Mount Tehama, also called Brokeoff Volcano, is the deeply eroded remnant of an extinct andesitic stratovolcano in Lassen Volcanic National Park and a major ancient center of the Cascade Volcanic Arc. Active roughly between 590,000 and 387,000 years ago, it grew into a massive volcano before hydrothermal alteration weakened its core and glaciers and streams cut it away, exposing its interior. Surviving remnants include Brokeoff Mountain, Mount Conard, Mount Diller, Pilot Pinnacle, and Diamond Peak. Mount Tehama also marks the shift of volcanism toward Lassen Peak, a process that continued into recent geologic time and helped shape the region’s later volcanic history.