12.4: Southern Mojave Basin

The Mojave Desert is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily in southeastern California and southwestern Nevada, with small portions extending into Arizona and Utah.

The Mojave Desert, together with the Sonoran, Chihuahuan, and Great Basin deserts, forms a larger North American Desert. Of these, the Mojave is the smallest and driest. The Mojave Desert displays typical basin and range topography, generally having a pattern of a series of parallel mountain ranges and valleys. It is also the site of Death Valley, which is the lowest elevation in North America. The Mojave Desert is often colloquially called the "high desert", as most of it lies between 2,000 and 4,000 feet (610 and 1,220 m).

Figure 12.18: Photo of the Cima Volcanic Field Near Baker, California. Image by Jeremy Patrich is licensed under CC-BY 4.0.

The 20 million acres (81,000 km2; 31,000 sq mi) desert supports several human activities, including recreation, ranching, and military training. The Mojave Desert also contains various silver, tungsten, iron, borax, and gold deposits.

Physical Geography

The Mojave Desert is a desert bordered to the west by the Sierra Nevada mountain range and the California montane chaparral and woodlands, and to the south and east by the Sonoran Desert. The boundaries to the east of the Mojave Desert are less distinctive than the other boundaries because there is no presence of an indicator species, such as the Joshua tree (Yucca brevifolia), which is endemic to the Mojave Desert. The Mojave Desert is distinguished from the Sonoran Desert and other deserts adjacent to it by its warm temperate climate, as well as flora and fauna such as ironwood (Olneya tesota), blue Palo Verde (Parkinsonia florida), and the chuparosa (Justicia californica). Along with these other factors, these plants differentiate the Mojave from the nearby Sonoran Desert.

The Mojave Desert is bordered by the San Andreas fault to the Southwest and the Garlock fault to the North. The mountains elevated along the length of the San Andreas fault provide a clear border between the Mojave Desert and the coastal regions to the West. The Garlock fault separates the Mojave Desert from the Sierra Nevada and Tehachapi mountains, which provide a natural border to the Mojave Desert. There are also abundant alluvial fans, which are called bajadas, that form around the mountains within the Mojave Desert and extend down toward the low altitude basins, which contain dried lake beds called playas, where water generally collects and evaporates, leaving large volumes of salt. These playas include Rogers Dry Lake, and China Lake. Dry lakes are a noted feature of the Mojave landscape. The Mojave Desert is also home to the Devils Playground, about 40 miles (64 km) of dunes and salt flats going in a northwest-southeasterly direction. The Devil's Playground is a part of the Mojave National Preserve and is between the town of Baker, California, and Providence Mountains. The Cronese Mountains are within the Devil's Playground.

There are very few surface rivers in the Mojave Desert, even given the low precipitation level: two major rivers generally flow underground. The intermittent Mojave River, which begins in the San Bernardino mountains and disappears underground in the Mojave Desert. The Amargosa River also flows partly underground through the Mojave Desert along a southward path. The Manix, Mojave, and the Little Mojave lakes are all large but shallow.  Soda Lake is the principal saline basin of the Mojave Desert. Natural springs are typically rare throughout the Mojave Desert, however, there are two notable springs, Ash Meadows, and Oasis Valley. Ash Meadows is formed from several other springs, which all draw from deep underground. Oasis Valley draws from the nearby Amargosa River.

Weather & Climate

According to the Köppen climate classification system, Death Valley National Park has a hot desert climate (BWh). Extremes in temperatures throughout the seasons characterize the climate of the Mojave Desert. Freezing temperatures as well as strong winds are not uncommon in the winter, as well as precipitation such as rain and snow in the mountains. In contrast, temperatures above 100 °F (38 °C) are not uncommon during the summer months. There is an annual average precipitation of 2 to 6 inches (51 to 152 mm), although regions at high altitudes such as the portion of the Mojave Desert in the San Gabriel mountains may receive more rain. Most of the precipitation in the Mojave comes from the Pacific Cyclonic storms that are generally present passing Eastward in November to April. Such storms generally bring rain and snow only in the mountainous regions, because of the effect of the mountains, which creates a drying effect on its leeward slopes.

During the late summer months, there is also the possibility of strong thunderstorms, which bring heavy showers or cloudbursts. These storms can result in flash flooding.

Figure 12.19: Climate Date for Death Valley National Park (Data from NOAA). Image by Jeremy Patrich is licensed under CC-BY 4.0.

Field Trip: Badwater Basin

Let’s head on a field trip to one of the hottest places on Earth! Either scan the QR code or visit this link to see Professor Patrich explore Badwater Basin in Death Valley, California. (Video length: 4min).

Flora & Fauna

The flora of the Mojave Desert consists of various endemic plant species, notably the Joshua Tree, which is a notable endemic and indicator species of the desert, (which unlike most plants, is pollinated by moths, not bees or birds). There is more endemic flora in the Mojave Desert than almost anywhere in the world. Mojave Desert flora is not a vegetation type, although the plants in the area have evolved in isolation because of the physical barriers of the Sierra Nevada and the Colorado Plateau. Predominant plants of the Mojave Desert include creosote bush (Larrea tridentata), brittlebush (Encelia farinosa), desert holly (Atriplex hymenelytra), white burrobush (Hymenoclea salsola), and most notably, the Joshua tree (Yucca brevifolia).

Figure 12.20: Joshua Tree National Park. Image by Wikimedia is licensed under CC BY-SA 4.0.

Additionally, the Mojave Desert is also home to various species of cacti, such as silver cholla (Cylindropuntia echinocarpa), Mojave prickly pear (O. erinacea), beavertail cactus (O. basilaris), and many-headed barrel cactus (Echinocactus polycephalus). The Mojave Desert is generally abundant in winter annuals.  The plants of the Mojave Desert each generally correspond to an individual geographic feature. As such, there are distinctive flora communities within the desert.

Notable species of the Mojave Desert include bighorn sheep (Ovis canadensis), mountain lions (Puma concolor), black-tailed jackrabbits (Lepus californicus), and desert tortoises (Gopherus agassizii). Various other species are particularly common in the Mojave Desert, such as the LeConte's thrasher (Toxostoma lecontei), banded gecko (Coleonyx variegatus), desert iguana (Dipsosaurus dorsalis), chuckwalla (Sauromalus obesus), and regal horned lizard (Phrynosoma solare). Species of snake include the rosy boa (Lichanura trivirgata), Western patch-nosed snake (Salvadora hexalepis), and Mojave rattlesnake (Crotalus scutulatus). These species can also occur in the neighboring Sonoran and Great Basin deserts.

The animal species of the Mojave Desert have generally fewer endemics than its flora. However, endemic fauna of the Mojave Desert includes Kelso Dunes jerusalem cricket (Ammopelmatus kelsoensis), the Kelso Dunes shieldback katydid (Eremopedes kelsoensis), the Mohave ground squirrel (Spermophilus Mohavensis) and Amargosa vole (Microtus californicus scirpensis). The Mojave fringe-toed lizard (Uma Scoparia) is not endemic, but almost completely limited to the Mojave Desert. There are also aquatic species that are found nowhere else, such as the Devils Hole pupfish, limited to one hot spring near Death Valley.

Figure 12.21: A Spawning Pair of Salt Creek Pupfish, Death Valley. Image is in the public domain.

Physical Geology

The rock that underlies the Mojave Desert was likely created under shallow water in the Precambrian. Sedimentary processes left large deposits of limestones, silicates, and dolomites. During the Paleozoic era, the area that is now the Mojave was again likely submerged under a greater sea. During the Mesozoic era, major tectonic activities such as thrust faulting and folding resulted in distinctive shaping as well as intrusion by magma.  During the Cenozoic, more tectonic deformation occurred whilst the Mojave was partly submerged. Major volcanic activity is thought to have occurred during the Oligocene. Large downpours during the Miocene likely significantly eroded the rock in the Mojave and accelerated deposition.

The Mojave Desert is a source of various minerals and metallic materials due to magma instusion. Due to the climate, there is an accumulation of weathered bedrock, fine sand, and silt, both sand and silt sediments becoming converted into colluvium. The deposits of gold, tungsten, and silver have been mined frequently prior to the Second World War. Additionally, there have been deposits of copper, tin, lead-zinc, manganese, iron, and various radioactive substances but they have not been mined for commercial use.

Case Study - The Geology of Death Valley

The park has a diverse and complex geologic history. Since its formation, the area that comprises the park has experienced at least four major periods of extensive volcanism, three or four periods of major sedimentation, and several intervals of major tectonic deformation where the crust has been reshaped. Two periods of glaciation (a series of ice ages) have also added drainage of water into the area, although no glaciers ever existed in the ranges now in the park.

Little is known about the history of the oldest exposed rocks in the area due to extensive metamorphism (alteration of rock by heat and pressure). Radiometric dating gives an age of 1,700 million years for the metamorphism during the Proterozoic. About 1,400 million years ago a mass of granite now in the Panamint Range intruded this complex. Uplift later exposed these rocks to nearly 500 million years of erosion, yet dating them to 1.8 billion years of age.

Figure 12.22: Long Valley Basin Cross-Section. Image by Jeremy Patrich is licensed under CC-BY 4.0.

The Proterozoic sedimentary formations of the Pahrump Group were deposited on these basement rocks. This occurred following uplift and erosion of any earlier sediments from the Proterozoic basement rocks. The Pahrump is composed of arkose conglomerate (quartz clasts in a concrete-like matrix) and mudstone in its lower part, followed by dolomite from carbonate banks topped by algal mats as stromatolites, and finished with basin-filling sediment derived from the above, including possible glacial till from the hypothesized Snowball Earth glaciation. The very youngest rocks in the Pahrump Group are basaltic lava flows.

Basement & Pahrump Group

A rift opened and subsequently flooded the region as part of the breakup of the supercontinent Rodinia in the Neoproterozoic (by about 755 million years ago) and the creation of the Pacific Ocean. A shoreline like the present Atlantic Ocean margin of the United States lay to the east. An algal mat-covered carbonate bank was deposited, forming the Noonday Dolomite. Subsidence of the region occurred as the continental crust thinned and the newly formed Pacific widened, forming the Ibex Formation. An angular unconformity (an uneven gap in the geologic record) followed.

A true ocean basin developed to the west, breaking all the earlier formations along a steep front. A wedge of clastic sediment then began to accumulate at the base of the two underwater precipices, starting the formation of opposing continental shelves. Three formations developed from sediment that accumulated on the wedge. The region's first known fossils of complex life are found in the resulting formations. Notable among these are the Ediacaran fauna and trilobites, the evolution of the latter being part of the Cambrian Explosion of life.

The sandy mudflats gave way about 550 million years ago to a carbonate platform (like the one around the present-day Bahamas), which lasted for the next 300 million years of Paleozoic time Death Valley's position was then within ten or twenty degrees of the Paleozoic equator. Thick beds of carbonate-rich sediments were periodically interrupted by periods of emergence. Although details of geography varied during this immense interval of time, a north-northeastern coastline trend generally ran from Arizona up through Utah. The resulting eight formations and one group are 20,000 feet (6 km) thick and underlay much of the Cottonwood, Funeral, Grapevine, and Panamint ranges.

Rifting & Deposition

In the early-to-mid- Mesozoic the western edge of the North American continent was pushed against the oceanic plate under the Pacific Ocean, creating a subduction zone. A subduction zone is a type of contact between different crustal plates where heavier crust slides below lighter crust. Erupting volcanoes and uplifting mountains were created as a result, and the coastline was pushed to the west. The Sierran Arc started to form to the northwest from heat and pressure generated from subduction, and compressive forces caused thrust faults to develop.

A long period of uplift and erosion was concurrent with and followed the above events, creating a major unconformity, which is a large gap in the geologic record. Sediments worn off the Death Valley region were carried both east and west by wind and water No Jurassic- to Eocene-aged sedimentary formations exist in the area, except for some possibly Jurassic-age volcanic rocks.

Compression & Uplift

Basin and Range-associated stretching of large parts of crust below southwestern United States and northwestern Mexico started around 16 million years ago and the region is still spreading. This stretching began to affect the Death and Panamint valleys area by 3 million years ago. Before this, rocks now in the Panamint Range were on top of rocks that would become the Black Mountains and the Cottonwood Mountains. Lateral and vertical transport of these blocks was accomplished by movement on normal faults. Right-lateral movement along strike-slip faults that run parallel to and at the base of the ranges also helped to develop the area. Torsional, or twisting forces, probably associated with northwesterly movement of the Pacific Plate along the San Andreas Fault (west of the region), is responsible for the lateral movement.

Igneous activity associated with this stretching occurred from 12 million to 4 million years ago. Sedimentation is concentrated in valleys (basins) from material eroded from adjacent ranges. The amount of sediment deposited has roughly kept up with this subsidence, resulting in the retention of the same valley floor elevation over time.

The Spillover Theory

Pleistocene ice ages started 2 million years ago and melt from alpine glaciers on the nearby Sierra Nevada Mountains fed a series of lakes that filled Death and Panamint valleys and surrounding basins (see the top of the timescale image). The lake that filled Death Valley was the last of a chain of lakes fed by the Amargosa and Mojave Rivers, and the Owens River.

The end-basin in a chain of lakes that began with Mono Lake, in the north, and continued through basins down the Owens River Valley, through Searles and China Lakes and the Panamint Valley, to the immediate west. Lake Manly, formed in Death Valley. The lake was nearly 100 miles (160 km) long and 600 feet (180 m) deep.

The large lake that covered much of Death Valley's floor, which geologists call Lake Manly, started to dry up 10,500 years ago. Salt pans and playas were created as ice age glaciers retreated, thus drastically reducing the lakes' water source. Only faint shorelines are left.

As the area turned to desert, the water evaporated, leaving an abundance of evaporitic salts, such as common sodium salts and borax, which were later exploited during the modern history of the region, primarily 1883 to 1907.

Figure 12.23: Map Showing the System of the Pleistocene Lakes in Eastern California. Image is in the public domain.

Zabriskie Point

Millions of years prior to the actual sinking and widening of Death Valley and the existence of Lake Manly, another lake covered a large portion of Death Valley including the area around Zabriskie Point (the location was named after Christian Brevoort Zabriskie, vice-president and general manager of the Pacific Coast Borax Company in the early 20th century). This ancient lake began forming approximately nine million years ago. During several million years of the lake's existence, sediments were collecting at the bottom in the form of saline muds, gravels from nearby mountains, and ashfalls from the then-active Black Mountain volcanic field. These sediments combined to form what we today call the Furnace Creek Formation. The climate along Furnace Creek Lake was dry, but not nearly as dry as in the present. Camels, mastodons, horses, carnivores, and birds left tracks in the lakeshore muds, along with fossilized grass and reeds. Borates, which made up a large portion of Death Valley's historical past were concentrated in the lakebeds from hot spring waters and alteration of rhyolite in the nearby volcanic field. Weathering and alteration by thermal waters are also responsible for the variety of colors represented there.

Figure 12.24: The Badlands, at Zabriskie Point in Death Valley. Image by Jeremy Patrich is licensed under CC-BY 4.0.

Regional mountains building to the west influenced the climate to become more and more arid, causing the lake to dry up, and creating a dry lake. Subsequent widening and sinking of Death Valley and the additional uplift of today's Black Mountains tilted the area. This provided the necessary relief to accomplish the erosion that produced the badlands we see today. The dark-colored material capping the badland ridges (to the left in the panoramic photograph) is lava from eruptions that occurred three to five million years ago. This hard lava cap has retarded erosion in many places and possibly explains why Manly Beacon, the high outcrop to the right, is much higher than other portion of the badlands. (Manly Beacon was named in honor of William L. Manly, who along with John Rogers, guided members of the ill-fated party of Forty-niners out of Death Valley during the California Gold Rush of 1849.)

The primary source of borate minerals gathered from Death Valley's playas is Furnace Creek Formation. The Formation is made up of over 5000 feet (1500 m) of mudstone, siltstone, and conglomerate. The borates were concentrated in these lakebeds from hot spring waters and altered rhyolite from nearby volcanic fields.

Field Trip: Zabriskie Point

Let’s head on a field trip to one of the most majestic views in all of Death Valley National Park! Either scan the QR code or visit this link to join Professor Patrich for a lecture at Zabriskie Point in Death Valley, California. (Video length: 3min).

Cultural Geography

Much of early Mojave history remains unrecorded in writing since the Mojave language was not written in precolonial times. They depended on oral communication to transmit their history and culture from one generation to the next. Disease, outside cultures and encroachment on their territory disrupted their social organization. Together with having to adapt to a majority culture of another language, this resulted in interrupting the Mojave transmission of their stories and songs to the following generations.

In the 1880s, extracting borax, a valuable mineral, from this remote location demanded a robust transportation system. Traditional mule teams proved inadequate for the 165-mile trek, so Francis Marion Smith pioneered the twenty-mule team in 1883. These specially bred animals, arranged in ten pairs, pulled massive wagons laden with borax ore. The critical water wagon, holding 1,200 gallons, ensured survival in the harsh environment. Skilled "mule skinners" navigated treacherous terrain and managed the demanding animals on ten-day round trips battling extreme heat and flash floods.

The twenty-mule team revolutionized borax transportation, enabling large-scale mining by significantly reducing costs. It became a symbol of American industrial prowess. However, the discovery of richer deposits closer to rail lines and the arrival of the "Death Valley Scotty" railroad in 1889 marked the team's decline. The final official trip occurred in 1898.