Lassen Peak, a prominent volcanic dome in Northern California, is the southernmost active volcano within the Cascade Range. The mountain is the main attraction of Lassen Volcanic National Park and remains a focus of geological study. The last major eruption sequence at Lassen Peak began in 1914, with the most powerful and destructive events occurring in May 1915, and activity diminishing by 1917.
The 1914 to 1917 Eruptive Sequence
The eruptive episode commenced on May 30, 1914, with a small phreatic (steam-driven) explosion near the summit. This initial event created a new vent and marked the beginning of a year-long phase characterized by more than 150 similar, non-magmatic steam blasts. These explosions were caused by superheated groundwater flashing to steam as it interacted with hot rock below the surface.
The activity escalated significantly in May 1915, signaling a shift from steam-driven to magmatic eruptions. By mid-May, highly viscous dacite lava had welled up to fill the summit crater, forming a temporary lava dome. This unstable mass partially collapsed on May 19, sending an avalanche of incandescent blocks down the mountain’s upper eastern flank.
The hot avalanche descended onto a deep snowfield, instantly melting the snow and generating a massive lahar (volcanic mudflow). This destructive flow traveled over 18 kilometers down Lost Creek, devastating the landscape in its path. This event served as a precursor to the climactic explosion that followed three days later.
The most powerful explosion occurred on May 22, 1915, blasting rock fragments and pumice high into the atmosphere. The massive vertical column of ash and gas reached an altitude of over nine kilometers above the vent. A partial collapse of the column resulted in a devastating pyroclastic flow that swept six kilometers down the volcano’s northeast side, flattening three square miles of forest.
The explosion also generated additional lahars that flowed over 20 kilometers down both Lost Creek and Hat Creek drainages. Fine volcanic ash from the eruption column drifted eastward, with measurable deposits reported as far away as 500 kilometers. Intermittent, less intense eruptions continued sporadically until the sequence concluded around the middle of 1917.
Understanding Lassen Peak’s Geological History
Lassen Peak is classified as a Pelean-type plug dome volcano, created by the slow, viscous extrusion of magma. It is situated on the northern end of the Cascade Volcanic Arc, which extends from British Columbia to Northern California. The mountain itself arose from the remnant northern flank of the much older ancestral stratovolcano, Mount Tehama.
The lava that forms Lassen Peak is dacite, a silica-rich magma that is highly viscous. This high viscosity traps volcanic gases, causing pressure to build until it is released in explosive eruptions, such as the one in 1915. Lassen Peak was constructed quickly about 27,000 years ago during a period of intense activity.
The broader Lassen Volcanic Center has a history of activity spanning more than 825,000 years, featuring multiple volcanic domes and vents. For example, the Chaos Crags dome complex, located a few kilometers north of Lassen Peak, experienced its last major eruption about 1,100 years ago. These events demonstrate the long-term, episodic nature of volcanism in the region.
Current Volcanic Status and Monitoring
Lassen Peak is considered an active volcano, meaning it has the potential to erupt again, although its current status is quiescent. The United States Geological Survey (USGS) designates the Lassen Volcanic Center with a “Very High” threat potential. This designation is based on the likelihood and potential impact of future eruptions, highlighting the necessity of continuous surveillance.
The California Volcano Observatory (CalVO), part of the USGS, maintains a robust monitoring network throughout the area. This network includes seismometers that continuously track minor earthquakes, which can signal the movement of magma or fluids beneath the surface. While small earthquake swarms are common, they typically occur within the expected background range.
Scientists also use GPS stations to measure ground deformation, which indicates swelling or deflation as magma or gas pressures change. Geothermal features in the national park, such as the fumaroles and hot springs in Bumpass Hell, are monitored for changes in temperature and gas composition. These measurements provide subtle, but observable, signs of the magmatic system’s activity.
Despite the historical activity and the high threat potential, the volcano’s current alert level remains at “NORMAL,” with an Aviation Color Code of GREEN. This classification confirms that the volcano is in a non-eruptive state and that all monitored parameters are within established background levels. The comprehensive monitoring system is designed to detect unrest weeks to months before any potential eruption.