Plinian–style explosive volcanic activity at Glacier Peak took place during two intervals about 13,000 ago. Glacier Peak has erupted at intervals of about 500 to 2,000 years during the past 6,000 years and at least twice during the past 2,000 years. Eruptions of 2,000 and 1,100 years ago generated fans of pyroclastic flow deposits near the volcano and lahar assemblages downstream.
13,000 years ago
Shortly after glaciers retreated at the end of the most recent ice age, about 13,000 years ago, Glacier Peak generated a sequence of six explosive eruptions. Two of the eruptions were Plinian and one ejected more than five times as much tephra as the May 18, 1980 eruption of Mount St. Helens. That eruption was one of the largest in the Cascade Range since the end of the last ice age. Tephra deposits associated with these volcanic events are more than a foot thick near Chelan, Washington and an inch thick in Western Montana. These eruptions also produced voluminous pyroclastic-flow deposits and lahars that extended to the Puget Sound along the Stillaguamish River valley.
Between 5,000 and 7,000 years ago
Eruptions of Glacier Peak resumed by about 7,000 years ago and continued periodically until some time before about 5,000 years ago. These eruptions included lava flow and dome growth followed by flow/dome collapse, pyroclastic flows, fine-grained ashfalls associated with pyroclastic flows, sporadic small explosive eruptions, and lahars. These deposits are called the Dusty assemblage. This eruptive period is poorly constrained in time but appears to have continued off and on for several hundred years to as long as 2,000 years. During this time, thin tephra layers were deposited to the east and at least 300 km (190 mi) northwest of Glacier Peak into Canada. One or more lava domes grew and pyroclastic flows filled valleys east and west of the volcano, and lahars extended far down the Sauk, Suiattle, and Skagit River valleys.
2,400 to 3,400 years ago
At least two eruptive episodes occurred during this interval. Lava dome growth, dome collapse, pyroclastic flows, and laharscharacterized the eruptions. Around 3,000 years ago lahars travelled as far as 30 km (18 mi) down the White Chuck drainage. These lahars likely occurred as a result of lava dome growth and associated pyroclastic eruptions melting snow and ice to produce slurries that travelled down the valley.
2,000 years ago
About 2,000 years ago, Glacier Peak erupted to produce a lava dome and pyroclastic flows derived from repeated collapse of the growing dome. The pyroclastic flows and associated lahars to flowed into the upper the Suiattle River where they formed a thick fill between Chocolate Creek and Vista Creek valleys. At about the same time, lahars extended at least 15 km (10 mi) down the White Chuck River valley, probably as a result of freshly erupted pyroclastic debris melting snow and ice. Sometime after 1,800 years ago, a large lahar, possibly triggered by a landslide high on the volcano, traveled at least 30 km (19 mi) down the White Chuck River.
1,100 years ago
Approximately 1,100 years ago a sequence of eruptions produced lahars and pyroclastic flows that inundated several valleys both east and west of Glacier Peak. Additionally at about this time, lahars flowed down the White Chuck River valley and moved at least 30 km (19 mi) downstream from the volcano to about 15 km (10 mi) upstream of Darrington.