Mount Rainier's lava is either andesitic or basaltic. These lavas have formed features such as hexagonal columns and ridges and are mostly light to dark gray, although some areas are black or red.
The United States Geological Survey considers Mount Rainier to be among the most dangerous volcanoes in the nation. While volcanic ash, lava flows and pyroclastic flows of mixed rock and gases could cause great damage, the greatest potential for loss of human life is in Rainier's proclivity for forming lahars. The risk of these volcanic mudflows is high because of the unstable geology of the mountain's slopes and Rainier's snow and glacier cover, which could melt rapidly during an eruption. About 150,000 people live along the paths of former lahars.