USGS Logo Geological Survey Bulletin 1292
The Geologic Story of Mount Rainier


An active volcano changes continually. Repeated eruptions build the cone by piling one lava flow on top of others, or on top of other volcanic formations. Simultaneously, the combined processes of erosion wear the volcano down. The relative importance of the two processes — one building, the other destroying — is reflected in the volcano's shape. The scarred and deeply gouged sides of Rainier's cone show that erosion has been dominant here for a longtime. Is Mount Rainier now doomed to continued piecemeal destruction until the lofty cone is reduced to a featureless mound? Will future eruptions of lava restore some of the volcano's bulk? Or will the volcano erupt violently some day, and then collapse as did Mount Mazama to form the deep basin of Crater Lake? The answers may not be known for centuries — or they may appear tomorrow.

Rampart Ridge
AN OLD LAVA FLOW from Mount Rainier which forms Rampart Ridge west of the meadow at Longmire. The thick lava flowed down an old valley floor and cooled and solidified. Rivers then eroded new valleys along both sides of the flow. These new valleys, subsequently glaciated, are today followed by the Nisqually River and Kautz Creek. Thus, the area of a former valley floor is now a ridge. (Fig. 21)

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Last Updated: 01-Mar-2005