Lassen Peak's lava dome formed about 27,000 years ago from a series of eruptions over a few years, undergoing significant glacial erosion between 25,000 and 18,000 years ago. The bowl-shaped depression on the volcano's northeastern flank, called a cirque, was eroded by a glacier that extended out from the dome. By 18,000 years ago, Lassen Peak started to form a mound-shaped dacite lava dome, pushing its way through Tehama's destroyed northern flank. As the lava dome grew it shattered overlaying rock, which formed a blanket of angular talus around the emerging steep-sided volcano. Likely resembling the nearby 1,100-year-old Chaos Crags, Lassen Peak reached its present height in a relatively short time, probably in just a few years. Within the past 1,000 years or so, activity at Lassen Peak has produced six dacite lava domes, erupted tephra and pyroclastic flows, and built Cinder Cone and the Fantastic Lava Beds. It also created the rockfalls at Chaos Jumbles.