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 of primarily of feldspar , pyroxene , and olivine
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Mount Tehama