Mount Lassen is considered an active volcano, even though it is not currently erupting. Located in the Cascade Range of Northern California, Lassen Peak is part of the larger Lassen Volcanic Center. Its “active” designation is based on its recent history of eruptions and the presence of an underlying magmatic system. The entire volcanic center is monitored closely by scientists for any signs of future unrest, stemming from its well-documented explosive period in the early 20th century.
How Geologists Classify Volcanic Activity
Geologists classify a volcano’s status based on its eruptive history, using terms like active, dormant, and extinct. The scientific definition for an “active” volcano is one that has erupted within the current geologic epoch, known as the Holocene (the last 11,700 years). Lassen Peak, having erupted in the last few hundred years, firmly fits this category.
The term “dormant” is often used for a volcano that is currently quiet but retains the potential to erupt again. Scientifically, a dormant volcano is considered a “potentially active” system that is simply between eruptions. Lassen is often described as dormant.
An “extinct” volcano is one that scientists believe is highly unlikely to erupt again because its magma plumbing system has solidified. Lassen Peak’s last eruptive sequence concluded in 1921, placing it indisputably within the active classification. The presence of active geothermal features like hot springs and steam vents within the National Park further confirms a functioning heat source remains beneath the surface.
The Eruptive History of Lassen Peak
Lassen Peak is a dacitic lava dome, the largest of its kind on Earth, and part of the broader Lassen Volcanic Center. The most recent and significant activity occurred over a seven-year period, establishing Lassen Peak as the most recently active volcano in the contiguous United States prior to the 1980 eruption of Mount St. Helens. This eruptive sequence began unexpectedly on May 30, 1914, with a steam explosion that created a small crater at the summit.
Over the next 11 months, more than 180 similar steam-driven explosions enlarged the summit crater. The eruption changed dramatically in May 1915 when new magma reached the surface, beginning with lava blocks seen bouncing down the mountain’s flanks. On May 19, 1915, a large explosion shattered the growing lava dome, creating an avalanche of hot rock and snow. The rapid melting of the deep snowpack transformed this avalanche into a massive mudflow, or lahar, which rushed four miles down the slope and traveled an additional seven miles along Lost Creek.
The most powerful explosion occurred on May 22, 1915, sending a column of ash and gas more than 30,000 feet into the atmosphere. A partial collapse of this column generated a swift, superheated pyroclastic flow that devastated a three-square-mile area on the northeast flank, now called the Devastated Area. This hot flow mixed with remaining snow, creating a second, larger lahar that traveled 15 miles down Lost Creek, flooding the lower Hat Creek Valley. Steam-driven explosions continued intermittently until 1921, marking the end of the last eruptive episode.
Current Monitoring and Signs of Unrest
Lassen Peak requires continuous monitoring by the U.S. Geological Survey (USGS) and the California Volcano Observatory (CalVO). This network tracks subtle changes in the volcano’s behavior to detect unrest that may precede a future eruption.
Monitoring Tools
- Seismometers track earthquake activity, particularly swarms of small, shallow earthquakes caused by the movement of magma or pressurized hydrothermal fluids.
- GPS stations and tiltmeters measure ground deformation, detecting slight swelling or sinking of the slopes as underground pressures change.
- Gas sensors monitor emissions like carbon dioxide and sulfur dioxide, which can increase significantly as magma rises closer to the surface.
Currently, the Lassen Volcanic Center is at the “Normal” alert level, exhibiting background, non-eruptive activity. This normal state includes small, frequent earthquake swarms typically caused by heated geothermal fluids rather than magma movement, which is common for a volcano with an extensive hydrothermal system. A sustained increase in seismicity, rapid ground swelling, or a substantial increase in volcanic gas output would signal a change in status, indicating the magmatic system is becoming restless.
Potential Hazards Associated with Lassen
A future eruption at Lassen Peak would likely involve hazards similar to those seen during the 1915 events, posing a localized but significant threat. The most destructive hazard is the lahar, a powerful mudflow of volcanic debris and water. Because Lassen Peak often has significant snowpack, hot volcanic material could rapidly melt the snow, generating lahars that travel great distances down creek beds and river valleys, including those draining into the Sacramento Valley.
Pyroclastic flows are fast-moving avalanches of hot gas, ash, and rock fragments that pose a serious danger near the volcano. These flows are generated by the collapse of an eruption column or a lava dome and destroy everything in their path. The third major hazard is ash fall, where fine particles are carried by high-altitude winds and deposited downwind.
While severe hazards like pyroclastic flows are concentrated near the summit, ash fall could impact communities hundreds of miles away, disrupting air travel and critical infrastructure. Even small eruptions, the most common type in the Lassen Volcanic Center, can produce basaltic lava flows and localized ash fall. The most hazardous areas are those immediately downhill from likely eruption sites, particularly the drainages that channel mudflows.