Hot springs and fumaroles dot this misnamed geothermal field, home to the world’s largest complex of underground heat-trapping power plants.
A large patch of magma, rich in silicon dioxide, broke through the earth’s crust under Coastal ridge of northern California about 1.3 million years ago. Today, this shallow body of rock is still hot, and the 45-square-mile (120-square-kilometer) area above it is known as Geysers. It is the world’s largest energy-producing geothermal field.
Although nearby Clear Lake Volcanic Field still erupting several thousand years ago, this geothermal field never had geysers. Geysers is a misnomer that originated in the 19th century with settlers in the area who misunderstood the hot springs and fumaroles that boil and flow in Big Sulfur Creek Canyon. From the middle of the 19th to the middle of the 20th century, a popular resort hotel operated here. Native residents of the area visited the springs since prehistoric times.
Located about 8 miles (13 kilometers) northeast of Geyserville, the site is now home to 18 power plants that use steam to drive turbines to generate electricity. The white roofs of several power plants are visible in this natural-color satellite image taken by the Operational Land Imager (OLI) on Landsat 8 on January 10, 2022.
Steam turbines in the area can generate 725 megawatts of electricity, enough to power a city the size of San Francisco. Geyser Power Plants typically supply the power needs of Sonoma, Lake, and Mendocino counties, as well as parts of Marin and Napa counties. As of 2018, turbines in the Geyser region produced 50% of California’s geothermal energy.
Geothermal energy is produced by extracting heat from inside the Earth either by natural steam or very hot water. It is a reliable renewable energy source. It has the advantage of being available whether the sun is shining or the wind is blowing.
Three elements are needed to produce geothermal energy: magma close to the surface, fractured or permeable rock, and fluids that can circulate through the heated rock. In the geysers, the rock that erupted more than a million years ago lies just 0.6 miles (1 kilometer) below the surface. The deeper parts, below 1.5 miles (2.5 kilometers), can reach temperatures of 750 °F (400 °C).
While most geothermal power plants operate from reservoirs, according to US Geological Survey, the Geyser system is one of only two steam-dominated systems in the world. This means that the formation directly produces steam that can be used more efficiently to produce electricity.
A small steam generator was first used to generate electricity here in the 1920s, and the first modern geothermal well was drilled in 1955. Over the next several decades, further drilling and development by various operators brought the Geysers to their peak production in 1987. At that time, 21 power plants with a total capacity of more than 2,000 megawatts were operating. After that, power production began to decline as the steam tank began to leak.
In the mid-1990s, plant operators turned to a practice called advanced geothermal systems (EGS) to maintain energy production and extend field life. Water is injected at high pressure to reopen natural cracks in the rock and allow hot water or steam to flow into the well. Today there are two pipelines deliver treated wastewater to refill the reservoir from Lake County and the City of Santa Rosa.