L'origine dei paesaggi granitici di Joshua Tree (California, USA)

Joshua Tree is a place where geology reigns supreme. Visitors cannot help but feeling in awe with the majestic landscape around and ask themselves: what geological processes produced such beautiful rock formations?

The over 3000 square kilometers [about 1200 square miles] of the park are sprinkled by piles of massive boulders of granite – rock formations known as tors – towering the surrounding sparsely vegetated hilly landscape . These formations vary in size from isolated groups of boulders to entire mountains.

The origin of these rock formations has in part to do with the climate of the area, but there is also a sweet geological explanation, related to the nature of the granitiche rocks that crop out in Joshua Tree. If you venture to the outskirt of the park, the tors rapidly disappear when we pass from the granites to the metamorphic rocks surrounding them: the Pinto gneiss. The spectacular contact, whom I have talked about in one of the first posts of this blog, clearly shows that the tors do occur only in the granites, while the gneiss produce a much gentler landscape.

This is because granites are much more resistant to erosion than the surrounding metamorphic rocks, because they consist of weathering-resistant igneous minerals like quarzo e feldspar, resulting from the crystallization of magma.

Not only that. Granites are typically very compact and offer few pathways for water to infiltrate and alter the rock. Nevertheless, even granites have their Achilles’ heel:

Even massive granite boulders like the one above are, indeed, crosscut by joints, regular sets of fractures that formed after the crystallization of magma. Granites crystallize at a depth of several miles into the crust. As erosion removes the several miles thick pile of rocks above the granite, the pressure acting over it decreases and the granite body starts to fracture. In Joshua Tree there are at least three dominant sets of fractures, one roughly horizontal and other two varying from vertical to steeply inclined, separating the granite into a series of rectangular blocks, as you can see in the slider below.

These fractures are the pathways used by percolating meteoric water to infiltrate deep within the otherwise intact granite. The feldspar minerals contained in granite slowly break down in the presence of water, reacting to produce clay minerals, which are the primary constituents of mud and soils. Granites slowly turns into a loose, unconsolidated material, known as regolith, that starts to form into the fractures and gradually replaces the intact granite cores, away from fractures. As water further reacts with these cores, the original cubes of granite gradually become more smoothed, starting to round along the corners.

When this mix of boulders and loose regolith reaches the surface, erosion easily removes the unconsolidated material, leaving behind piles of boulders, the intact material far from the joints that resisted weathering. Soil erosion happens very easily in Joshua Tree, thanks to the barren landscape, with little to no vegetation able to hold soil against the occasional flash floods. Indeed, tors are typical of arid climates, not only deserts but also periglacial environments like tundra.

Here you can see summarized in four stages (the slider below has four slides), the processes leading to the development of tors from granite outcrops.

Tors are my favorite geomorphological landform, because – to me – they are a reminder of the different forces that contribute to shape our planet: endogenous processes (magma emplacement and crystallization) and the exogenous ones (erosion and climate), acting together like a sculptor and its block of marble.

This post was possible thanks to the support of Sandra McLaren, Silvio Ferrero, and Ellie Coffelt (thank you!). In particular, it was inspired by Ellie, who supported geologyistheway because “our website answered our questions from our hike today”. If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!

Bibliografia
Powell, R. E., Matti, J. C., & Cossette, P. M. (2015). Geology of Joshua Tree National Park geodatabase (No. 2015-1175). US Geological Survey.
Joshua Tree National Park
Hike the Parks – Joshua Tree NP
Geology of Joshua Tree NP