What defines a volcanic field? That's actually a tough question. In a general sense, a volcanic field is a geographical area that features a high concentration of volcanic vents that have their own magma supply, whether one-off eruptions of small scoria cones or repeated eruptions of a composite volcano. However, they're a little different from large volcanoes that have satellite or subsidiary vents; those are usually considered offshoots of a central volcanic system, while fields may have multiple sets of 'plumbing'. They may also have multiple compositions, ranging from basalt all the way to rhyolite. Another characteristic of volcanic fields is their area. Instead of just being confined to one location, volcanic fields may span hundreds or even thousands of square miles. 

California has multiple volcanic fields, both active and dormant or extinct. Monitored fields with documented eruptions in the last few thousand years include Clear Lake, Coso, and Mono Lake; older ones include the Golden Trout, Cima, Lavic Lake, and Big Pine (shown in the photo). Every one of these fields demonstrates a broad variability in eruption compositions and types. For example, the Mono Lake Volcanic Field began with a basalt eruption through the lake, and its most recent activity took place about 300 years ago when lake-bottom sediments forming much of Paoha Island were uplifted by the intrusion of a rhyolitic cryptodome. The Coso Volcanic Field also started with basaltic lava flows about 6 million years ago, then went on to add andesite, dacite, rhyodacite, and high-silica rhyolite eruptions between 4 million and 40,000 years ago.

So what's underpinning these fields? Because different types of magma require different storage timescales, it's not unusual for volcanic fields to have multiple potential magma sources - some shallow and long-lived, some deeper and active only for a short time. For example, the silica-rich magma storage (the 'felsite') feeding The Geysers geothermal area near Clear Lake Volcanic Field is thought to have have fed activity for millions of years, while the basalt or basaltic andesite dikes thought to have fed scoria cone and maar eruptions passed through the crust relatively quickly and left much less residual magma or heat. Residual heat from cooling silicic magma also fuels geothermal activity in the form of mudpots and fumaroles at the Coso Volcanic Field, making it another of California's prime sites for geothermal power production.

Monitoring volcanic fields is challenging, given their size and the dispersed nature of their eruptions. The next eruption might occur near the most recent ones, or it could develop elsewhere. Fortunately, we have a home-field advantage here in California: volcanic eruptions tend to occur in areas that are already seismically active, and our state's extensive seismic and geodetic (GPS) monitoring networks bring in data to help us keep an eye on both volcanic and seismic hazards. So the next volcanic field eruption won't be out of left field!