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Much of Mount Rainier is composed of andesite lava flows. Lava flows may accompany explosive eruptive activity, but they occur more often after explosive activity declines. The term "andesite" refers to the chemical composition of the rock. Andesitelavas tend to be moderately viscous and rather slow moving: on gentle slopes, they may move much more slowly than a person can walk. Although people and animals can escape them, lava flows destroy everything in their paths either by fire, impact, or burial.
The primary hazard to people from lavaflows is low, but a more serious hazard arises when such flows come into contact with snow and ice. The result is rapid melting, which is capable of generating floods and lahars. Some lahars from Mount Rainier may be the indirect products of lava flows.
The only lava flows known to have been erupted from Mount Rainier in the past 10,000 years are those that built the summit cone, which was constructed within the past 5,600 years. Young lavas that erupted from the new summit cone widely underly the Emmons and Winthrop Glaciers, supporting their smooth, continuous slopes up to the summit. Locally the solidified lava shows through the ice as dark rock ribs.