Glacier Peak - Eruptive History

Eruptive History

Despite its elevation of 10,541 feet (3,213 m), Glacier Peak is a small stratovolcano. Its relatively high summit is a consequence of its location atop a high ridge, but its volcanic portion extends only 1,600–3,200 feet (500–1,000 m) above the underlying ridge. Another Cascade Arc volcano with similar geomorphology is the Mount Meager massif in southwestern British Columbia, Canada, which is situated on a 1,300 ft (400 m) ridge of nonvolcanic, crystalline and metamorphic rock.

Of the five major volcanoes in Washington, only Glacier Peak and Mount St. Helens have had large eruptions in the past 15,000 years. Since both volcanoes generate magma of dacitic origin, the viscous magma builds up since it cannot flow through the eruptive vent. Gradually, the pressure grows, culminating in an explosion that ejects materials such as tephra, which in its simplest form, is ash.

Tephrochronology and radiocarbon dating indicate that Glacier Peak eruptions occurred in 1700 AD ± 100 years, 1300 AD ± 300 years, 900 AD ± 50 years, 200 AD ± 50 years, 850 BC, 3150 BC, and in 3550 BC. The Volcanic Explosivity Index (VEI) for three of these was 2 to 4, small compared to the 5 of the 1980 eruption of Mount St. Helens. They were characterized mainly by a central vent eruption, followed by an explosive eruption. These eruptions varied in outcome; some produced lahars, some pyroclastic flows, and others lava domes.

A little more than 13,000 years ago, a sequence of nine tephra eruptions occurred within a period of less than a few hundred years. Associated with these eruptions were pyroclastic flows. Mixed with snow, ice and water, these formed lahars that raced into three nearby rivers, filling their valleys with deep deposits. Subsequently the mudflows drained into both the North Fork of the Stillaguamish River (at that time an outlet of the Sauk River) and Skagit Rivers. In Arlington, 60 miles (97 km) downstream, lahars deposited seven feet of sediment. Subsequent erosion of lahar deposits near Darrington led to the current river system with the Stillaguamish River separated from the Sauk/Skagit Rivers. Lahar debris was deposited along both the Skagit and Stillaguamish Rivers all the way to Puget Sound. A small portion of the erupted tephra was deposited locally. However, most of the tephra reached higher levels of the atmosphere, and was transported by the wind hundreds of miles. Deposits from this congregation were as thick as 1 foot (0.30 m) near Chelan and 0.3 inches (7.6 mm) near Missoula, Montana.

Since these events, Glacier Peak has produced several lahars. The largest events were 5,900 and 1,800 years ago and were associated with dome-building eruptions. In both cases, the lahars traveled down the Skagit River to Puget Sound.