largest) would have more widespread effects and could deposit enough tephra to collapse roofs in nearby downwind communities. Owing to prevailing wind patterns, tephra fall during future eruptions is most likely east of Glacier Peak. But tephra could affect communities in all directions from the volcano depending on wind patterns during an eruption. Five tephra eruptions from Mount St. Helens in 1980, for example, deposited ash north, east, west, and south of the volcano. Damage from tephra can be mitigated by such actions as shutting down and covering equipment, frequently replacing air filters in machinery, wearing dust masks, and avoiding unnecessary travel.

click here for larger version of this map Areas at risk from lahars, lava domes, pyroclastic flows, and associated phenomena from Glacier Peak. Map modified from R.B. Waitt and others, U.S. Geological Survey Open-File Report 95-499.

Annual probability of tephra fall exceeding 0.5 inch thick from an eruption of Glacier Peak. Communities east of the volcano are more susceptible to tephra fall because the wind is normally from the west. Glacier Peak has produced large tephra eruptions, but not frequently.

PREPARING FOR THE NEXT ERUPTION

Future eruptions from Glacier Peak will almost certainly be preceded by an increase in earthquake activity, and possibly by measurable swelling of the volcano and emission of volcanic gases. In cooperation with the USGS, the University of Washington ’s Geophysics Program continuously monitors earthquakes that could portend Glacier Peak ’s next eruption. The USGS also works with Federal, State, Provincial, and local agencies to prepare for disruption that might accompany renewed activity. A coalition of these agencies, known as the Mount BakeröGlacier Peak Facilitating Committee, has drafted a plan outlining how agencies will work together in the event of unrest at Mount Baker or Glacier Peak. If Glacier Peak were to reawaken, the USGS would rapidly deploy additional monitoring instruments and, together with these agencies, establish a local volcano observatory and command center that would keep nearby communities informed of developments.

WHAT YOU CAN DO

• Learn about the volcano hazards that could affect your community, and determine whether you live, work, play, or go to school in a volcano-hazard zone.

A few moments spent now could help prevent the next eruption from becoming a disaster for you, your family, and your community.