Young, Explosive Eruptions at Clear Lake, California

Mount Konocti and the southeastern Clear Lake volcanic field, seen from the northern lakeshore near Glenhaven. Photo: Jessica Ball, U.S. Geological Survey, 2021.

How the volcanic field formed

Millions of years ago, as the San Andreas Fault began to form, three tectonic plates met at a triple junction in southern California — today's Mendocino triple junction. There, heat from the mantle passed easily into the crust and made magma. As the junction moved north along the coast, volcanic fields — clusters of volcanoes and volcanic rocks — erupted behind it: eight of them, from near the Mexican border to north of the San Francisco Bay area. The youngest volcanic rocks in western California are in the Clear Lake volcanic field in the north and in a field near the Salton Sea in the south.

Explosions of steam

Eruptions have happened around, and through, Clear Lake for 2 million years. The latest period of activity probably began about 40,000 years ago, with mainly explosive eruptions on and near faults in and around the lake. Where hot magma met groundwater, it released hot steam explosively, blasting out craters called maars and throwing ash, pumice and lava fragments into the air to fall back over the land.

• The deposits show in many places around the southeast end of the lake — especially in roadcuts — as repeated layers of ash, popcorn-sized pumice and rock.
• Some blasts threw football-sized lava bombs as far as 4 kilometers (2.5 miles) beyond the craters.
• The eruptions probably happened between 40,000 and 8,000 years ago, so they may have overlapped with ancient human occupation: archaeological evidence and Indigenous oral histories show that people may have watched some of them.

Maar craters on and around the peninsula below Buckingham Peak, seen from Mount Konocti. Photo: J. Ball, U.S. Geological Survey.

Where the maars are

At least 11 maar craters have been mapped, mostly at or very near the lakeshore. Many gave the lake its scalloped shoreline, and several hold their own bodies of water. At least seven formed on or near faults, which suggests the faults gave magma a path to the surface. The spread of maar deposits and the shape of the shoreline suggest that more craters may lie hidden under the water, at places such as Konocti Bay, Clearlake Oaks and the lake's northern shore.

Faults, maar craters and maar deposits (purple) around Clear Lake. Map: U.S. Geological Survey; geologic data from Bard and others (2022).

Looking south over Clear Lake to Mount Konocti, a collection of lava domes. Along the shore below, maar craters form the semicircular inlets of Soda Bay, Horseshoe Bend and Konocti Bay. Photo: J. Ball, U.S. Geological Survey.

How a maar eruption works

A maar eruption happens when rising magma meets groundwater. The magma heats the confined water above its boiling point; if the pressure on the rock holding it is suddenly relieved, or the water breaks out, it flashes to steam. The steam expands explosively, shattering the magma and blasting a circular crater at or below the ground. (Maar comes from a 19th-century German dialect word first used for circular lakes.)

This phreatomagmatic activity — magma interacting with water — happens where groundwater is plentiful. Falling lava, pumice and ash build tuff rings or tuff cones around the crater. Maar eruptions are also known for angled jets of material that look like rooster tails, for lava bombs, and for pyroclastic density currents — violently turbulent, ground-hugging clouds of ash, gas and lava fragments.

They are violent, but usually not large:

The eruption cloud rising from the east Ukinrek Maars crater, Alaska, April 6, 1977. Photo: R. Russell, U.S. Geological Survey.

Reading the layers

Maar deposits are pebble- to boulder-sized pieces of lava, local (country) rock, pumice and volcanic ash, laid down in layers that drape the ground. The material is thrown out as ballistics, falls from the plume, or is carried by pyroclastic density currents, and the layers pinch and swell around a crater, showing where jets and flows concentrated debris.

The type, size and sorting of the fragments and the structures within a deposit answer questions such as: was the eruption driven mainly by steam, or did it involve magma? Did the fragments fall from the sky or tumble along the ground? How much energy did it release? In the wall of one of the Konocti Bay craters, geologists note changes in the size, composition, thickness and shape of the layers and combine them into an account of the eruption.

Layers in a maar deposit hold different amounts and sizes of ash, pumice, lava and local rock. The mostly pumice layer came from eruptions of mostly fresh magma; the layers with local rock were probably made by crater-excavating explosions. The measuring stick is 1 meter (3.3 feet) long. Photo: J. Ball, U.S. Geological Survey.

A 9-meter (30-foot) outcrop of layered maar deposits on Kono Tayee Point, on the north side of Clear Lake. Photo: J. Ball, U.S. Geological Survey.

Eruption history

Eruptions in the field fall into four general periods, the youngest beginning about 40,000 years ago. Styles ranged from explosive to effusive (flowing), building small mountains, long lava flows and towering lava domes. Radiocarbon dates from organic material in the maar deposits show that the maars formed mainly in the last 20,000 years — young compared with the rest of the field.

Could it erupt again?

The USGS considers the field active: radiocarbon dates put maar eruptions within the last 10,000 years, and The Geysers geothermal field lies 25 kilometers (15 miles) to the southeast, so there is still magma below. Geophysical surveys suggest magma at about 5 kilometers (3.1 miles) beneath parts of the field, and future intrusions and eruptions are possible. Another maar eruption could endanger people around the lake.

• Most known maar deposits lie right beside the lake, but some pyroclastic density currents and tephra fall may have reached 5 kilometers (3.1 miles) from their vents.
• At Ubehebe Craters, a maar volcano in Death Valley, research shows pyroclastic density currents may travel as far as 15 kilometers (9.3 miles).
• Based on eruptions such as Ukinrek Maars, plumes 6,000 meters (20,000 feet) or more high could disrupt the region's people, infrastructure and transportation — and affect major airports such as Oakland and San Francisco.

These are potential hazards. As of 2024 there were no clear signs of volcanic unrest in the seismic, gas or ground-deformation data of the California Volcano Observatory, which watches the area with seismometers, satellite data, and frequent sampling of gas vents and springs. Work continues on how recent, and how violent, the youngest eruptions were.

Sources

Based on Jessica L. Ball, Seth Burgess and Dawnika Blatter, "Young explosive eruptions from the Clear Lake volcanic field," U.S. Geological Survey Fact Sheet 2025–3020, U.S. Geological Survey; a work of the United States government in the public domain. The photographs and map are reproduced from the fact sheet.