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Geology and History Summary for Mount Rainier

Mount Rainier is an active volcano of the Cascade Range in Washington State, 50-70 km (30-44 mi) southeast of the Seattle–Tacoma metropolitan area. Volcanism occurs at Mount Rainier and other Cascades arc volcanoes because of the subduction of the Juan de Fuca Plate off the western coast of North America.

Mount Rainier is not the first volcano to have grown in its present location. The edifice of modern Mount Rainier assembled over the last half million years by the accumulation of hundreds individual lava flows, but an ancestral Mount Rainier stood in the same place from 1 to 2 million years ago. Before this, magmas both erupted and accumulated beneath the surface. Some of this magma solidified as the 18- to 14-million-year-old Tatoosh Granodiorite, which formed when large volumes of magma rose into the subsurface and slowly cooled. Volcanoes were probably also fed by these magma bodies, but about 10 million years ago, the western margin of North America was uplifted and erosion stripped volcanic rocks overlying the granodiorite. Parts of Rainier also sit upon three older formations of volcanic rocks and associated sediments: Fife's Peak (26 to 22 Ma), Stevens Ridge (26 Ma), and Ohanapecosh (36 to 28 Ma). About 40 million years ago, much of what is now western Washington lay beneath the sea, and widespread volcanism began due to the process of subduction off the margin of North America. The volcanoes of the Ohanapecosh Formation were tall enough to stand above sea level.

Age measurements of lavas and ashes from Mount Rainier show that the most recent lava flows erupted close to 2,200 years ago, and that pyroclastic flows erupted as recently as 1,100 years ago. Although there are several 19th century reports of dark clouds at the summit, interpreted by observers as small eruptions, no ash or other related volcanic deposits have been found to confirm such recent activity. Scientific examination of sparse pumice formerly thought to have erupted between 1820 and 1850, instead shows that this is 2,200 year old pumice. Mount Rainier has been active for the last 500,000 years; so the 2,200–year interval since the last known lava eruption is less than half a percent of the lifespan of the volcano.

Mount Rainier is chiefly made of andesite and some dacite lava flows and has erupted sizeable amounts of pumice throughout its history, though not as voluminously or as frequently as Mount St. Helens. Pyroclastic flows are a relatively minor component of Mount Rainier's eruptive products, and lava domes are almost unknown because its andesitic magmas are more fluid than the stickier dacite magmas of Mount St. Helens. Some portions of the upper volcano host sizeable areas of hydrothermally altered rock, but most of these rocks collapsed 5,600 years ago to form a massive lahar, known as the Osceola Mudflow. Two craters, each about 0.4 km (0.25 mi) across, emanate steam and are not eroded by the large summit glaciers are evidence of the volcano’s youth. The recentness of eruptions and the frequency of those recent eruptions indicate that Mount Rainier is an active volcano that will erupt again.