Brokeoff Volcano represents the Pleistocene component of the Lassen volcanic complex, an area of active volcanism for more than three million years. It formed during a period of eruptive activity approximately 600,000 years ago on a series of faults; the largest of these eruptions was about 50 times as powerful as the 1980 eruption of Mount St. Helens. During the Pleistocene, constantly renewed lava of andesite flowed from the central vent of the volcano. The eruption of overlapping andesitic lava flows accumulated to build the volcano. By the time it reached its peak size, the volcano was immense, measuring at its base between and wide and more than tall. Activity at Brokeoff lasted for approximately 200,000 years. Eruptions varied from ejection of pyroclastic material to andesitic lava flows built primarily of feldspar, pyroxene, and olivine.