8.5: Rift-Related Volcanism in Eastern California

Crustal Thinning and Volcanism

As normal faulting thinned the crust throughout the Basin and Range continental rift, asthenospheric mantle was brought closer to the surface. As this material rose closer to the surface, pressure was reduced and the material underwent decompression-induced partial melting. Such partial melting of the ultramafic peridotite of the mantle produces a magma that is basaltic in composition. This magma is less dense than the surrounding rock and continues to rise through the crust to the surface. If the path to the surface is direct and there is little interaction with the crust, then it will erupt as basaltic lava, producing flood basalts, cinder cones, and basaltic lava flows. Alternatively, if there is a lot of crustal interaction, the lava that erupts will be more felsic. Examples of both are found in eastern California.

Video \(\PageIndex{1}\): Basin and Range Volcanic Processes

The following video provides further explanation of how melting and magma generation occurs in the Basin and Range; some of these processes are discussed in Chapter 2, Rocks and Minerals, and Chapter 9, Sierra Nevada.

Volcanism began in the southern Owens Valley approximately 6 million years ago, just after the most intense period of extension in the Basin and Range, and continued until 1.1 - 0.04 Ma. Notable examples of young volcanic features are in Death Valley (Ubehebe Crater; Figure \(\PageIndex{1}\)) and all along the Owens Valley (e.g. Red Hill near Coso Junction; Figure \(\PageIndex{2}\)). These volcanic structures are typically located along normal faults, which serve as conduits or paths for magma.

Figure \(\PageIndex{1}\): The Ubehebe Crater, Death Valley National Park. This is a cinder cone with a large depression in the center. "Ubehebe Crater, Death Valley, California" by Daniel Mayer, is licensed under CC-SA 3.0.

Figure \(\PageIndex{2}\): A view from the south of Red Hill, a small volcanic cinder cone located in the Southern Owens Valley. Little Lake is in the foreground, and the high Sierra Nevada Range in is located to the west. "Red Hill" by Martha House, is licensed under CC-BY-NC.

Video \(\PageIndex{2}\): Fossil Falls California

This video explores an area in southern Owens valley called “Fossil Falls”. This area was a path through which Pleistocene lakes drained and contains interesting sculpted shapes as a result of the water that flowed over it. The video will explain the landscape of the area, and will also give an overview of the power of water to erode rock

To the north, in Long Valley, dramatic examples of extension-related volcanism abound. An important feature is the Bishop Tuff, a widespread deposit that was produced by the eruption of the Long Valley Caldera 760,000 years ago.

Figure \(\PageIndex{3}\): Simplified map of the geology of the Long Valley Caldera. Outcrops of the Bishop tuff are on the north and southeast, and related volcanic features are on the perimeter of the caldera. "Geologic Map of Long Valley and Mono Basin region" by USGS, is in the public domain. Access a detailed description.

As described in Figure \(\PageIndex{3}\), this pyroclastic eruption produced ash fall that is preserved to the north and southeast of the caldera. Other volcanic features associated with this volcanic system include Mammoth Mountain to the west, Glass Mountain to the northeast, and the Inyo-Mono craters to the north. Scientists estimate that the eruption of this caldera produced the Bishop Tuff and also created more than 100 km³ (24 mi³) of tephra that was dispersed widely as ashfall. Evidence suggests that more than 200 km³ (48 mi³) of pyroclastic flows were deposited outside the caldera, and approximately 350 km³ (85 mi³) ponded and welded together within the caldera. Outside the caldera, these pyroclastic flows swept over an area of more than 2,200 km² and buried the pre-existing ground to depths of nearly 200 m (660 ft.) just south of the caldera. The tuff blankets a large area of the northern Owens Valley, and fragments associated with the eruption of this caldera were detected as far east as Nebraska (Figure \(\PageIndex{3}\)). The appearance of this tuff is highly variable. In some places, it’s very ashy and unconsolidated (the lower part of Figure \(\PageIndex{4}\)), while in others it’s welded into a rock solid enough to support rock climbers (the upper part of Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Two different examples of the Bishop Tuff. In the upper image, dark colored welded tuff contains petroglyphs carved through the dark coating (Sky Rock Petroglyphs near Bishop, CA). This work by Ryan James (dangerismycat via Flickr), is licensed under CC BY-NC-ND 2.0.In the lower image, an example of the air fall pumice and pyroclastic layer of the tuff shows rills from water erosion and has a light-colored appearance. "Bishop Tuff" by USGS, is in the public domain.

A large caldera complex is the southernmost expression of a chain of explosion pits, volcanic domes and lava flows of the Inyo-Mono craters, which extends as far north as Mono Lake (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): The Inyo-Mono Craters chain of volcanic domes and craters extends southward from Mono Lake to Mammoth Mountain in the south. Pahoa Island is in the center of the lake. Long Valley is shown in the lower left (SE) corner of the image. "Mono-Inyo Craters satellite image-annotated" by NASA, is in the public domain. Access a detailed description.

The well-known ski area, Mammoth Mountain, sits on the edge of a large caldera; skiers who ride the gondola can see active steam vents on the side of this young dacite dome. This chain of volcanic features, which includes dacite domes such as Mammoth Mountain, rhyolite and obsidian domes, and explosion craters (Figure \(\PageIndex{6}\)) is thought to be linked via large dike structures that are fault controlled; their location along the major normal faults indicates that fault features may have served as conduits for magma.

Figure \(\PageIndex{6}\): Three different examples of volcanic features found in the Inyo-Mono Craters chain. Top Image: Mammoth Mountain forms a large sloped volcanic features with the Ritter Range to the west in the background and Lake Mary in the foreground. "Ritter Range and Mammoth Mountain" by Hike395, is licensed under CC BY 3.0. In the middle image, the rough surface of the rhyolitic Panum Crater is viewed from the air and is surrounded by a tephra apron. i"Panum Crater" work by USGS, is in the public domain. In the bottom image, the Inyo Craters and South Deadman Dome are viewed from the south. The rough surfaces of the dome emerge from above the surrounding forest, while the explosion pits of the craters to the south sink below "Inyo Craters" by USGS, is in the public domain.

While the volcanism that produced this chain of features began approximately 50,000 yrs ago, the most recent eruptions occurred during the Holocene. For example, volcanic Paoha Island in Mono Lake is only about 300 years old.

Query \(\PageIndex{1}\)

Volcanic Hazards

The Long Valley region, as well as other sites within the Basin and Range Province, present significant volcanic hazard potentials. Particular threats from the Long Valley Caldera region include fast-moving pyroclastic flows and surges. The current hazard zone associated with this caldera is based on explosive eruptions from vents located along the Inyo-Mono chain in the past 10,000 years that are known to have ejected magma and generated pyroclastic flows or surges. The hazard zone shown in Figure \(\PageIndex{7}\), is centered along the south moat of the caldera, which is the location of epicenters of many swarms of earthquakes since 1980 and the area of most intense ground movement. It has been mapped to extend northward from the Long Valley Caldera to Conway Summit, and along the eastern edge of the Sierra Nevada Range and the western limits of the northern White Mountains. Scientists suggest that future eruptions might occur from this restless zone, and pyroclastic flows and surges could travel as far as 15 km from a new vent. The South Moat, which is located to the south of the town of Mammoth Lakes, is a potentially hazardous volcanic vent that could produce a large pyroclastic flow that would extend as far north as Conway Summit on the north side of Mono Lake.

Figure \(\PageIndex{7}\): A map of the hazard zone for pyroclastic flows and surges around existing explosive vents along the Mono-Inyo Craters volcanic chain and from potential vents located in Long Valley's south moat. The hazard zone extends approximately 60 km (~35 miles) northward from the Long Valley Caldera to Conway Summit, and along the eastern edge of the Sierra Nevada Range and the western limits of the northern White Mountains. Hazards Summary for Mono-Inyo Craters by USGS/C.D. Miller, is in the public domain.

Video \(\PageIndex{3}\): Volcanology and the Long Valley Caldera

This video discusses the last large eruption at Long Valley Caldera.

Volcanic eruptions are not the only hazard in this area: gas emissions have produced high concentrations of carbon dioxide (CO₂) that can accumulate in the winter in tree wells, around buildings, and immediately below the snow surface. For example, high CO₂ levels have built up in soil and are killing trees on the flanks of Mammoth Mountain. First noted in 1990, the areas of tree kill total about 170 acres in six general areas, including the most visually impressive tree-kill area adjacent to Horseshoe Lake on the south side of Mammoth Mountain. The soil gas in the tree-kill areas is composed of 20 - 90% CO_2. Note that there is less than 1% CO₂ in soils outside the tree-kill areas.

The ultimate source of much of the CO₂ is magma beneath Mammoth Mountain. The onset of CO₂ emissions is closely linked to a major seismic swarm beneath the mountain in 1989. Scientific studies indicate that there may be a large reservoir of gas deep below the mountain.

Geothermal Activity (Coso and Round Valley)

Once the crust has thinned, the mantle can rise up and replace it, bringing heat close to the surface. This is shown in very high heat flows in the Basin and Range area. In 2023, 98% of the geothermal electricity in the United States was produced in Basin and Range states, and volcanic areas in California and Nevada were the most productive. The Casa Diablo hydrothermal plant, for example, takes advantage of the heating power of the magma below the Long Valley Caldera. Here, the hydrothermal system is recharged with water primarily from snow-melt in the Sierra Nevada highlands along the western rim of the caldera. This water infiltrates to depths of a few kilometers where it is heated to at least 220 °C (428 °F) by hot rock near geologically young intrusions of magma. The thermal water rises along fractures to depths of 1-2 km in the western part of the caldera, just north of the town of Mammoth Lakes, and then flows eastward within permeable rock layers to discharge points along Hot Creek and around Crowley Lake in the eastern half of the caldera. This process creates many natural hot springs in the region.

Query \(\PageIndex{1}\)

References

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