The Joshua Tree National Park is very unique looking as is the desert in general. Probably the most impressive scenery is the gigantic rocks that make up the landscape. In order for these rocks to form you have to combine a multitude of different natural forces such as molten rock and fault lines.
Joshua Tree is crisscrossed with hundreds of faults, and is a great place to see raw rocks and the effects of earthquakes. The famous San Andreas Fault runs along the south side of the park, and can be seen from Keys View. Blue Cut Fault in the center of the park can be seen from the hilltop behind Lost Horse Mine. The fault forms the straight base of the Hexie Mountains east of Queen Valley.
Fault zones are important factors in localizing natural springs. Movement by faults causes impervious zones of shattered rock fragments to form an underground dam forcing ground water to rise. The Oasis of Mara at the visitor center in Twentynine Palms marks the Pinto Mountain fault. This fault is at least 73km long, however it could be up to 90km long. The park has four other fault-caused oases that support the native California Fan Palms. These oases supply food and water to a wide variety of wildlife and point to the connection between the park’s geology and its wildlife habitat.
Geologists believe the face of our modern landscape was born more than 100 million years ago. Molten liquid, heated by the continuous movement of Earth’s crust, oozed upward and cooled while still below the surface. These plutonic intrusions are a granitic rock called monzogranite.
The monzogranite developed a system of rectangular joints. One set, oriented roughly horizontally, resulted from the removal—by erosion—of the miles of overlying rock, called gniess (pronounced “nice”). Another set of joints is oriented vertically, roughly paralleling the contact of the monzogranite with its surrounding rocks. The third set is also vertical but cuts the second set at high angles. The resulting system of joints tended to develop rectangular blocks. Good examples of the joint system may be seen at Jumbo Rocks, Wonderland of Rocks, and Split Rock.
As ground water percolated down through the monzogranite’s joint fractures, it began to transform some hard mineral grains along its path into soft clay, while it loosened and freed grains resistant to solution. Rectangular stones slowly weathered to spheres of hard rock surrounded by soft clay containing loose mineral grains. Imagine holding an ice cube under the faucet. The cube rounds away at the corners first, because that is the part most exposed to the force of the water. A similar thing happened here but over millions of years, on a grand scale, and during a much wetter climate.
After the arrival of the arid climate of recent times, flash floods began washing away the protective ground surface. As they were exposed, the huge eroded boulders settled one on top of another, creating those impressive rock piles we see today.
Visitors also wonder about the “broken terrace walls” laced throughout the boulders. These are naturally occurring formations called dikes. Younger than the surrounding monzogranite, dikes were formed when molten rock was pushed into existing joint fractures. Light-colored aplite, pegmatite, and andesite dikes formed as a mixture of quartz and potassium minerals cooled in these tight spaces. Suggesting the work of a stonemason, they broke into uniform blocks when they were exposed to the surface.
Of the dynamic processes that erode rock material, water, even in arid environments, is the most important. Wind action is also important, but the long-range effects of wind are small compared to the action of water.
The erosion and weathering processes operating in the arid conditions of the present are only partially responsible for the spectacular sculpturing of the rocks. The present landscape is essentially a collection of relict features inherited from earlier times of higher rainfall and lower temperatures.
