Joshua Tree Geology Tour


We have been busy this last week. Great weather with high temps in the upper 70 and lower 80s and crystal clear skies. Yesterday that all changed with a southern California winter storm. The main part of the storm is on the western side of the mountains, the desert doesn’t get much rain but the wind has picked back up and the mountains are hidden by clouds. There was rain overnight and flash flood warnings, but just wet and cloudy today so far. Hopefully the rains will bring wildflowers in the next couple of weeks before we leave?? Yesterday and today enjoying staying in, restocking, washing and making some stew since the temps this weekend are to be below average with highs in the 60s. Also catching up on our blog!!

Of course, we went back to Joshua Tree. We took the self-guided geology tour, an 18 mile off road tour of various geological formations.   When you realize that many of the rocks and formations have been created over 1.7 BILLION years, it really makes our life span seem very insignificant. Being from the coast, we understand erosion as it relates to the beach, but to realize that the mounds of rock all around Joshua Tree were once mountains that have since eroded is fascinating.

Some of the mountains are still intact. This is because the rocks in these mountains are harder than others and erode much slower. All mountains are still eroding, it is just a matter of how fast the erosion occurs. The intact mountains are formed from 1.7 billion year old Pinto Gneiss which is a metamorphic rock. That means that this rock was formed from pressure, heat and chemical activity and is much harder and erosion resistant.


The mounds of uncovered rocks are 85 million year old White Tank Monzogranite. This rock was formed when molten magma intruded the gneiss. It cooled much slower being 15 to 20 miles below the surface and is therefore less dense than the gneiss and erodes much faster. You can really see that the rock is softer it looks and feels much like concrete, rough and grainy. While the gneiss is smooth. The erosion of monzogranite and sediments around it that have created the “piles” of rocks and interesting formations. So, each “pile” used to be a mountain and after erosion of that mountain these piles are what is left. Just imagine what it looked like millions of years ago!!


Close up of monzogranite

The white tank monzogranite cracked while cooling from molten magma and created joints. Over time some of these joints refilled with more molten magma and created dikes which are still another type of rock either aplite (light colored) or pegmatite (darker). This is what created the veins in some of the large boulders. It erodes slower than the monzogranite and sometimes protrudes out of the large boulders. Where these joints did not refill they eroded and formed the boulder formations. Some look like children’s blocks stacked on each other while others have rounded off and look like marbles. This is due to various stages of erosion.


Notice the dikes across the boulders.



You can also see strange pits and hollows on the rock surface where irregularities in the rock trapped water. This moisture promotes chemical breakdown of the rock to clay which holds more moisture. As they erode they produce shade which in turn promotes more moisture and before you know it (millions of years) you have a large hollow in the rock. Some as large as a cave, they are much cooler and a nice respite from the sun and heat of a hot day.  These hollows create some of the distinctive images in the park.

Famous Skull Rock
Face Rock on far right end of formation
Our picnic spot on the geology tour

Beyond these two kinds of mountains and rocks on the geology trail, there is a third much younger mountain. This mountain is believed to be as young as 2 or 3 million years old and is made of black basalt. This is believed to be formed from molten magma intruding into the monzogranite but not quite reaching the surface. The Basalt is more resistant to weathering than the monzogranite. The former layer of monzogranite that covered the basalt has since eroded and become part of the valley surrounding these young mountains.





This is an elementary geology report which could be better explained by some of my former co-workers (geologist) at Clark Environmental. But is what I understood from our brief class via the self-guided tour. Well worth the time.  Although I did not mention it earlier, of course earths movement especially here on the San Andros fault line contributed (a lot) to the mountains formation and cracking of rocks over time.

Below you can see the difference kinds of rock side by side.


and the difference in erosion rates on the edge of this mountain.


Peace and love.

4 thoughts on “Joshua Tree Geology Tour”

  1. What a great account of JTree. When we go, and this is just a must after seeing your pictures, your words will be out guide. Much love!!

  2. “…just remember the world is 4.543 billion years old and you somehow managed to exist at the same time as David Bowie.” (These rock photos are the best!)

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