9.5: Volcanic Features of the Sierra Nevada

Young Volcanics

Although the Sierra Nevada is largely defined by its ancient geological formations, the region also hosts a series of younger volcanic features that have formed within the last few million years. These young volcanics are a result of relatively recent tectonic and volcanic activity, which is often associated with the extensional processes occurring in the adjacent Basin and Range Province. This ongoing tectonic activity has caused the Earth's crust to stretch and thin, leading to the formation of faults and the upwelling of magma. The Mono-Inyo Craters volcanic chain, located near the eastern edge of the Sierra Nevada, is one of the most notable examples of this young volcanic activity. This chain consists of a series of rhyolitic domes, craters, and lava flows that have erupted over the past few thousand years, with the most recent eruptions occurring approximately 600 years ago.

The Mono-Inyo Craters are particularly significant because they provide a clear record of volcanic activity in the region during the Holocene epoch. The rhyolitic domes, such as Panum Crater, are composed of highly viscous lava that erupted explosively, forming steep-sided volcanic features. These eruptions were accompanied by pyroclastic flows and the deposition of tephra, which blanketed the surrounding area. The region remains geologically active, with ongoing seismic activity suggesting that magma continues to move beneath the surface, indicating the potential for future eruptions. In addition to the Mono-Inyo Craters, the Long Valley Caldera, just south of Mono Lake, is another prominent feature associated with young volcanism but does not reside within the Sierra Nevada Province.

Figure \(\PageIndex{1}\): South Inyo Crater Lake near Mammoth, CA by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

The Inyo Craters, located near the eastern edge of the Sierra Nevada (the southernmost Inyo Crater is shown in Figure 9.5.1) are a series of three volcanic craters formed during explosive eruptions approximately 600 years ago. These craters, part of the larger Mono-Inyo Craters volcanic chain, were created when magma interacted with groundwater, resulting in steam-driven explosions that blasted through the surface. The largest of the Inyo Craters, South Inyo Crater, measures about 600 feet in diameter and 200 feet deep. The craters are surrounded by deposits of pumice and ash, remnants of the eruptions that shaped them. The Inyo Craters serve as a vivid reminder of the relatively recent volcanic activity in the Sierra Nevada and remain a point of interest for both geologists and visitors exploring the region’s dynamic volcanic landscape.

Additionally, Obsidian Dome, located near the Inyo Craters, is a prominent volcanic feature formed during an eruption approximately 600 years ago. This dome is composed primarily of obsidian, a naturally occurring volcanic glass, and rhyolite, which were extruded during a series of explosive eruptions (Figure 9.5.2). The eruptions forced viscous lava to the surface, where it cooled and solidified into a steep-sided dome. Obsidian Dome rises about 300 feet above the surrounding landscape and spans roughly a mile in diameter. Its rugged surface is marked by large blocks of obsidian and pumice, creating a striking contrast with the surrounding forested terrain. Obsidian Dome is an excellent example of the type of volcanic activity that has shaped the Mono-Inyo Craters region and remains a popular destination for visitors interested in the geological history of the Sierra Nevada.

Figure \(\PageIndex{2}\): Obsidian Dome near Inyo Craters by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

Figure 9.5.3 exhibits a hand-sized sample of snowflake obsidian, which is a unique variety of volcanic glass that forms when molten lava cools rapidly, preventing the formation of a crystalline structure. What sets snowflake obsidian apart are the white or grayish "snowflake" patterns, which are actually clusters of cristobalite, a type of quartz. These patterns form when the obsidian slowly begins to crystallize over time, allowing small crystals of cristobalite to develop within the glassy matrix. As the obsidian cools, the cristobalite crystals grow and spread, creating the distinctive snowflake-like inclusions. The contrast between the deep black of the obsidian and the light-colored cristobalite makes snowflake obsidian a striking and sought-after material for both collectors and jewelers. This process of slow crystallization within the volcanic glass typically occurs after the initial formation of the obsidian, as the lava continues to cool and stabilize.

Figure \(\PageIndex{3}\): Snowflake obsidian from Obsidian Dome by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

Beyond the Mono-Inyo Craters, another noteworthy example of young volcanism within the Sierra Nevada is the volcanic activity in the area surrounding Mammoth Mountain. Although Mammoth Mountain itself is part of the Long Valley volcanic field, its formation is distinctly tied to the volcanic processes that have influenced the eastern Sierra Nevada. Mammoth Mountain, a large dacitic dome complex, began forming around 110,000 years ago and has seen multiple eruptions over its history. While not erupting in recent history, Mammoth Mountain continues to exhibit volcanic unrest, including earthquake swarms, ground deformation, and the release of volcanic gases, indicating that the area remains a site of potential future volcanic activity. These young volcanic features illustrate the ongoing tectonic and volcanic processes that continue to shape the Sierra Nevada, highlighting the region's dynamic geological evolution and potential for future volcanic events.

Table Mountains and Inverted Topography

Inverted topography is a fascinating geomorphological phenomenon common in the American West, and the Sierra Nevada is home to several striking examples. This process occurs when volcanic deposits, such as lava flows or ash-flow tuffs, descend into river valleys and other topographic lows, filling them with resistant volcanic rock. Over time, the surrounding softer materials, like sedimentary rocks that once formed the valley sides, erode more quickly than the harder volcanic rock. This differential erosion causes the former valleys, now capped with erosion-resistant volcanic material, to stand elevated as mesas or ridges, effectively reversing the original topography. As a result, areas that were once topographic highs become valleys, while former valleys are transformed into elevated features (Figure 9.5.4).

The Table Mountains in the Sierra Nevada, particularly those near Sonora in Tuolumne County, provide some of the most striking examples of inverted topography. During the Miocene epoch, approximately 10 million years ago, volcanic eruptions filled ancient river valleys with thick layers of basaltic lava. As the lava solidified into hard rock, it resisted erosion far better than the surrounding sedimentary materials. Over millions of years, erosional processes removed the softer surrounding rocks, leaving behind the distinctive flat-topped ridges we see today. The Stanislaus Table Mountain is a classic example of this process, where the ancient Stanislaus River was buried by a lava flow that now forms a prominent ridge, shaping the course of modern rivers and streams in the area.

Inverted topography in the Sierra Nevada is not only a geological curiosity but also offers important insights into the region's volcanic and erosional history. The preservation of these lava-capped ridges allows geologists to reconstruct the ancient landscapes of the Sierra Nevada, providing clues about the climate, tectonics, and volcanic activity that occurred millions of years ago. These features serve as natural laboratories for studying the processes of erosion, weathering, and landscape evolution. The Table Mountains and other examples of inverted topography in the Sierra Nevada illustrate the long-lasting impact of volcanic activity on the landscape and the intricate interplay between tectonic forces and surface processes.

Figure \(\PageIndex{4}\): Schematic illustrations of the formation of inverted topography by the National Park Service is in the public domain. Access a detailed description.

There are a number of examples of this inverted topography in the Sierra Nevada providing yet more evidence of an ancestral Sierra Nevada mountain range that predates the one we see today.

Big Table Mountain and Kennedy Table Mountain in Madera County are prominent examples of inverted topography in the Sierra Nevada, illustrating the dramatic effects of volcanic activity and erosion on the landscape. These table mountains were formed during the Miocene epoch when basaltic lava flows filled ancient river valleys of the San Joaquin River and its tributaries. Over millions of years, the surrounding softer sedimentary rocks eroded away, leaving the resistant basalt-capped ridges elevated above the current landscape. Big Table Mountain, located near the city of Auberry, and Kennedy Table Mountain, situated closer to the San Joaquin River, both exhibit the flat-topped, steep-sided morphology typical of inverted topography. Figure 9.5.5 shows Big Table Mountain on the horizon, with the board flat top, and the San Joaquin River just below. This image was taken from Pincushion Peak and is looking to the southeast. Figure 9.5.6 shows an aerial view of Big Table Mountain (highlighted in yellow). One might note the sinuous shape that these table mountains take on further demonstrating lava flow direction and the ancient San Joaquin River channel.

Figure \(\PageIndex{5}\): Looking southeast from Pincushion Peak towards Big Table Mountain, Madera County, CA by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

Figure \(\PageIndex{6}\): Aerial imagery showing Big Table Mountain in Fresno and Madera Counties, CA. Created in Google Earth Pro by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

The connection of these table mountains to the San Joaquin River highlights the powerful interplay between volcanic activity and fluvial processes in shaping the Sierra Nevada's landscape. As the San Joaquin River carved its course through the Sierra Nevada, volcanic eruptions periodically filled its valleys with lava, which later became the high-standing table mountains after extensive erosion of the surrounding terrain. These features not only offer striking visual evidence of the region's geological history but also provide insights into the ancient river systems and the volcanic events that shaped them. Together with other examples like the Stanislaus Table Mountain, Big Table Mountain and Kennedy Table Mountain underscore the significance of inverted topography in understanding the Sierra Nevada's complex volcanic and erosional history.

Late Cenozoic Uplift of the Sierra Nevada

The Late Cenozoic uplift of the Sierra Nevada, beginning around 5 million years ago, has played a critical role in shaping the region's current landscape and volcanic features. This uplift, driven by tectonic forces related to the subduction of the Farallon Plate and the extension of the Basin and Range Province, has elevated the Sierra Nevada to its present height, dramatically increasing the relief of the range. The uplift has caused the Sierra Nevada to tilt westward, resulting in the gentle western slope and the steep eastern escarpment that characterizes the range today. This tilting has also influenced the drainage patterns of the region's rivers, with many rivers cutting deep, steep-sided canyons into the western slope as they flow toward the Central Valley.

The uplift of the Sierra Nevada has also had significant implications for the region's volcanic history. As the mountains rose, they altered the stress patterns in the Earth's crust, creating conditions that favored volcanic activity along the eastern margin of the range. This activity is particularly evident in the formation of the Long Valley Caldera, one of the largest and most active volcanic systems in the western United States. The caldera was formed by a massive eruption approximately 760,000 years ago, which released an estimated 600 cubic kilometers of ash and debris, blanketing large parts of what is now the western United States. The uplift of the Sierra Nevada has also contributed to the preservation of volcanic features such as the Mono-Inyo Craters and the various lava-capped Table Mountains, which were protected from erosion as the landscape around them was uplifted and dissected.

The Late Cenozoic uplift of the Sierra Nevada is also closely linked to the region's ongoing tectonic activity. The uplift has created a series of normal faults along the eastern edge of the range, which have accommodated the extension of the Basin and Range Province and contributed to the formation of the steep eastern escarpment. These faults are still active today, as evidenced by the numerous earthquakes that occur along the eastern Sierra Nevada, including the historic Lone Pine earthquake of 1872. The combination of uplift, volcanic activity, and tectonic deformation in the Sierra Nevada makes this region a key area for studying the interplay between tectonic processes and surface features. The ongoing uplift and deformation of the Sierra Nevada continue to shape the landscape, providing a dynamic environment where the forces of nature are on full display.

Devils Postpile

Devils Postpile, a striking geological formation located in the Sierra Nevada, is renowned for its impressive basalt columns and the nearby Rainbow Falls. Though not designated as a full national park, Devils Postpile National Monument is a protected area that showcases one of the finest examples of columnar jointing in the world. Columnar joints are captivating geological formations arising from the cooling and contraction of volcanic or igneous rock, most commonly basalt. These distinct hexagonal or pentagonal structures are a testament to the intricate interplay of geology and physics. It all begins with the eruption of molten rock rich in minerals like silica and iron. As the lava flows and spreads, it gradually loses heat to the surrounding environment, causing it to cool and solidify. During this cooling process, the lava contracts, creating internal stress that leads to the formation of cracks. These cracks develop perpendicular to the cooling surface, resulting in a polygonal pattern that eventually forms the characteristic hexagonal or pentagonal columns. The geometry of these columns is a consequence of the efficient packing of polygons, minimizing gaps and maximizing stability. The size and regularity of these columns are influenced by the cooling rate and the composition of the rock. Slower cooling rates yield larger columns, while faster cooling rates produce smaller ones. Basalt, with its particular mineral composition, is a common rock type associated with these formations. Columnar joints are a testament to the beauty of nature's artistic expressions in rock, born from the gradual cooling and solidification of molten material.

Devils Postpile’s columns tower up to 60 feet and display a striking symmetry. However, this was not the case an estimated 80,000 to 100,000 years ago a lava vent began spewing hot basaltic lava into the Reds Meadow Valley near present-day Upper Soda Springs, a few miles north of the Monument. Basalt lava is rich in iron and magnesium and is typically much hotter than other types of lava. Because of these traits, basaltic lava tends to have a lower viscosity and will flow more quickly than other lava types. The lava flowed down the valley like a river until it was blocked by a natural dam, probably a glacial moraine left down-valley by a receding glacier during a previous ice age. The lava began filling the valley behind this dam, creating a lava lake 400 ft. deep in some areas. Such depth is uncommon among lava flows and plays a crucial role in the formation of the long columns we see today and that are exhibited in Figure 9.4.7.

Figure \(\PageIndex{7}\): Devils Postpile by Cole Heap is licensed under CC BY-NC 4.0. Access a detailed description.

As the lava flow ceased, the molten rock began cooling into solid rock. Shallow parts of the lava flow would have solidified first, with deeper parts of the lava lake requiring much more time to release the massive amount of stored thermal energy. As the lava lake cooled and solidified from a molten soup to solid rock it began to contract. Contraction stresses developed because the cool solid form of basalt has a lesser volume than the hot liquid form. Cracks, also called joints by geologists, began to form. Jointing releases internal stress created by the cooling and associated contraction. In some locations, such as at the Devils Postpile, the jointing formed columns. Jointing would have begun at the top and bottom, as well as all around the edges of the lava lake, where the lava made direct contact with a cooler surface. The cracks would have extended inwards over time as the more insulated locations within the lava lake finally released enough thermal energy to change from a liquid to a solid state. Figure 9.5.8 is a schematic of this described process of columnar joint formation.

Figure \(\PageIndex{8}\): Diagram showing the formation of columnar jointing by the National Park Service is in the public domain. Access a detailed description.

The Devils Postpile used to be much taller than what we see today. Powerful erosive forces have been at work during the last 80,000 to 100,000 years carving, shaping and demolishing remnants of the lava flow. Freeze-thaw cycles help break apart the columns. Earthquakes knock columns down into the talus slope below. The river slowly eats away at pieces that fall into the water. But no force has left a greater footprint on the Postpile than that of glaciers. In fact, the beautifully straight hexagonal columns hidden within the depths of the lava flow would not been seen had glaciers not excavated the formation. Figure 9.5.9 exhibits these hexagons from on top of Devils Postpile. Several distinct glacial periods have occurred since the Postpile was formed and each has dug deeper and deeper into the dense, heavy rock known officially as basaltic trachyandesite. The last major glacial period ended about 18,000 years ago. Glacial polish and striations evident on top of the Postpile are from this last glaciation.

Figure \(\PageIndex{9}\): On top of Devils Postpile by Cole Heap (2023) is licensed under CC BY-NC 4.0. Access a detailed description.

Volcanic Legacy and Geomorphic Evolution of the Sierra Nevada

This section explored the more recent volcanic legacy and related geomorphic evolution of the Sierra Nevada, focusing on the dynamic interplay between volcanic activity and tectonic processes that shaped the region’s landscape. Key features such as the Mono-Inyo Craters, Obsidian Dome, and Devils Postpile highlighted the relatively recent volcanic activity that left a lasting imprint on the eastern Sierra Nevada. Additionally, the concept of inverted topography, exemplified by the Table Mountains, illustrated how volcanic deposits transformed ancient river valleys into elevated ridges over millions of years.

The section also delved into the Late Cenozoic uplift of the Sierra Nevada, which intensified erosion and exposed these volcanic features, contributing to the region's dramatic topography. From the towering basalt columns of Devils Postpile to the unique snowflake obsidian found near Obsidian Dome, each aspect of young volcanism provided a comprehensive overview of how volcanic and tectonic forces combined to create the unique, varied, and young portions of the Sierra Nevada landscape.

References

1. Bateman, P. C. (1992). Plutonism in the central part of the Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 1483.
2. Bateman, P. C. (1992). Plutonism in the central part of the Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 1483.
3. Hildreth, W., & Fierstein, J. (2016). Eruptive history of Mammoth Mountain and its mafic periphery, California. U.S. Geological Survey Professional Paper 1812.
4. Huber, N. K. (1981). Amount and timing of Late Cenozoic uplift and tilt of the central Sierra Nevada, California—evidence from the upper San Joaquin River basin. U.S. Geological Survey Professional Paper 1197.
5. Lindgren, W. (1911). The Tertiary Gravels of the Sierra Nevada of California. U.S. Geological Survey Professional Paper 73.
6. Slemmons, D. B., & Brock, V. E. (1975). Late Cenozoic volcanism and tectonism in the western Great Basin. Geological Society of America Bulletin, 86(11), 1485-1492.