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Geology and History Summary for Glacier Peak

Glacier Peak (3214 m, 10544 ft) is a stratovolcano composed mainly of dacite, which is located about 100 km (65 mi) northeast of Seattle and 110 km (70 mi) south of the International Boundary with Canada. It lies in the rugged and scenic Glacier Peak Wilderness Area, in the Mt. Baker-Snoqualmie National Forest and rises only a few thousand feet above neighboring peaks that reach 2,700 m altitude. From coastal communities Glacier Peak appears as a mere high point along a snowy saw-toothed skyline. Volcanism occurs at Glacier Peak and other volcanoes in the Cascades arc due to subduction of the Juan de Fuca plate off the western coast of North America.

The age of the volcanic cone of Glacier Peak itself is poorly known. The early eruptive deposits have been eroded by subsequent glaciation and buried by younger volcanic domes. The oldest lava flows known to have originated from Glacier Peak cap ridges north and east of the volcano and have been radiometrically dated to between 200,000 and 600,000 years old. Since the continental ice sheets receded from the region approximately 15,000 years ago, Glacier Peak has erupted repeatedly during at least six episodes. Two of these eruptions were among the largest in the Cascades during this time period.

All the domes and dome fragments at Glacier Peak are aligned approximately north-south across the summit of the volcano. This suggests that a north-south structural trend has controlled the emplacement of the domes for at least the last several thousand years.