A Geological History of Joshua Tree National Park

The area encompassed by Joshua Tree National Park has been restless for at least 1.5 billion years. What we see today is the product of at least two widely separated episodes of mountain building. The latest of these episodes was followed by uplift and very deep erosion and then by further uneven uplift along faults. Exposed by this erosion were two rock bodies originally formed deep below Earths surface, the 1.5 billion-year-old Pinto gneiss and the 150-million-year-old White Tank monzogranite which intruded the Pinto gneiss as molten magma.



Giant piles of white monzogranite rock stand tall among the forests of Joshua trees. In the center of the photo, you can see the giant dome near Barker Dam. This image was taken along the Barker Dam Nature Trail.

Many other geologic events probably occurred in this area throughout the vast time before and after the formation of the gneiss and the intrusion of the monzogranite. However, the rock record of these events has been lost through the deep erosion that has occurred. What remains are primarily the roots of old and very ancient mountains. The rocks that we see in the park have been faulted, jointed, weathered, and eroded to produce the geologic scenery of Joshua Tree National Park.

The monzogranite forming the rock piles seen throughout much of the park was once a molten mass which was forced upward, or intruded, the overlying, older Pinto gneiss. The magma cooled at a depth of about 15 miles below the surface and crystallized to form solid rock. Erosion over the ages has stripped away the overlying Pinto gneiss, exposing the monzogranite outcrops as you see them. 

The mountains along the west side of the park are composed primarily of the darker gneiss, which is more resistant to erosion than monzogranite. Within the monzogranite, those areas with more widely-space joint cracks weather more slowly than others and form the high rock piles called inselbergs. 

In some piles, well defined joint systems are obvious; while in other piles, smaller boulders have collapsed and obscured the underlying joint pattern.

The rounded rock masses found in the White Tank monzogranite are probably more extensive and better displayed at Joshua Tree National Park than anywhere else in the world. What is the story behind these rock sculptures? Massive rock bodies like this monzogranite commonly display sets of cracks. Called joints, that intersect at roughly right angles. 

Here's an example of joint systems that have formed on the light monzogranite rock in the Mojave Desert. In the distance, you'll see an amazing view of Queen Valley.
Notice the contrast, even right next to each other, of the rock formations. Some appear rough, jagged and asymmetrical while others appear smooth, rounded and symmetrical.

The nearly vertical cracks probably occurred when the rock mass contracted while cooling; movement along fault lines may also have contributed. As erosion removed the overlying rock, nearly horizontal cracks were created when the rock mass expanded upward. Jointing produces more or less cube shaped blocks of rock. Why then are most of the rock masses quite rounded? While the monzogranite was still below the surface, water containing carbon dioxide moved through the joint cracks. Little by little, the rock was dissolved along the joints by this solution and fell apart into individual mineral grains. Since this process is more effective on corners and edges, as there is more surface area than on faces, the cubes slowly changed into rock spheres.

Notice how the layers of light monzogranite rock form a symmetrically balanced stack.

This scene in Lost Horse Valley shows how green Joshua tree buds, sandy brown rock formations and purple mountains majesty dominate the landscape.

The white Tank monzogranite formed originally as magma deep below Earth’s surface and rose as great balloon-like masses to a depth of around 15-20 miles below the surface, where the magma cooled and crystallized. The large mineral grains are a product of slow cooling, which allows more time for crystals to grow. Many of the monzogranite boulders in this area have light colored bands of rock cutting across them. These bands of rock, called dikes, were formed when molten magma filled opening joints in the monzogranite. These dikes are composed either of aplite (light color) or pegmatite. They are more resistant to weathering and erosion than monzogranite and tend to protrude above it as low walls.



The white jagged spines that protrudes from the monzogranite formations are known as "dikes." They are simply intrusions of harder rock that didn't erode at the same rate as the host rock formation.

The Blue cut fault is one of a number of earthquake faults found in and around Joshua Tree National Park. The blue Cut fault extends for about 50 miles through the Little San Bernardino Mountains, under Pleasant Valley, and into the Pinto Basin. This fault is named for the blue granodiorite that is exposed on the mountainside to the southwest and marks the main branch of the fault. Through activity on a branch of this fault, the land was uplifted to form the steep, straight, southern edge of the Hexie Mountains, and on the other side the land was dropped to create Pleasant Valley.

The banded and folded Pinto gneiss, probably the oldest type of rock in the park, is approximately 1.5 billion years old. Gneiss is a metamorphic rock, whereas the basalt of Malapai Hill and the monzogranite are igneous rocks, the product of cooled and crystallized magmas.  Geologists believe that the Pinto gneiss was formed from preexisting sedimentary and igneous rocks. At some point the rocks were subjected to a deep burial where they underwent metamorphism to the present state. Metamorphism consists of changes in mineral composition, grain size, and orientation due to increases in pressure, heat, and chemical activity. Direct pressure causes certain mineral grains to segregate and band together, it is the alternate banding of light and dark minerals that defines a gneiss.

The roots of a mountain range formed about 1.5 billion years ago are revealed by the Pinto gneiss. The remainder of this range was eroded away long ago. The White Tank monzogranite that was formed around 150 million years ago has been deeply eroded as well. For the past several million years, uplift of most of the park area has been occurring in association with activity on the San Andreas Fault to the south. Along with this uplift, there has been further erosion, revealing more resistant rock as hill and less resistant as valleys. Local vertical movement on the north branch of the Blue Cut fault has uplifted the Hexie Mountains and depressed the adjacent Pleasant Valley. Across the valley Malapai Hill can be seen, formed by a shallow intrusion of basalt. Finally we see the mountains and hills weathering and their remains being eroded and deposited as debris flows, alluvial fans, bajadas, and the deposits of a dry lake.

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© Brad Biringer 1999-2005
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