Volcanic Eruption - Glacier Peak

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Despite its elevation of 10,541 feet, Glacier Peak is a small stratovolcano. Its high summit is located atop a high ridge, but its volcanic portion extends only 1,600 - 3,200 feet above the underlying ridge.
Of the 5 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.
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 nearby rivers, filling there valleys with deep deposits. Subsequently the mudflows drained into both the North Fork of the Stillaguamish River and Skagit River.
Lahars
When lahars reach populated areas, they can bury structures and people. An example was the Armero tragedy at Nevado del Ruiz where 23,000 died from an enormous mudflow. Lahars from Glacier Peak pose a similar threat to the small communities of Darrington and Concrete and a lesser threat to the larger and rapidly growing towns of Mount Vernon and Burlington, as well as other communities along the lower Skagit and Stillaguamish Rivers.
A 2005 study conducted by the United States Geological Survey identified nine Cascade volcanoes, including Glacier Peak, as "very-high-threat volcanoes with inadequate monitoring".