Beautiful Mammoth Mountain, as seen from US Highway 395 during winter.
Volcanoes in California expected to erupt again
The state of California has numerous volcanoes, and the largest of them all is Long Valley Caldera. Often called a supervolcano, it doesn’t quite meet the official criteria, although it’s close enough that after Earth’s three true supervolcanoes, this is one of the next-largest on the planet.
California has volcanoes in the Cascades Range in the north, in the Mojave Desert in the south, and in a few other locations, and the ones discussed on this page are on the eastern side of the Sierra Nevada Mountains near the border with Nevada. Near this caldera, which is considered dormant due to not having erupted since 50,000 years ago, there are other volcanoes that are considered active, which have erupted within the past few hundred years.
After discussing the other volcanoes in the surrounding area, Long Valley Caldera is discussed last below, including what scientists know about possibilities for future eruptions.
This article uses some basic volcanology terms including caldera, cinder cone, and others. For definitions and explanations, see my article Types of Volcanoes and Volcanology Terms.
Trees killed by volcanic gases emitted from Mammoth Mountain.
This volcano’s peak reaches 11,053 feet (3,369 meters), and it is a lava dome complex that lies close to the southwest rim of Long Valley Caldera, although is considered to be a completely separate volcano, and also separate from the nearby Mono-Inyo Craters, which are discussed below.
The most eruptive activity of any type was 700 years ago. These were phreatic eruptions, which occur when magma heats water and an explosion of steam occurs, which can be mixed with ash, rock, and pieces of lava. The last that magma erupted from this volcano was about 57,000 years ago.
Associated with Mammoth Mountain are two cinder cones to the southwest called the Red Cones, which last erupted about 8,900 years ago. Further west from the mountain and northwest of Red Cones is Devil’s Postpile, which is a fascinating volcanic feature formed 100,000 years ago, that is now a national monument.
Mammoth Mountain is famous for its ski areas. The nearest town with a hotel is Mammoth Lakes, just a few miles away.
Because of onging unrest in recent decades with earthquakes, gas emissions, and uplift of the ground, USGS closely monitors Mammoth Mountain, and also Long Valley Caldera.
Volcanic rock formations (called "tufa") in Mono Lake.
Mono Lake Volcanic Field
To the west is Yosemite National Park. To the south is the Mono-Inyo Craters, and then Mammoth Mountain.
Mono Lake is a saltwater lake, and it is actually saltier than the ocean. Within Mono Lake and on its north shore are volcanic vents. Islands within the lake are lava domes. The most recent volcanic activity occurred about 300 years ago underneath the surface of the water and formed Paoha Island.
The Inyo Craters, part of a volcanic chain extending from Mono Lake in the north southward to Mammoth Mountain, and near the western edge of Long Valley Caldera. 
The Mono-Inyo Craters
An 18-mile (29 km) chain of lava domes, lava flows, and craters extends between Mono Lake to the north and Mammoth Mountain to the south. The northern portion closest to Mono Lake is called Mono Craters, and the portion nearest to Mammoth Mountain is called the Inyo Craters.
The most recent eruptive activity occurred about 600 years ago, forming lava flows and lava domes. USGS has given this area a threat potential rating for future eruptions of high.
Map of Long Valley Caldera by USGS.
Long Valley Caldera
This large caldera measures 20 x 10 miles (16 x 32 km). It is often called a supervolcano, although this isn’t quite accurate. The true definition of a supervolcano is a volcano that has expelled in an eruption 1,000 cubic kilometers of material. About 760,000 years ago, Long Valley Caldera had a massive eruption (called the Bishop Tuff Eruption) that expelled 600 cubic km. This is incredible, and although it falls short of the defintion of a supervolcano, it still covered about one quarter of the USA in volcanic ash.
The volcano does not fit within the standard four types of volcanoes – shield volcanoes, stratovolcanoes, lava domes, and cinder cones. Its operation is different, and in fact is largely a mystery. It’s possible that its existence is formed similarly to a supervolcano, although the eruptions coming from it have not been as large as the three mentioned. Perhaps the standard for a supervolcano needs to be changed to a volcano that has erupted 500 cubic km of material. If that was the case, Long Valley Caldera in California, Valles Caldera in New Mexico, Mount Aso in Japan, and a very few others in the world would qualify that otherwise are just referred to as calderas, and are sometimes called supervolcanoes despite not meeting the official criteria.
The last eruptive activity to occur within the caldera was about 50,000 years ago. Anciently the caldera held a large lake, although it drained thousands of years ago. The caldera remains thermally active with numerous hot springs, and there is constant seismic activity. The ground continues to rise an inch (2.5 cm) or more every year. A power plant makes use of the heat from this volcano to generate enough power for 40,000 homes.
USGS has given the caldera a threat potential rating for future eruptions of high. The possibility of another massive eruption cannot be entirely ruled out, although the next time the volcano erupts, the expectation is that the eruption will be small to medium in size.