Geothermal 101

Copyright © Graphics Courtesy of Geothermal Education Office

The Basics of Geothermal

Geo (earth) thermal (heat) energy is a clean, renewable and reliable source of power that reduces our nation’s dependence on fossil fuels.   Geothermal power plants use the earth’s natural heat to generate electricity that is environmentally sound and sustainable.







What is geothermal energy?

Copyright © Graphics Courtesy of Geothermal Education Office

Here’s a simple description of how it works:   Water far beneath the surface that is heated by hot rocks. This steam and hot water then collects in underground reservoirs, if the proper geologic conditions are present. Geothermal reservoirs are common along areas where the earth’s tectonic plates meet or an area referred to as the “Ring of Fire.”   In the United States, geothermal reservoirs are common in the west, including Alaska and Hawaii.

In order to use this hot water to generate electricity, wells are drilled into the reservoirs which might be hundreds or thousands of feet under the earth’s surface.

The geothermal production wells then move the hot fluid to the power plant and convert the thermal energy of the water to electrical energy. After providing its heat for production, the geothermal fluid is injected into the reservoir.

To gain more specific information about how our facilities work visit this page.

Geothermal steam vents naturally from the earth in the Long Valley Caldera.

The Benefits and Limitations of Geothermal Power

Electricity from geothermal resources is clean, safe and sustainable. In addition to saving fossil fuels and reducing air emissions, geothermal facilities use minimal land space and can be designed to blend in with their environment. They also provide reliable, baseload power that is not dependent on the weather. Geothermal facilities provide significant economic benefits to their local communities in the form of property taxes, lease payments and royalties. In addition, they offer good paying jobs and regularly purchase products and services from other local businesses.

With all these advantages, you might wonder why there aren’t more geothermal power plants. Unfortunately, they can’t be built just anywhere. There must be a proven geothermal resource and the facility must be located near that resource.   It’s also important to note that geothermal facilities require a substantial up-front capital investment that involves exploration to confirm the geothermal resource as well as purchasing equipment and building the pipelines and power plants.



Mono County’s Geothermal Resources

Mono County, California is the home of the Long Valley Caldera, which covers an area of approximately 10 miles by 20 miles.   The caldera was formed about 730,000 years ago by a cataclysmic volcanic eruption. This eruption ejected an estimated 150 cubic miles of material. (This can be compared to the 1980 eruption of Mount St. Helens in Washington State, which ejected an estimated 0.67 cubic miles of material.)

Numerous more recent eruptions formed the Inyo volcanic chain.   The resurgent dome was created by magmatic activity and continues to grow today.

The geothermal system in the caldera is most noticeable in surface expressions such as steam fumeroles and mudpots on the flanks of the resurgent dome. The Casa Diablo Geothermal Field supplies the Mammoth Geothermal Complex power plants with hot geothermal fluid and is located at the southwest edge of the resurgent dome.   It is thought that the heat source for this geothermal system is under the volcanic domes in the northwest part of the caldera.

In general, subsurface water flows from west to east in the caldera.   Rain and snow melt from the Mammoth Mountain area and the western boundary of the caldera flows down through the caldera ring fractures.  

At depths greater than 3,500 feet, the water is heated by contact with hot basement rocks to about 450 degrees Fahrenheit.   The hot water begins to flow upward through faults from beneath the Rhyolite Plateau and flows eastward toward Casa Diablo.

The geothermal fluids eventually flow out at the surface at springs in the eastern part of the caldera. These include fish hatchery springs, Little Hot Creek, Hot Creek and Crowley Lake.

The production wells at Casa Diablo are completed at the relatively shallow depth of about 450 feet. These production wells use pumps to move the hot water that is flowing into the western portion of the field to the power plants. The cooler geothermal fluid, after being utilized by the power plants, is injected back into the reservoir via wells on the east side of the field. The injection wells are approximately 2,000 feet and intersect a deep fault zone.

Production well at Mammoth Pacific







Mammoth Geothermal Complex’s Geothermal Leases

Long Valley Caldera Faults, Springs and Fumaroles 

Mammoth Geothermal Complex does not own all the land where the geothermal fluids flow. Instead, we enter into lease agreements with local landowners that allow us to produce geothermal fluid from the reservoirs beneath their land. Mammoth Geothermal Complex pays royalties to the landowners - with an overwhelming majority of Mammoth Geothermal Complex’s production being performed on federal lands managed by the Bureau of Land Management (BLM). Approximately 25 percent of the royalties paid by Mammoth Geothermal Complex to the BLM is routed back to Mono County’s general fund.

Through payments to the federal government, Mammoth Geothermal Complex maintains additional leases on lands in the Mammoth area that are not currently in geothermal production. Similar to Mammoth Geothermal Complex’s royalty payments, a portion of these lease payments is also routed back to Mono County.

These federal leases, as well as the national Geothermal Steam Act, require that Mammoth Geothermal Complex diligently explore the leased lands for geothermal resources for possible commercial production. As a part of that effort, Mammoth Geothermal Complex is working on the CD IV development project.


Long Valley Caldera Faults, Springs and Fumaroles