Mount Rainier[7] is a massive stratovolcano located 54 miles (87 km) southeast of Seattle in the state of Washington, United States. It is the most topographically prominent mountain in the contiguous United States and the Cascade Volcanic Arc, with a summit elevation of 14,411 feet (4,392 m).[1][2] Mt. Rainier is considered one of the most dangerous volcanoes in the world, and it is on the Decade Volcano list.[8] Because of its large amount of glacial ice, Mt. Rainier could potentially produce massive lahars that would threaten the whole Puyallup River valley.[9]

Mount Rainier, at 4392 m the highest peak in the Cascade Range, forms a dramatic backdrop to the Puget Sound region. Large Holocene mudflows from collapse of this massive, heavily glaciated andesitic volcano have reached as far as the Puget Sound lowlands. The present summit was constructed within a large crater breached to the northeast formed by collapse of the volcano during a major explosive eruption about 5600 years that produced the widespread Osceola Mudflow. Rainier has produced eruptions throughout the Holocene, including about a dozen during the past 2600 years; the largest of these occurred about 2200 years ago. The present-day summit cone is capped by two overlapping craters. Extensive hydrothermal alteration of the upper portion of the volcano has contributed to its structural weakness; an active thermal system has caused periodic melting on flank glaciers and produced an elaborate system of steam caves in the summit icecap. Reported 19th-century eruptions have not left identifiable deposits, but a phreatic eruption may have taken place as recently as 1894.