The Geologic Story of Mount Rainier / A look at the geologic past of one of America's most scenic volcanoes

Mount Rainier, also known as Tahoma or Tacoma, is a large active stratovolcano located 59 miles (95 km) south-southeast of Seattle, in Mount Rainier National Park. With a summit elevation of 14,411 ft (4,392 m), it is the highest mountain in the U.S. state of Washington, and of the Cascade Range of the Pacific Northwest, it is the most topographically prominent mountain in the continental United States and the first in the Cascade Volcanic Arc.

Mt. Rainier is considered one of the most dangerous volcanoes in the world, and it is on the Decade Volcano list. Because of its large amount of glacial ice, Mt. Rainier could produce massive lahars that could threaten the entire Puyallup River valley. "About 80,000 people and their homes are at risk in Mount Rainier’s lahar-hazard zones."

Mount Rainier is a stratovolcano in the Cascade Volcanic Arc that consists of lava flows, debris flows, and pyroclastic ejecta and flows. Its early volcanic deposits are estimated at more than 840,000 years old and are part of the Lily Formation (about 2.9 million to 840,000 years ago). The early deposits formed a "proto-Rainier" or an ancestral cone prior to the present-day cone. The present cone is more than 500,000 years old.

The volcano is highly eroded, with glaciers on its slopes, and appears to be made mostly of andesite. Rainier likely once stood even higher than today at about 16,000 ft (4,900 m) before a major debris avalanche and the resulting Osceola Mudflow approximately 5,000 years ago. In the past, Rainier has had large debris avalanches, and has also produced enormous lahars (volcanic mudflows), due to the large amount of glacial ice present. Its lahars have reached all the way to Puget Sound, a distance of more than 30 mi (48 km). Around 5,000 years ago, a large chunk of the volcano slid away and that debris avalanche helped to produce the massive Osceola Mudflow, which went all the way to the site of present-day Tacoma and south Seattle. This massive avalanche of rock and ice removed the top 1,600 ft (500 m) of Rainier, bringing its height down to around 14,100 ft (4,300 m). About 530 to 550 years ago, the Electron Mudflow occurred, although this was not as large-scale as the Osceola Mudflow.

After the major collapse approximately 5,000 years ago, subsequent eruptions of lava and tephra built up the modern summit cone until about as recently as 1,000 years ago. As many as 11 Holocene tephra layers have been found.