Summary

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The volcano has produced a larger volume (about 300 km³ or 70 mi³) of eruptive material during the past million years than any other Cascade stratovolcano except Mount Shasta (about 350 km³ or 85 mi³). Mount Adams lies in the middle of the Mount Adams volcanic field—a 1,250 km² (about 500 mi²) area comprising at least 120, mostly basaltic volcanoes that form spatter and scoria cones, shield volcanoes, and some extensive lava flows. The volcanic field has been active for at least the past one million years. Mount Adams was active from about 520,000 to about 1,000 years ago and has erupted mostly andesite. Eruptions have occurred from ten vents since the last period of glaciation about 15,000 years ago. Approximately 6,000 and 300 years ago, debris avalanches from the southwest face of Mount Adams generated clay-rich lahars that swept more than 30 km² (11 mi²) south of the volcano along the White Salmon River. The summit of Mount Adams contains a large section of unstable altered rock that can spawn future debris avalanches and lahars.