California, known for its seismic activity, also hosts a significant number of volcanoes. The state is situated in a geologically active region, where tectonic plate movement creates conditions conducive to volcanic processes. Some of these volcanic features are actively monitored for signs of change.
California’s Volcanic Landscape
The presence of volcanoes in California is largely a consequence of plate tectonics, specifically its location along the Pacific Ring of Fire. This region is characterized by the interaction of several major tectonic plates, where the Pacific Plate and other smaller plates meet the North American Plate. In northern California, the Gorda Plate is subducting, or sliding beneath, the North American Plate, leading to volcano formation.
Areas within California also exhibit characteristics of divergent plate boundaries and hot spots, where magma rises closer to the surface. These diverse tectonic settings contribute to the variety of volcanic structures found across the state. California contains all five types of volcanoes: cinder cones, complex volcanoes, composite/stratovolcanoes, shield volcanoes, and spatter cone volcanoes.
Major Volcanic Sites
California features several notable volcanic areas, each with distinct geological characteristics.
The Long Valley Caldera, located in eastern California, is one of North America’s largest calderas, measuring approximately 20 miles long and 11 miles wide. Formed about 760,000 years ago by a massive eruption, this area displays ongoing hydrothermal activity, including hot springs and fumaroles.
Mount Shasta, a prominent stratovolcano in northern California, is the second-highest peak in the Cascade Range. It is composed of at least four overlapping cones and is the most voluminous stratovolcano in the Cascade Volcanic Arc. Its last recorded eruption occurred in 1786, and it features steam vents and several glaciers.
Lassen Peak, in Lassen Volcanic National Park, is a large lava dome that erupted from 1914 to 1917. An explosive event on May 22, 1915, spread ash hundreds of kilometers east. The park showcases four types of volcanoes, with Lassen Peak being one of the world’s largest plug dome volcanoes.
The Mono-Inyo Craters form a 25-mile-long chain of lava domes, flows, and explosion craters stretching from Mono Lake southward. These rhyolitic features have experienced eruptions within the last 600 years, including activity around Mono Lake between 1720 and 1850 A.D.
Further south, the Salton Buttes are five rhyolitic lava domes at the southern end of the Salton Sea. Associated with a large geothermal field, they represent some of California’s youngest volcanic features, formed between approximately 1,800 and 5,900 years ago.
Tracking Volcanic Activity
California’s volcanoes are under continuous observation by scientific agencies to detect signs of unrest. The U.S. Geological Survey (USGS) and the California Volcano Observatory (CalVO) monitor these sites across California and Nevada.
Scientists employ various instruments, including GPS receivers, seismometers, and multi-gas spectrometers, to track volcanic activity. These tools help detect ground deformation, seismic activity (earthquakes), and changes in gas emissions, which can precede an eruption by weeks or months. Continuous monitoring helps establish a baseline of normal activity, allowing scientists to identify shifts that might indicate volcanic unrest. This data helps issue timely notifications and alerts, ensuring communities and authorities are informed and prepared for potential volcanic events.