the rocks of Joshua Tree National Park

A Fiery Tale of Magma and Plate Tectonics Over Millions of Years

Monzogranite (a specific type of granite) decorates the landscape of JTNP. The monzogranite is composed of the minerals quartz (smokey white in color), potassium feldspar (pinkish in color), plagioclase feldspar (plain white in color), and biotite mica (black in color). The presence of biotite distinguishes it from the more general granite. [Image modified from eps.mcgill.ca and from nps.gov]

Joshua Tree National Park is located at the crossroads of two tectonic plates: the North American and the Pacific Plate. The San Andreas fault, a deep fracture caused by the movement of the Earth’s crust, is the boundary that separates them. In the high desert where an earthquake can occur at any time, the landscape is decorated with boulders of igneous rock, once called plutons (blobs of molten magma that live under the surface of the Earth). These granite boulders are connected to the iconic, seemingly outer-space landscape in a way that has long drawn fascinated visitors, scientists, hippies, and soul-searching artists to the California Desert.

But how did these unique rocks form?

Watch the motion of the oceanic plate as it dives under the less dense continental plate in a subduction zone. [video from the IRIS Earthquake Science Consortium]

As was mentioned, Earth’s crust is broken into pieces (tectonic plates). There are two types of crust: continental crust, which is about (40 km (25 miles) thick) and made of granite rocks rich in aluminum, oxygen, and silicon, and oceanic crust, about 6 km (4 miles) thick and composed primarily of dark, basalt rocks rich in silicon, magnesium, and oxygen. Although the oceanic crust is much thinner than the continental crust, its mineral composition causes it to be much denser than the continental crust. Because continental crust is less dense than oceanic crust, it floats higher on the mantle (the layer beneath the crust), just like a piece of Styrofoam floats higher on water than a piece of wood does. These plates which float on top of the mantle (the layer below the crust), are constantly moving at an average rate of two inches per year. This is about the rate at which the everyday person’s fingernails grow.

Let’s rewind. 270 million years ago, most of the land on Earth was part of a single super-continent called Pangea. As the plates were moving, a plate of oceanic crust collided with a plate of continental (land) crust, at what geologists call a convergent plate boundary. Because oceanic crust is always heavier than continental crust, the oceanic crust dove underneath the continental crust creating a specific type of convergent plate boundary called a subduction zone.

Water carried down toward the mantle by the oceanic crust gets boiled by the heat from Earth’s interior creating magma that intrudes the overlying rock like the water in an  Italian stovetop coffeemaker (moka pot) boils upward into the overlying coffee grounds. [Right image from pintrest.au] — Water carried down toward the mantle by the oceanic crust gets boiled by the heat from Earth’s interior creating magma that intrudes the overlying rock like the water in an Italian stovetop coffeemaker (moka pot) boils upward into the overlying coffee grounds. [Right image from pintrest.au]

While the oceanic plate was subducting under the continental plate, it carried some left-over seawater down with it. The heat from the interior of the Earth boiled this water, causing it to rise into the overlying rock. This caused the rock above to melt, forming magma (underground lava) chambers.

The magma grew upward, intruding itself further into the surrounding rock and seeping into fractures (cracks) within the rock until it finally cooled forming a big blob (pluton) of stone a few miles below the surface. Over millions of years, the ground above these plutons eroded away, exposing them on the Earth’s surface.

watch as the magma creeps upward through the crust forming plutons. Overlying sediment gradually erodes, exposing the giant boulders. [video from nps.gov]

Monzogranite boulders at Hidden Valley trail, Joshua Tree National Park (2021)

Geologic processes continue to shape the landscape of the California Desert. Many faults (cracks that mark the boundary between two tectonic plates) around the world are not active; it’s as if they are sleeping. The San Andreas, near Joshua Tree National Park however, is active and very much awake. An earthquake could occur at any moment in this fiery landscape of magmatic rock, changing the geology of this Mars-like terrain on a large scale very quickly. In the meantime, slow and steady geologic processes like weathering (breaking down of rock through chemical and mechanical processes) and erosion (the movement of rock), shape the enviornment day by day for millions of years to come.

Written by Amanda Pascali, Geoscientist in the Park at Joshua Tree National Park - in collaboration with the Geological Society of America, Conservation Legacy, the National Park Service, and AmeriCorps (2021)