Mount Rainier is a powerful, dynamic volcano built during the past 500,000 years by lava from thousands of eruptions. Gray and reddish rock on the mountain records those flows, which cooled to form the present volcano. During those eruptions, glaciers far larger than today’s enveloped Mount Rainier and worked together with volcanic forces to shape the terrain. Present-day ridges formed where molten material built up between glaciers, with lava flows channeled along either side of thick ice. At this location, the ground stands at the toe of a lava flow that, while still hot, ponded against the edges of converging glaciers. Mount Rainier has repeatedly undergone sudden and drastic change. Mud, ash, and ice have surged in enormous flows, while landslides, glaciers, rivers, and other geologic forces have also broken the mountain down. Similar active and unstable terrain is likely in the future and can dramatically affect the people and land of the Pacific Northwest.