California is widely known for its seismic activity, but the state also sits atop a dynamic landscape of active volcanoes. Several volcanic centers are currently monitored for potential unrest. This volcanic presence is a direct result of the state’s unique position at the convergence of multiple tectonic plates. Understanding the nature of this volcanism is important for appreciating the full scope of California’s geologic hazards.
What Defines Active Volcanism
Geologists define an active volcano as one that has erupted within the last 10,000 years, corresponding to the Holocene epoch. The term “active” means the volcano has the potential to erupt again, not that it is currently erupting. A volcano that has not erupted in this timeframe but is still capable is called dormant. A volcano is considered extinct only when its magma supply is completely cut off.
Modern classification also considers volcanoes active if they display signs of ongoing unrest, such as increased seismic activity or gas emissions. These subtle changes indicate that magma is moving beneath the surface, even if a full eruption is not underway. Consequently, several volcanic centers in California are classified as active because of their relatively recent eruptive history and persistent subsurface thermal activity. This distinction helps scientists prioritize which systems require the most intensive monitoring efforts.
California’s Major Volcanic Centers
California hosts at least seven volcanic centers designated by the U.S. Geological Survey as having a high or very high threat potential. These areas include diverse volcanic structures, from towering stratovolcanoes to massive calderas. The Lassen Volcanic Center represents the southernmost extent of the Cascade Range volcanoes and last erupted explosively in 1915. This center contains Lassen Peak, a large plug dome volcano, and exhibited activity as recently as the early 20th century.
Mount Shasta, a prominent stratovolcano, is another very high threat area in the northern part of the state. It is a large, cone-shaped mountain built up by layers of lava and ash, with its last confirmed eruption occurring around 1786. Further south, the Long Valley Caldera, located near Mammoth Mountain, is a large, collapsed depression formed by a massive eruption approximately 760,000 years ago. Although its last major activity was 16,000 to 17,000 years ago, the region exhibits ongoing unrest through ground deformation and earthquake swarms.
The Medicine Lake Volcano, located northeast of Mount Shasta, is a broad shield volcano with a caldera at its summit. Its last eruption was about 950 years ago, and it is classified as a high threat center. Additionally, the Mono-Inyo Craters, a chain of cinder cones and lava domes stretching north from Long Valley, last erupted around 300 to 600 years ago. These regions collectively demonstrate California’s capacity for various types of volcanic hazards, including lava flows, ash fall, and pyroclastic flows.
The Tectonic Engine Driving California Volcanism
The presence of active volcanism in California is directly linked to the state’s complex relationship between the Pacific and North American tectonic plates. In northern California, the Cascadia Subduction Zone is the primary driver for volcanoes like Mount Shasta and the Lassen Volcanic Center. Here, the small Gorda Plate is sliding beneath the North American Plate, a process that introduces water into the mantle. This water lowers the melting point of the rock, generating magma that rises to the surface, forming the Cascade volcanic arc.
A different mechanism is responsible for the Long Valley Caldera and the surrounding volcanic fields. This region experiences extensional or “transtensional” tectonics, where the crust is being pulled apart in an area known as the Eastern California Shear Zone. As the crust stretches and thins, the underlying hot mantle material rises closer to the surface, reducing the pressure and causing it to melt. This rifting-related volcanism contrasts with the subduction-driven activity to the north.
Current Monitoring and Risk Assessment
The U.S. Geological Survey (USGS) operates the California Volcano Observatory (CalVO) to continuously track the state’s most hazardous volcanic centers. CalVO employs a network of instruments to detect the subtle precursory signs of volcanic unrest. Ground deformation is monitored using GPS receivers and tiltmeters, which can detect small changes in the shape of the volcano as magma moves beneath the surface. Seismometers record the frequency and intensity of earthquakes, which often increase as magma breaks rock on its path upward.
Scientists also use gas sensors and thermal imaging to measure changes in the release of volcanic gases and heat from vents and fumaroles. This comprehensive monitoring allows the USGS to issue timely warnings using a four-tiered alert system for volcanic activity. The system uses color codes to communicate the current level of unrest and the potential for an eruption to the public and emergency managers:
- Normal (Green)
- Advisory (Yellow)
- Watch (Orange)
- Warning (Red)