Lassen Peak (10,457 feet) is the largest plug dome volcano in the world and is the southernmost volcano in the Cascade Range. It was created on the destroyed northeastern flank of now gone Mount Tehama, a stratovolcano that was at least a thousand feet higher than Lassen. Lassen is located in north central California and is the centerpiece of Lassen Volcanic National Park.
Unlike most plug dome volcanoes, Lassen is topped by craters. In fact a series of these craters exist around Lassen's summit (although two of these are now covered by solidified lava).
Geology
Following a post-Tehama pause in activity around 18,000 years ago, Lassen Peak started to form as a dacite plug quickly pushed its way through Tehama's destroyed north-eastern flank. As the plug pushed its way up, it shattered overlaying rock, which formed a blanket of talus around the emerging volcano. Lassen rose and reached its present height in a relatively short time.
History
Lassen Peak was named in honor of Danish blacksmith Peter Lassen who guided immigrants pass the peak to Sacramento Valley in the 1830s. Lassen's trail, however, never found general long-term use because it was considered unsafe. Nobles Emigrant Trial, named after William Nobles, which linked Applegate Trail in Nevada to Northern Sacramento Valley, replaced it.
In 1864 Helen Tanner Brodt became the first woman to reach the summit of Lassen Peak. A tarn (lake in a cirque basin) on Lassen was named Lake Helen in her honor.
Starting in 1914 and ending in 1921 Lassen came alive with a series of phreatic eruptions (steam explosions), dacite lava flows, and lahars (volcanic mud flows). There were 200-400 volcanic eruptions during this period of activity but nobody was reported killed or seriously injured.
On May 15, 1915 a dacite flow oozed 1000 feet down the western side of Lassen before it hardened. Observers 20 miles away said it looked like the peak was boiling over for two hours. Heat from the lava melted much of the snowpack on the mountain and destroyed the deep crater lake atop the peak, which created a lahar that filled Lost Creek and then Hat Creek.
Then at 4:30 PM on May 22, 1915 a violent explosion (referred to as "the Great Explosion") ejected a very large mushroom-shaped cloud of ash thousands of feet skyward. A pyroclastic flow shattered and burned trees on the northwest slope and reactivated the lahar there. Together these events created the Devastated Area which is still sparsly populated by trees due to the low nutrient and high porosity of the soil there.
In April, May and June 1917 a series of dramatic steam and ash explosions emanated from the summit area and blasted out a fourth summit crater there (on the northwest corner).
Since then, the USGS in cooperation with the United States Park Service have been monitoring Lassen and other volcanic and geothermal areas in the park.