The oldest rocks in Joshua Tree National Park are 1.4 to 1.7 billion-year old metamorphic rocks.
These are the original rocks that the magmatic intrusions traveled up through, heating and altering the surrounding rocks as the hot material migrated. The metamorphic rocks in Joshua Tree National Park are divided into four different subunits. The oldest is called the Joshua Tree Augen Gneiss. This rock was previously granite, an igneous rock that you may have heard of if you have ever shopped for countertops. This granite was subjected to high pressures and temperatures which caused the minerals in it to migrate into bands. Small, eye-shaped structures in the rock called augens formed alongside the mineral bands. On top of the Joshua Tree Augen Gneiss is a rock unit referred to as the metasedimentary suite of Placer Canyon. Metasedimentary means that before metamorphosis, this rock was sedimentary. It is mainly composed of the minerals quartz and dolomite. The next rock unit is another augen gneiss, which is found at Monument Mountain and can be recognized by its dark color and large crystals. Last is another metasedimentary unit of the Pinkham Canyon. It contains several different types of rocks: quartzite, schist, granofels, and dolostone.
Within the park, there are at least five different bodies of igneous rock. These rocks originated as intrusions of magma into the metamorphic rocks mentioned above. The oldest intrusions rival the metamorphic rocks in age, and the youngest are only tens of millions of years old.
Within the park, there are several mountain ranges: The Cottonwood, Little San Bernardino, Pinto, Eagle, Hexie, and Coxcomb Mountains. Low-lying valleys (structural valleys and erosional valleys) lie between these mountain ranges. Structural basins form along faults (planes of weakness where displacement occurs), often resulting in blocks of rock between two faults which drop relative to the adjacent mountains. One example of a structural basin is Pleasant Valley. Erosional valleys are easier to visualize. They form because of erosion, as their name suggests. This erosion is largely caused by water and/or wind slowly eating away at the rock.
San Andreas Fault System
Joshua Tree National Park has numerous joints (cracks in rocks) and faults, (cracks along which movement has occurred). One of the most notable fault zones in the region is the San Andreas Fault System. The San Andreas is a strike-slip fault, meaning that the masses of rock are moving past each other like two lanes of opposing traffic. This fault is located just outside the park, but you can observe it from Keys View. Here, the fault branches off into two smaller faults called the Mission Creek and Banning Faults. They are easy to see because there is a wedge of black rock called the Indigo Hills between them that has been forced upward.
Many of the rocks in Joshua Tree National Park have been subjected to chemical and physical weathering. Chemical weathering generally occurs inside the rock and includes chemical changes, such as those that happen as water interacts with the mineral grains. The water itself and anything that is dissolved in it can cause a chemical reaction that changes the minerals.
Physical weathering involves the mechanical breakdown of rock by wind, water, gravity, or other forces such as freeze-thaw cycles or displacement by roots. Frost-wedging occurs when water fills fractures or pore spaces in rock and freezes. Expansion of the water as it turns to ice can crack the nearby rock and continue to widen cracks over time during freeze-thaw cycles. Root-wedging occurs when the roots of plants infiltrate the rock, usually through pre-existing cracks, and cause these cracks to widen.
Much of the large-scale erosion that shaped the Joshua Tree National Park’s rocks is no longer occurring. Instead, the weathered formations that we see today occurred long ago. The landscapes left behind by these conditions are truly something to behold. In particular, large masses of rounded granite with interesting weathering patterns can be seen at places like Wonderland of Rocks, Split Rock, and Jumbo Rocks. Here, we can see something called spherical (or spheroidal) weathering.
Imagine a trickle of water slowly moving inside of granite. As the water comes into contact with a potassium feldspar, a common mineral which is known for the ease with which it reacts with water and hydrogen, a new, soft clay mineral (kaolinite) replaces the feldspar. Kaolinite tends to build up around edges of the granite. There is an abundance of surface area for the clay to build up in because of the abundant joints, or cracks, in the rock. Because the clay is soft, the clay-rich edges of the granite erode and become softer and rounded. Eventually, this process can round out large amounts of rock in strange and picturesque ways. Thin, rounded concentric sheets of rock also have a tendency to peel off in spheroidal masses, making the weathering of this rock appear even more curious.
Deserts have many unique structures that you can’t find in other places. This is largely because in arid desert landscapes, water is scarce. When rock erodes, there is not a lot of water to carry it away. When erosion happens in humid regions, on the other hand, streams and rivers often wash away rock debris from its source. In deserts, eroded rock stays nearby, forming some interesting geologic features. Some of these structures, like inselbergs, are relict structures, meaning that they formed million years ago. These structures tell us about the changes the land has undergone and give us insight into the past.
Pediments and Inselbergs
About seven to nine million years ago, the Mojave Desert was semi-arid, covered in soft rolling hills and vegetation, a vastly different scene than the dry, cactus-filled ground today. The granite bedrock was covered by soil which formed from water moving down through the rock, assisted by joints in the granite. Slowly, the jointed granite became more rounded, resulting in tall, elongated boulders encased by soil. As the landscape became drier, less water was available to chemically weather the rock and remove debris away, and soil stopped forming so rapidly. Rounded boulders and small rocks were left behind, no longer having enough water to carry away eroded material. Inselbergs can be found at Hidden Valley, Caprock, Jumbo Rocks, and Ryan Campground. They look like islands of rock.
The “ocean” that these islands rise from is made up of pediments. Pediments are rock surfaces which appear flat but have slopes from a half a degree to six degrees. There is some debate about how pediments form. Some believe that they formed at the same time as inselbergs, by the same erosional processes. A more common belief is that they form when mountains retreat. When we talk about retreating mountains, it doesn’t mean the mountains are picking themselves up and moving. Instead, the front of the mountains has been eroded by physical and chemical weathering, and a pediment is left on the land that mountain front used to occupy. Pediments are subject to winds and sheet floods that, for the most part, relatively clear them of debris. In order to be considered a pediment, any debris on the plane must be less than ten feet thick.
Alluvial Fans and Bajadas
Joshua Tree National Park has many mountains ranges with flat-lying areas between them, the park is an excellent place to find alluvial fans. Alluvial fans form at the base of mountain as eroded sediment carried within a stream is dumped out when the slope becomes flat. When it hits the flat-lying area at the foot of the mountains, it slows. The faster a stream flows, the more rocks and sediments it can carry, so when the stream slows down, it drops the debris that it is carrying. This debris spreads out in a shape that often looks like a wedge, or a fan. Alluvial fans are often poorly sorted, meaning that they have rocks ranging from small sediments to larger rocks. When multiple alluvial fans meet and overlap, they are referred to as a bajada.
Playas and Arroyos
Playas are flat, shallow ephemeral lakes that can extend for many kilometers. Often, they are only centimeters deep. Though they are lakes, playas are only filled with water very rarely. In fact, they are lakes only during the rainy season, and only for a few weeks a year.
Arroyos are streams in the same way that playas are lakes. They are filled with water even more infrequently. They are dry stream beds most of the year, except for a few days or even a few hours. They are usually only filled during rainy season floods and are deep, steep-sided channels cut into the surface.
Desert varnish consists of manganese, iron, and clays which form a thin, dark yellow to black layer on the surface of many of the rocks in Joshua Tree National Park. Desert varnish may form when bacteria draw manganese out of the wind-blown sediment and oxidize the manganese before cementing it to the surface of the rock. Or, particles in the air containing clays, manganese, and iron may be deposited on the surface of the rock without the help of bacteria.
The desert varnish in Joshua Tree National Park is over 2,000 years old. There is archaeological evidence to support this. Ancient petroglyphs are carved into the desert varnish, which date back to 2,000 years or more!
Some of the soils in Joshua National Park, along with other areas in the desert southwest, are alive! These soils are called cryptobiotic crusts, and they are mostly made up of a type of algae known as cyanobacteria. They can also contain other algae, moss, lichens, bacteria, and micro fungi. Though these creatures are tiny, they play a big role in ensuring the well-being of the ecosystem. For one, filaments, or threads, of cryptobiotic material move through the soil and leave behind an adhesive residue that holds the soil in place. They also protect from flash floods by absorbing rain and any nutrients it contains. This in turn makes the cryptobiotic crusts a source of nutrients (namely organic material and nitrogen) to the surrounding ecosystem. Unfortunately, these organisms are as delicate as they are important. They can easily be crushed by the foot of an off-trail hiker, and they take five to seven years to regenerate. It is important that we watch out for these little friends-of-the-desert.
In the late 1800’s, one of the oases in the park, Oasis of Mara, was a place for miners to stop when moving to and from surrounding gold mines. In fact, the park itself was subject to mining, and today there are over 2,000 abandoned mines and prospects in Joshua Tree National Park. Most of these 2,000 are prospects that were never mined. There are 288 mines within the park and 747 entrances to these mines. The mineral of interest to the miners was gold ore, which could be found in veins of quartz which run throughout the rock. Though the mining of the park started in the 1870’s, it peaked around 1920 or 1930. One of the most notable mines, the Lost Horse Mine, still has a well-preserved mill which can be seen along the Long Horse Mine Hike.
Joshua Tree National Park is located in the Mojave and Colorado Deserts of southern California and has an area of 794,300 acres. The first is called the rain shadow effect. To the west of the park, there are tall mountains. Air moving from the west towards the park is pushed up in order to get over the mountains. As the air rises, it becomes colder, and cold air can hold less water than warm air can. Because of this, the water molecules carried in the air as clouds will fall as precipitation. The newly dried air continues to move, having succeeded in getting over the mountain range. Because of the lack of water in the air, the area to the east of the mountains will rarely get rain, and this area is said to be in a rain shadow.
The other cause of arid conditions in the park is the Hawaiian High. This is a mass of air which has high pressure. In the summer, it hovers over the Pacific Ocean and blocks the movement of warm moist air to California. The Hawaiian High combined with the rain shadow effect ensure that Joshua Tree National Park does not receive much precipitation. However, despite this, it is a majestic and thriving environment.
Oases and Springs
Oasis and springs seem paradoxical in a desert, yet they are quite common in the park. They often form alongside the faults mentioned above. This is because when faults move, they grind against each other, pulverizing the rock along the fault. This condensed, pulverized rock often can’t let water through, so groundwater is trapped. The groundwater experiences an increase in pressure, which causes it to rise to the surface in nearby areas, often through cracks in the rock called joints that have been caused by the fault. This is why oases and springs in the park are often located near fault zones.