Geology and History

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.

Timeline of eruptions at Mount Rainier.

We know more about the recent volcanism at Mount Rainier because deposits postdate extensive glaciation and therefore are well preserved.

The modern glaciers that grace the slopes of Mount Rainier are much smaller descendants of the vast ice masses that repeatedly overwhelmed the Cascade Range during Pleistocene time.

Mount Rainier is behaving about as it has over the last half-million years, so all evidence suggests that the volcano will continue to erupt, grow, and collapse.

Lahars are common at Mount Rainier, because its mantle of snow and ice provides water when melted, and parts of the upper flanks of the volcano contain abundant loose, weak, hydrothermally altered rock.

Almost annually, torrential rain, glacial outbursts, and water-saturation of steep debris-covered slopes cause debris flows at Mount Rainier.

Mount Rainier National Park, established on March 2, 1899, includes 980 square kilometers (378 square miles) of rugged terrain on the west slope of the Cascade Mountains in central Washington.