Mount Lassen, California’s southernmost active volcano in the Cascade Range, last experienced a major eruptive episode between 1914 and 1917. This was the last significant volcanic event in the Cascade Range until the 1980 eruption of Mount St. Helens. The climactic explosions occurred in May 1915. Lassen remains an active volcanic center with the potential for future eruptions.
The Start of the 1914-1917 Activity
The eruptive sequence began on May 30, 1914, with a steam-driven explosion near the summit of Lassen Peak. This initial blast was a phreatic eruption, caused by superheated groundwater flashing into steam as it encountered hot rock beneath the surface, rather than fresh magma. The explosion created a small new crater at the summit and ended an approximately 27,000-year period of quiet for the peak.
Over the following year, the volcano experienced more than 150 phreatic explosions. These persistent steam explosions gradually enlarged the summit crater to about 1,000 feet across. Magma was rising and heating the groundwater system, but the process had not yet breached the surface in a magmatic eruption. This period of steam-only eruptions continued until May 1915.
The Major Explosive Events of 1915
The eruption became magmatic in mid-May 1915, when incandescent blocks of lava were observed on the mountain flanks. By May 14, a dome of sticky dacite lava had welled up to fill the summit crater. The first major event of this new phase occurred on the evening of May 19, when a large explosion shattered the fresh lava dome.
The dome collapse resulted in an avalanche of hot rock and lava blocks that tumbled onto the deep snowpack covering the upper slopes. The intense heat rapidly melted the snow, generating a massive mudflow, known as a lahar, that traveled more than 11 miles down Lost Creek. This lahar caused flooding and damage in the surrounding valleys.
The climactic event took place on the afternoon of May 22, 1915. Lassen Peak exploded, hurling rock fragments and pumice high into the air, creating a volcanic ash column that rose more than 30,000 feet. The explosion also produced a pyroclastic flow—a fast-moving current of hot gas and rock—that swept down the mountain and devastated an area up to four miles away. Fine ash was carried by the wind and reported as far east as Elko, Nevada, over 300 miles away.
Current Status and Future Potential
Lassen Peak is still considered an active volcano, even though the last activity was a final series of steam explosions in 1917. The United States Geological Survey (USGS) and the California Volcano Observatory (CalVO) classify the Lassen Volcanic Center as a very high threat area. The continued presence of active steam vents, hot springs, and bubbling mudpots within Lassen Volcanic National Park indicates that a dynamic magmatic system remains beneath the surface.
Scientists closely monitor the area for signs of unrest using a network of instruments. Seismometers track earthquake activity, which signals magma movement. GPS receivers measure ground deformation, and gas emission sensors monitor the release and composition of volcanic gases. These tools serve as early warning indicators of a potential eruption.
While no immediate eruption is expected, the long-term geological record suggests that the Lassen area will erupt again. The time between major eruptions in this volcanic center can span centuries or even millennia, making the probability of an eruption in any given year low. Scientists closely monitor the mountain, understanding that the area’s history points toward future volcanic activity.