According to the USGS, there are more than 500 volcanic vents in California. The great majority of these are located in the northernmost portion of the State where the southern end of the Cascade Range extends across the Oregon border. Seven hundred miles long, the Cascade Volcanic Arch began to form 37 million years ago as the Juan de Fuca Plate and its associated microplates (all remnants of the Farallon Plate) were subducting beneath the North American Plate; this process continues today as demonstrated by the spectacular eruption of Mt. St. Helens in 1980.
The Cascades of California result from the subduction of the Gorda Plate, a southern fragment of the Juan de Fuca Plate. Mt. Shasta (14,179 feet) is second in size among modern Cascade volcanoes (exceeded only by Mt. Rainier) and, over the past 4000 years, has been the second most active of the major Cascade volcanoes (exceeded only by Mt. St. Helens). Lassen Peak (10,461 feet), which last erupted from 1914-1917, sits among 30 volcanic domes and peaks within its own National Park and Medicine Lake Volcano, a large shield volcano northeast of Mt. Shasta, hosts Lava Beds National Monument on its northeast flank.
Until the Juan de Fuca and associated Plates have been completely recycled (shutting off the fuel for volcanism), the Cascades will continue to form and erupt. In addition, the entire coast of the Pacific Northwest (from British Columbia to Northern California) will remain susceptible to earthquakes and tsunamis, generated along the subduction trench.