Exploring the Hot Springs Mountains

The volcanic rocks that compose much of the range were primarily derived from the ancestral Cascade arc that contributed to abundant volcanic activity in the area millions of years ago. The ancestral Cascade volcanic arc began forming during the early Miocene (Busby et al., 2008), as the dense oceanic Farallon plate was subducting beneath the western margin of the less dense continental North American plate (present day western Nevada and eastern California).

This subduction and subsequent melting in the upper mantle above the subducting oceanic plate resulted in the formation of a volcanic arc, comparable to the present-day Cascade volcanoes of northern California, Oregon, Washington, and southern British Columbia. The oceanic Farallon plate has since subducted beneath the North American plate, leaving the Juan de Fuca plate to the north and the Cocos plate to the south. These two plates are now separated by a transform plate boundary you may have heard of: the San Andreas Fault. To learn more about plate tectonics, refer to the presentation  Plate Tectonics, Faults, and Folding—An Introduction to the Structure of the Earth, Crustal Movement, and Associated Land Forms . 

Geologists have identified many different rock units in the northern Hot Springs Mountains and mapped the Desert Peak quadrangle and the area surrounding the Bradys geothermal area in great detail. This region continues to be explored and mapped today. Both the  Preliminary geologic map of the Desert Peak quadrangle  and the  Preliminary geologic map of the Bradys geothermal area  are available for free download from NBMG and contain cross sections, geothermal well locations, and over 100 different geologic units. Some of the more prominent geologic units in the area include older aphanitic basalt lavas, basaltic andesite lavas, a variety of tuffaceous rocks, and silicified siltstone and diatomite units (Faulds et al., 2012; Faulds et al., 2017).

Many of the rocks exposed in the area are covered with desert varnish, making them appear dark and glossy, similar to other rocks found around the Fortymile Desert. Desert varnish is commonly seen in arid regions and forms a thin rust-to-black coating on exposed rock surfaces. The varnish is composed primarily of manganese (Mn) and iron (Fe), but can contain clay minerals, oxides, or other trace elements ( Encyclopedia Britannica ). Microorganisms that live on the rock surface take manganese from their surrounding environment, oxidize it, and cement it to the rock surfaces. Desert varnish takes thousands of years to accumulate and commonly acted as a canvas for Native Americans to carve petroglyphs ( National Park Service ). Such petroglyphs can be seen to the southeast, across the Lahontan Valley at  Grimes Point Archaeological Site .

During the Pleistocene, the Hot Springs Mountains were an island in ancient Lake Lahontan, a glacial lake that filled and connected many basins in the western Great Basin. Pyramid Lake, Walker Lake, and the Carson and Humboldt sinks are remnants of ancient Lake Lahontan. Evidence of shorelines can be seen on the lower elevations of the hills and ridges in the region and give a rough idea of the area that was underwater.

The shorelines may appear slightly uneven in places – this is a result of the deformation of the Earth due to the weight of the water in the lake depressing the land and the land rebounding when the lake dried out (Bills et al., 2007). The lake reached a highest stand (elevation 4369 ft; 1332 m) at about 15,700 years ago (dePolo et al., 2018). As the climate warmed, the ancient lake dried up leaving dry playas and lake beds behind. During wet years some of these playas and lake beds hold excess water for short periods, serving as a brief glimpse into a wetter past.

Nevada jointfir (scientific name Ephedra nevadensis) is a native shrub found throughout much of the Great Basin. Nevada jointfir was used for medicine and food by native Americans and was brewed as tea by the Mormons, which is why this plant is commonly referred to as Mormon tea. Seeds were eaten by the Panamint and Owens Valley Paiute while the Moapa Paiute and Shoshone used tea brewed from the twigs to treat venereal diseases. The Shoshone also drank the tea to encourage urination and used ground up twigs to treat wounds ( USDA PLANTS Database ). According to the USDA, Nevada jointfir is considered to be toxic and should be used with caution. 

The Nevada jointfir is a spreading shrub that can reach up to 4 feet (1.2 meters). It has no leaves and exhibits jointed, evergreen stems, or twigs. It is primarily dioecious (having distinct male and female individual plants), and if there is enough moisture in the soil, the plants will produce cones in the spring months. Pollination is driven by wind, and the plant produces seeds, which are carried off by small mammals and deposited in shallow seed caches. The Nevada jointfir roots are woody and can grow up to 6.5 feet (2 meters) in depth, which may spread laterally and produce other plants (Anderson, 2004).  

Habitat is varied throughout the Great Basin and can be found on slopes, ridgetops, and floodplains between ~2800 and 7000 feet (~853 and 2130 m) in elevation. The plant has a high saline tolerance and is commonly found along the edges of salt deserts and playas (Anderson, 2004).

The basket evening primrose (scientific name Oenothera deltoides) is one of the more spectacular wildflowers of the Great Basin. This beautiful, short-blooming annual is known by many other names, including birdcage evening primrose, dune primrose, devil’s lantern, and lion-in-a-cage (Blackwell, 2006). The basket evening primrose grows on sandy dunes and flats up to an elevation of 5000 feet (1524 meters) and is commonly seen blooming in the mid-to-late spring months around Pyramid Lake and other sandy locations in western Nevada.

As the name suggests, the basket evening primrose flowers begin to open in the late afternoon or evening. The 4-petaled flowers are large (1.5 – 2.5 in; 3.8 – 6.4 cm) and white, with yellow centers. The bright white, fragrant flowers fade to a pinkish-lavender color in the morning (Blackwell, 2006). When the basket evening primrose dies back, the outer stems dry out and curl upward and inward, leaving a basket or birdcage around the seed pod. 

Purple Nama (Nama aretioides) is a beautiful, reddish-purple ground covering annual that is found throughout west central Nevada in sandy to gravely washes and flats up to an elevation of 6500 feet (1980 m). The small, 5-petal flowers with yellow centers grow close to the ground above narrow, fuzzy leaves (Blackwell, 2006). Purple Nama blooms from May through June and creates a brightly colored, eye-catching carpet that can be seen covering some of the hillsides in the Hot Springs Mountains.

Chorizanthe, known more commonly as spineflowers, are a genus of plants in the buckwheat family and are hard to miss with their bright green, spiny appearance. They are very small and can be easily covered by rocks and debris. Desiccated and dead Chorizanthe are brown to black in color and blend into their surroundings.  

Despite the dryness, there is water here. From small streams that run from geothermal springs and rare snowmelt, to streams that come to life following seasonal thunderstorms, this water gives life to its surroundings.

Wildlife have adapted to living within this high desert environment. When temperatures become extreme, either hot or cold, many critters will spend much of their time in underground burrows. Mammals, reptiles, and insects alike will all use burrows to protect themselves from the harsh environment they call home ( U.S. Fish & Wildlife Service ). Use caution when exploring their habitat. What may appear to be a small hole in the sand could be the front door to an extensive burrow, winding below the sand and dirt. 

Among some of the wildlife that can be found here is the great horned owl. The great horned owl (Bubo virginianus) is a large bird of prey that ranges in height from 18 to 25 inches (~46 to 64 cm) and has a wingspan of up to 6 feet (1.8 m)! They generally display various shades of brown feathers with a prominent white throat and have large ear tufts, which resemble horns, spaced apart on their heads. Their eyes are strikingly bright yellow and quite large. The great horned owl lives throughout North America and can be found in a variety of landscapes including desert mountains, forests, and even urban areas (Peterson, 1961). They nest in open areas or trees and commonly use nests that other birds have built since they do not build their own. These raptors are nocturnal and roost during the day among rocks or in trees. They are territorial birds who remain solitary except when breeding and nesting with their mate. Great horned owls feed primarily on small rodents, which they hunt by perching on trees, rocky outcrops, or cliffs and listening for signs of prey. Once they locate their prey, they will silently swoop down, catching their meal with their large talons ( Nevada Department of Wildlife ).   

Some fun facts about the great horned owl, courtesy of the Nevada Department of Wildlife: great horned owls are Nevada’s largest nocturnal bird of prey. If the great horned owl was the size of a human, its eyes would be as big as oranges. Great horned owls can locate their prey solely by sound.

The Fortymile Desert was an incredibly long, dry, and challenging section of the California Trail. Emigrants traveling across the country often traversed this section in August, hoping to make it over the Sierra Nevada before snowfall. The Fortymile Desert extends from the end of the Humboldt River, in the Humboldt Sink, to the Carson River or the Truckee River. In the Humboldt Sink, there was a fork in the California Trail. Emigrants could choose the Carson route or the Truckee route, but each spur resulted in the same distance traveled before reaching fresh water ( California Trail Interpretive Center ). Emigrants wrote extensively about this dangerous stretch of trail, many noting the extreme heat, dead animal carcasses, abandoned wagons, and grave markers seen along the trail (Kreutzer, 2012).

Bradys Hot Springs, located at the base of the northwestern portion of the Hot Springs Mountains, were once known as the Emigrant Springs of the Fortymile Desert. To some emigrants, they were called the "Spring of False Hope", because the water at this location was so hot. Oxen of the wagon trains traveling across the Fortymile Desert could smell the water of the hot springs before reaching them. As they approached the springs, they rushed forward to drink, but the water was scalding hot and undrinkable. Many livestock died here, and travelers had to collect water and wait for it to cool before being able to drink. Later on, emigrants left water in casks to cool for those who followed (Price and LaPointe, 2010).

The hot springs in the Bradys geothermal field have been used for at least the past 150 years starting with emigrants crossing the Fortymile Desert. Later on, between 1930 and 1950, a resort and spa operated here. Geothermal exploration began in the 1950s and has continued off and on over the past several decades. The first power plant at Bradys began operating in 1992. Today, a combined dual flash and binary geothermal power plant has a total installed capacity of 26 MWe (Megawatts electric) (Faulds et al., 2017). When passing by the Bradys geothermal field, travelers on Interstate 80 will often see steam escaping from the ground on cooler days. The ground in many places is quite warm in this area, and reservoir temperatures were estimated to be between 356 and 379°F (180 and 193°C) at 3280 to 6560 feet (1000 to 2000 m) in depth by Benoit et al. (1982).

While many geothermal features can be seen in the Bradys geothermal field, both Desert Peak and Desert Queen are blind geothermal systems. A blind geothermal system is a system where there are no surface indications of hot water (hot springs or fumaroles) and very few fossilized spring deposits (Faulds et al., 2010) such as sinter. 

The Desert Peak geothermal system has a reservoir temperature of 405°F (207°C) and powers two binary geothermal plants with a combined capacity of 26 MWe. This blind geothermal system displays some silicified sand of late Pleistocene age, located about 4300 feet (1310 m) west of the production wells. These sands were probably the result of outflow from a prehistoric spring into ancient Lake Lahontan (Faulds et al., 2010). This system was first identified by a drilling program in the 1970s (Benoit et al., 1982) and utilized two original production wells and one injection well in its first 30 years of operation. A drilling program in the late 2000s helped to increase production (Faulds et al., 2010).

The ore deposits at the Desert Queen, Fallon Eagle, and a third unnamed mine were located in quartz veins within strongly jointed diorite rocks. The diorite is overlain by silicified sedimentary and volcanic rocks, which are in turn unconformably overlain by a series of basalt lava flows and tuffs, with interspersed sedimentary deposits. Rocks in the district are displaced by several normal faults, which all appear to be younger than the mineralization (Willden and Speed, 1974). The three mines in the district were rather extensive during mining operations based on the size of tailings piles still visible and old maps of the workings.

During the cold war, a handful of these now abandoned mines were surveyed for use as fallout shelters, should the United States ever come under nuclear attack. Records of these surveys have been used in recent years to determine the extent of the mines and potential for suitable bat habitat. 

There are over 300,000 abandoned mine land features scattered across the state of Nevada, many of which pose significant risk to human and animal life. Hazards associated with abandoned mines include falling, cave-ins, explosives, poisonous or deadly animals (rattlesnakes and cougars), rotten timbers, water, and bad air.  The Nevada Division of Minerals  (NDOM) runs the state’s  Abandoned Mine Lands Program  (AML) and has worked toward cataloging and securing these hazards since the late 1980s. According to NDOM, there have been zero reported injuries or fatalities associated with AML features since 2013; over 100,000 historic mining features (both hazardous and not) have been inventoried; and they have cataloged over 23,500 hazardous AML features with an 80% securing rate. In 2020 alone, NDOM had inventoried 213 AML sites and secured 480 hazards (statistics courtesy of Sean Derby, Chief of the Abandoned Mine Lands Program, NDOM and current as of October 1, 2020). NDOM works with other state and federal agencies in Nevada to catalog and secure these old mines, and they hire interns each summer to assist with inventory and securing. Some of these closures have taken place in the Desert mining district and include bat gates, which promote the continued use of an abandoned mine for bat habitat but keep people from entering and disturbing sensitive habitat and putting important bat colonies at risk. If you would like to learn more about the importance of bats and the challenges they face, please refer to the 2019 Earth Science Week field trip guide,  Lode and Lode and Behold! Geology and Natural Resources of the Truckee Meadows and Virginia City .