Snow and ice-covered Mount Baker, located in northern Washington, is the highest peak in the North Cascades (3,286 m or 10,781 ft) and the northernmost volcano in the conterminous United States. It is the only U.S. volcano in the Cascade Range that has been affected by both alpine and continental glaciation. The stratovolcano is composed mainly of andesite lava flows and breccias and was largely formed prior to the most recent major glaciation (Fraser Glaciation), which occurred between about 25,000 and 10,000 years ago.
The most recent major eruption at Mount Baker, about 6,500 years ago, was accompanied by a major flank-collapse event that produced a lahar that reached Bellingham Bay 100 km (60 mi) away via the Nooksack River. In 1975-76, Sherman Crater, immediately south of the summit, exhibited signs of renewed volcanic activity as a result of magma intruding into the volcano but not erupting. This activity resulted in monitoring that was more intense than previously applied at any other Cascade Range volcano and produced important baseline data against which recent research has been compared. Sherman Crater has been the site of increased steam emission since 1975.