Because geologically recent volcanic activity in an area is the best guide to forecasting future eruptions, scientists study the lava flows, ash, and other deposits from past eruptions. Volcanoes in the Lassen area tend to erupt infrequently, and may be inactive for periods lasting centuries or even millennia. The most recent eruptions in the Lassen area were the relatively small events that occurred at Lassen Peak between 1914 and 1917. The most recent large eruption produced Chaos Crags about 1,100 years ago. Such large eruptions in the Lassen area have an average recurrence interval of about 10,000 years. However, the geologic history of the Lassen area indicates that volcanism there is episodic, having periods of relatively frequent eruptions separated by long quiet intervals. For example, the last large event before the Chaos Crags eruption was the one that built Lassen Peak 27,000 years ago.

A series of large avalanches possibly triggered by steam explosions have occurred on the north side of the Crags at least 300 years ago. These avalanches were of the "air cushion" type and developed speeds exceeding 100 miles per hour to form a wilderness of debris, the Chaos Jumbles, which covers an area of 2 1/2 square miles. Manzanita Lake was formed as a result of Manzanita Creek being dammed by the debris. Reflection Lake and Lily Pond are depressions in this debris.

Credits and Sources:

“Lassen Volcanic: Lassen’s Geology,” National Park Service,http://www.nps.gov/lavo/learn/kidsyouth/upload/Lassen-s-Geology.pdf, Accessed on June 29, 2015.