Unique Rock Formations

Joshua Tree National Park

Rocks near Willow Hole. NPS Photo.

The geologic landscape of Joshua Tree has long fascinated visitors to the Mojave Deserts. How were these steep-sided, bold outcrops, rising abruptly from the desert floor formed? What forces sculpted these fantastic shapes?

Diagram of monzogranite joint systems

Geologists believe the face of this modern landscape was born more than 100 million years ago. Molten liquid, heated by the continuous movement of the 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 gneiss (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.

Intersection Rock at Joshua Tree in the snow. NPS Photo.

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. Geologists call these unique rock formations inselbergs.

An inselberg is a residual relief feature--an isolated hill, knob, ridge, or small mountain that rises abruptly, like an island, from a gently sloping or virtually level surrounding plain (inselberg is a German word meaning "island mountain").

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.

Erosional forces change the shape of the stones 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. The ground surface is washed away leaving interesting rock formations.

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 erosional and weathering processes operating in the current arid conditions of the desert are only partially responsible for the spectacular sculpturing of the rocks. The present landscape is essentially a collection of relic features inherited from earlier times of higher rainfall and lower temperatures.