Scientific examination of sparse pumice formerly thought to have erupted between 18, instead shows that this is 2,200 year old pumice.
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. The volcanoes of the Ohanapecosh Formation were tall enough to stand above sea level.Īge 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. 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. 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). 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. 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. Before this, magmas both erupted and accumulated beneath the surface. 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. Mount Rainier is not the first volcano to have grown in its present location. 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 an active volcano of the Cascade Range in Washington State, 50-70 km (30-44 mi) southeast of the Seattle–Tacoma metropolitan area.
Mount Rainier, Washington with flowers from Paradise, Mount Rainier National Park.
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