Lassen Peak, located in northern California, is an active volcano within the Cascade Range. Its classification as active stems from its recent eruptive history and ongoing scientific observation.
Understanding Volcanic Activity
Volcanoes are classified based on their eruptive history and potential for future activity. An “active” volcano is defined as one that has erupted within the Holocene epoch, which began approximately 11,700 years ago. This includes volcanoes currently erupting and those likely to erupt again, often termed “dormant.” A dormant volcano is not currently erupting but is expected to erupt in the future, implying a period of quiescence rather than a permanent cessation of activity. In contrast, an “extinct” volcano has not erupted in the Holocene and is not expected to erupt again, often because its magma supply has solidified or been cut off.
Lassen’s Volcanic Past
Lassen Peak’s active status is established by its eruptive sequence that began in 1914. After approximately 27,000 years of quiet, steam explosions commenced on May 30, 1914, creating a summit crater. Over the next 11 months, over 180 phreatic explosions enlarged this crater to about 1,000 feet across.
The eruption changed in mid-May 1915, when incandescent lava blocks bounced down the flanks. On May 14, a new lava dome began to form, filling the summit crater. This dome partially collapsed on May 19, sending a hot avalanche of rock down the north flank.
The most powerful event occurred on May 22, 1915, when an explosive eruption ejected rock and pumice, forming a deeper crater. This blast produced an ash and gas column that rose over 30,000 feet. The column’s partial collapse generated a pyroclastic flow—a mixture of hot ash, pumice, rock, and gas—which swept down the volcano’s side, devastating three square miles. This flow then transformed into a lahar, or mudflow, as it incorporated melted snow, extending 15 miles down Lost Creek. While the most significant eruptions concluded by 1917, steam explosions continued for several years, indicating persistent heat beneath the surface.
Monitoring Lassen Today
The Lassen Volcanic Center is one of the closely observed volcanoes in the United States. The U.S. Geological Survey (USGS) California Volcano Observatory (CalVO) is responsible for continuous monitoring to detect any changes that might signal renewed unrest. This comprehensive surveillance system aims to provide timely warnings and assess potential hazards.
Scientists use a variety of instruments to track the volcano’s behavior. Seismometers are deployed throughout Lassen Volcanic National Park to measure seismic activity, as earthquakes can indicate magma movement beneath the surface. Ground deformation is monitored using Global Positioning System (GPS) receivers and Interferometric Synthetic Aperture Radar (InSAR), which can detect subtle changes in the ground’s shape caused by subsurface pressure changes. Gas emissions from fumaroles and hot springs are also regularly sampled and analyzed for changes in composition or flux, which can signal increased volcanic activity.
While Lassen Peak is considered active, current data from these systems do not indicate any immediate signs of an impending eruption. This ongoing scientific oversight provides information about the volcano’s current status.