California sits at the junction of several tectonic plates, a dynamic geologic setting that creates both dramatic earthquakes and the conditions for volcanism. The state’s volcanic activity is a direct consequence of the Cascadia Subduction Zone in the north and the complex system of faults and rifts elsewhere. While eruptions are infrequent on a human timescale, they are a normal and recurring feature of California’s deep geologic history. Understanding the timing and nature of past events helps scientists prepare for future possibilities.
Identifying California’s Last Volcanic Event
California’s most recent documented volcanic activity occurred at Lassen Peak, located in the northern part of the state. The eruptive episode began on May 30, 1914, with a series of small steam-driven explosions known as phreatic eruptions. These initial events happened when rising magma heated shallow groundwater, causing it to flash to steam and blast a new vent through the summit.
The activity escalated over a year, with more than 150 explosions occurring, until the character of the eruption changed in May 1915. A dacite lava dome filled the summit crater, and on May 19, an avalanche of hot rock and snow created a destructive mudflow, or lahar, that traveled more than 18 kilometers down Lost Creek. The most powerful explosion took place on May 22, 1915, producing a pyroclastic flow that devastated a six-kilometer area northeast of the peak. Intermittent, less intense eruptions continued until the middle of 1917, marking the end of the last volcanic event in California.
Major Volcanic Centers in California
While the Lassen Volcanic Center is the site of the most recent activity, California hosts several other significant volcanic areas that remain geologically active. The U.S. Geological Survey (USGS) identifies seven volcanoes with high to very high threat potential in the state. These centers are geographically dispersed and vary widely in their structure and history.
Mount Shasta, a prominent stratovolcano in Siskiyou County, is ranked as a very high threat due to its size and history of producing both lava flows and destructive lahars. Its last eruption is estimated to have occurred only 200 to 300 years ago. Further south, the Long Valley Caldera, located near Mammoth Lakes, is a massive depression formed by a super-eruption about 760,000 years ago. This region includes the Mono-Inyo Craters, a chain of smaller vents that saw its most recent eruption on Paoha Island in Mono Lake approximately 300 years ago.
The Clear Lake Volcanic Field, situated north of San Francisco, is not a single cone but a collection of vents known for its extensive geothermal activity. The heat from this field powers The Geysers, the world’s largest geothermal energy facility, with the most recent activity occurring between 8,500 and 13,500 years ago. The Coso Volcanic Field in the Mojave Desert has not erupted in about 1,800 years but exhibits numerous small earthquakes and hot springs, indicating ongoing underground heat.
Current Monitoring and Hazard Status
The task of tracking California’s restless volcanoes falls to the U.S. Geological Survey’s California Volcano Observatory (CalVO). CalVO monitors sixteen volcanoes that are considered potentially active to ensure public safety and provide early warnings. The observatory uses a variety of methods to detect the subtle changes that precede an eruption.
Monitoring involves tracking seismicity, as earthquake swarms can indicate magma moving beneath the surface. Scientists also use GPS and tiltmeters to measure ground deformation, which is the subtle swelling or sinking of the earth’s surface caused by the pressure of rising magma. Changes in the release and composition of volcanic gases, like sulfur dioxide and carbon dioxide, are also tracked using specialized spectrometers at vents and fumaroles.
CalVO communicates the current activity level using a four-tiered system of Volcano Alert Notices. The base level is “Normal,” signifying typical background activity in a non-eruptive state. If elevated unrest is detected, the status moves to “Advisory,” then to “Watch” for escalating unrest, and finally to “Warning” if a hazardous eruption is underway or imminent.