Joshua Tree is one of the most magnificent places where I have ever been. The desert landscape of southern California gifts us with many breath-taking geological features, and are home many plants and wildlife species (check our trip to the Cholla Cactus Garden, for instance!). We visited the national park in the distant 2014. We entered from Cottonwood Visitor Center, to the South. From here the road twists and turns through the valleys and alluvial fans of the Pinto Basin. Desert shrubs dominate the landscape, surrounded by black hills that stand out in the background.
These black hills are made of foliated and fractured metamorphic rocks known as the Pinto Gneiss. The Pinto Gneiss it one of the oldest outcropping rocks in the park, with an age of about 1.7 billion years. These rocks formed in the Proterozoic, before the development of complex lifeforms. At the time, they were intrusive, granitoid rocks. Metamorphism at temperatures exceeding 500 °C and pressures around 3-4 kbars transformed the original granite in gneiss. Their dark color is due to the presence of bands of black metamorphic minerals like biotite, cordierite, and andalusite. There is also garnet (red/brownish) and bands of quartz and feldspars (light-colored).
Play with the slider to see an interpreted and uninterpreted snapshot of the Pinto Gneiss!
Heading west on Pinto Basin Rd, we reach the area of White Tank. The desert landscape suddenly changes to a rough terrain sprinkled with rocky hills and massive, jointed boulders. As the locality name suggests, rocks here are light-colored compared to the Pinto gneiss.
We have left the gneiss and entered another rock type: granite. The granitic rocks of Joshua Tree have monzogranite to granodiorite composition. There are also diorites and gabbros in the area, but they are found mostly in the northeastern part of the park. The age of the granites range from the Jurassic to the Cretaceous (about 200 to 65 million years ago). These granitic rocks originated from the solidification of large volumes of magma at depth during the early history of convergence between the western margin of North America and the Pacific plate (then Farallon plate). Their light-color is related to the presence of many sialic minerals (quartz and feldspar) and the limited content of femics – here mostly biotite and hornblende. In the picture below, quartz is easily recognizable because it is grey and transparent. Feldspars are white to pinkish, and biotite is black. Biotite contains iron that weathers to iron oxides, forming, in other words, rust.
From the parking lot at the intersection between Pinto Basin Rd and Stirrup Tank Rd (here), the contact between the granites and the Pinto Gneiss is superbly exposed in the panorama. This is an intrusive contact: the granites emplaced at depth as molten magma within the gneiss that was metamorphosed and pushed upward. Between the gneiss and the granites there is an age difference of about 1.5 billion years! Millions of years of uplift and erosion have exposed this contact at surface, allowing us to observe the striking differences between these rock types: the granites are obviously lighter than the gneiss but they are also much more resistant to erosion. Indeed, they constitute some of the most prominent geographical features in Joshua Tree, emerging from the soil and the sparse vegetation as rocky outcrops known as tors and inselbergs. The granites are also crosscut by multiple sets of fractures, but that’s a topic for another post [edit: you can read my follow-up post here]!
One thing that amazed me of Joshua Tree (and North American deserts in general) is the beautiful exposure of geologic features, which can be followed continuously for tens of miles.
Structures can easily be followed from space on Google Earth! I am not going to sketch the image below (horizontal scale = 1600 m/1 mile): at this point, I think you can tell me where is the gneiss and where’s the granite!
This place remained in my hearth and in my opinion represents a must-see place in one’s lifetime!
Side note: the title of this post has been freely inspired by the sign with the geological description on the site. Go there and read it!
References and further reading