Lassen Peak, Plug dome volcano in Shasta County, United States.

Lassen Peak is a dacite lava dome volcano in Shasta County, northern California, reaching 3,187 meters (10,457 feet) according to 1981 data. Its upper slopes are bare tan and brown rock with a darker outcrop on the left and another mass right of the rounded summit, while snow patches lie across the top. Dense dark green conifers cover the lower slopes.

The eruption began in May 1914 and produced lahars, pyroclastic flows, and ash ejections over three years that destroyed surrounding forests. This volcanic activity led to the establishment of Lassen Volcanic National Park in 1916 to protect the area and its geological features.

The peak is named after Peter Lassen, a Danish blacksmith who led settlers from the east toward the Sacramento Valley around 1830; his route was soon abandoned as unsafe and replaced by the Nobles Emigrant Trail. Before settlers arrived, the area east and south of the mountain was Maidu tribal territory. Helen Tanner Brodt became the first woman to climb the summit in 1864, and a cirque lake below bears her name.

The ascent starts from a parking area near the main road and follows a well-marked trail that switchbacks to the summit. Hikers should start early to avoid afternoon thunderstorms and bring warm clothing, as temperatures drop noticeably with elevation gain.

Lassen Peak is crowned by several craters near its summit, two of which are buried under solid lava; the 1917 steam and ash explosions opened a fourth crater on the northwest side. The volcano is dormant, not extinct: a magma chamber still lies beneath it, and the California Volcano Observatory monitors the peak with sensors.