Part of the Pinnacles-Neenach volcanic field lies on the west side of the San Andreas Fault and part on the east — and the two halves are now about 300 kilometers apart. Western California's volcanic rocks, their locations, ages and chemistry, tell the story of how the state's plate boundaries changed over the past 40 million years.
Three kinds of plate boundary
Earth's crust is broken into plates of two kinds: oceanic crust, generally thinner, denser, younger and rich in iron and magnesium; and continental crust, generally thicker, older, less dense and rich in silicon and potassium. Plates ride on the mantle and meet at boundaries where they move toward, away from or past each other. California has all three types today:
| Boundary | Where | What happens |
|---|---|---|
| Divergent | offshore of northern California, to the northwest | the Pacific and Juan de Fuca plates move apart |
| Convergent | just offshore of northern California | the Juan de Fuca plate subducts beneath the North American plate |
| Transform | along much of the state | the Pacific and North American plates slide past each other, along the San Andreas and many other faults |
Where the Juan de Fuca, Pacific and North American plates meet, in northern California, is the Mendocino triple junction.
The three kinds of plate boundary around California: divergent (A), where the Pacific and Juan de Fuca plates move apart; convergent (B), where the Juan de Fuca plate subducts beneath North America; and transform (C), the San Andreas Fault. D marks the Mendocino triple junction. Illustration: U.S. Geological Survey.
How the San Andreas was born
- By about 28 million years ago, part of the spreading ridge between the Farallon and Pacific plates had reached North America and begun to subduct beneath it, consuming the Farallon plate.
- By 20 million years ago, much of the Farallon plate was gone, leaving the Juan de Fuca and Cocos plates as its remnants. The Pacific plate came into oblique contact with North America, creating a transform boundary — the beginnings of the San Andreas Fault — and the Mendocino triple junction.
The slab window
As a spreading ridge is subducted, the leading edge of the sinking plate falls away, leaving a gap — a "slab window" — in the oceanic crust beneath the continent. Hot mantle rises to fill it and can partly melt as it rises, and it can also melt the edges of the sinking slab or the base of the continental crust above. Magma from a slab window has a chemistry, and a location near the transform boundary, that tell geologists it did not come from an ordinary subduction zone. As the Pacific and North American plates slide past each other, the triple junction and its slab window migrate northwest. Western California's volcanic fields are the product of ridge subduction and melting above that window.
The volcanic fields
- Eight volcanic fields stretch from near the Mexican border to about 90 kilometers north of Sacramento, oldest near the middle of the state and youngest toward the ends of the San Andreas Fault.
- Many kinds of rock and eruption: small isolated flows, large lava domes, the eroded roots of volcanoes, ash from eruptions that spread across California, Hawaiian-style lava flows, cinder cones and maar craters. Some are reduced to scattered remnants; others, like thick lava flows and steep cinder cones, still look fresh.
- Torn apart by faults: earthquakes on faults like the San Andreas — famous for its damaging 1906 and 1989 earthquakes — have carried parts of some fields far from the rest, as at Pinnacles-Neenach.
- The most likely places for future eruptions are the youngest fields, Clear Lake and Salton Buttes, both active within the past 10,000 years.
Western California's eight volcanic fields: 1 Salton Buttes, 2 Morro-Islay, 3 Pinnacles-Neenach, 4 Quien Sabe–Burdell Mountain, 5 Berkeley Hills–Tolay, 6 Sonoma, 7 Clear Lake and 8 Sutter Buttes. Map: U.S. Geological Survey, base modified from Stanley and others (2002).
The U.S. Geological Survey's California Volcano Observatory closely monitors the parts of western California with the greatest potential for volcanism. Understanding when, where and how past eruptions happened helps scientists judge the potential for future ones and the hazards they might bring.
Sources
Based on "Plate Tectonics and Volcanism in Western California," U.S. Geological Survey Fact Sheet 2025–3013, a work of the United States government in the public domain. Its illustration and map are reproduced from the fact sheet.
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