Mount Lassen (Lassen Peak)

 mountain, volcano, natural monument

A volcanic cone, last erupted in 1917. The summit is 10,462 feet (3,189 meters.)
The Lassen region has been volcanically active for more than 3 million years. The Lassen "volcanic center" began to erupt about 600,000 years ago. From 600,000 to 400,000 years ago, eruptions built a large volcano, often referred to as "Brokeoff Volcano" or "Mount Tehama". Later, this volcano became inactive and was mostly eroded away, leaving remnants that include Brokeoff Mountain, Mount Conard, Mount Diller, and Diamond Peak. Subsequent eruptions in the Lassen volcanic center have formed more than 30 steep-sided lava domes (the Lassen dome field). The most recently active parts of the volcanic center are Lassen Peak and other young domes formed in the past 50,000 years. Clynne, et.al., 2000

Lassen Peak Volcanic Dome
Lassen Peak is the largest of a group of more than 30 volcanic domes erupted over the past 300,000 years in Lassen Volcanic National Park in northern California. These mound-shaped accumulations of volcanic rock, called lava domes, were created by eruptions of lava too viscous to readily flow away from its source. Eruptions about 27,000 years ago formed Lassen Peak, probably within only a few years. With a height of 2,000 feet and a volume of half a cubic mile, it is one of the largest lava domes on Earth. When Lassen Peak formed, it looked much like the nearby 1,100-year-old Chaos Crags Domes, with steep sides covered with angular rock talus. However, from 25,000 to 18,000 years ago, during the last ice age, Lassen's shape was significantly altered by glacial erosion. For example, the bowl-shaped depression on the volcano's northeastern flank, called a cirque, was eroded by a glacier that extended out 7 miles from the dome. -- Clynne, 1998

Eruptive Activity During the Past 1,100 Years
Three episodes of volcanism have occurred at the Lassen volcanic center in the past 1,100 years. These are the complex eruption at Chaos Crags, the eruptions at Cinder Cone, and the summit eruptions of Lassen Peak in 1914-1917. -- Clynne, 1990, IN: Wood and Kienle

Eruptive Activity 1914-1917
The most recent eruptive activity occurred at Lassen Peak in 1914-1917 A.D.. This eruptive episode began on May 30, 1914, when a small phreatic eruption occurred at a new vent near the summit of the peak. More than 150 explosions of various sizes occurred during the following year. By mid-May 1915, the eruption changed in character; lava appeared in the summit crater and subsequently flowed about 100 meters over the west and probably over the east crater walls. Disruption of the sticky lava on the upper east side of Lassen Peak on May 19 resulted in an avalanche of hot rock onto a snowfield. A lahar was generated that reached more than 18 kilometers down Lost Creek. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 kilometers northeast of the summit. The eruption also generated lahars that traveled more than 20 kilometers down Lost Creek and floods that went down Hat Creek. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9 kilometers above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 kilometers to the east-northeast The fall of fine ash was reported as far away as Elko Nevada, more than 500 kilometers east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917. -- Hoblitt, et.al., 1987

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