Arch Rock Trail in Joshua Tree National Park

In the northern part of the park, there are fairly large groupings of giant boulders that seemed to have just sprung out of the earth.  Or, some spaceship from another galaxy decided to toss them around from their hovering spaceship!  Just joking, of course, but that was a thought that popped into my head, because it just seemed like such an unlikely place to find such gigantic rocks!

Contrasting Texture

This image gives you an idea of why I said what I did about spaceships.  This happens to be an area where there are a lot of the boulders seemingly strewn about, but there are other areas where nary a rock can be seen.  However, they didn’t exactly “spring” from the earth, nor were they tossed from above.  These boulders are made of a type of granite called “monzogranite”.  These monzogranites formed beneath the earth starting about 245 million years ago, and with the youngest forming about 100 million years ago.  Because of the shifting of the geological plates in the area, cracks were formed in the monzogranite over the years, and hot magma from below the earth’s crust seeped upward via a process called intrusion and helped form these shapes underground.  They also broke some of the monzogranite into much smaller particles.  Eventually, erosion caused the smaller particles to wear away, leaving this giant boulders protruding from the earth. 

White Tank Granite

Along Arch Rock Trail, the granite is called White Tank.  Scientists believe the White Tank granite intruded into the overlying rock about 135 – 150 million years ago in this area.  This was during the time of the dinosaurs, so maybe the boulders didn’t seem quite so large to them!

Minerals in the Granite

This is a close up view of the granite making up the boulders along Arch Rock Trail.  White Tank Granite is made up of three basic minerals – quartz,  feldspar, and biotite.  Also, in the past, this rock was called White Tank Monzogranite and White Tank Monzonite.  Different names, but it’s all made up of the same minerals.

Arch Rock Trail View

I couldn’t help but look up and enjoy the view as I explored along this trail.  I was continually fascinated by the diversity of landscape that makes up this park!

Rock Formation

Here’s a close up view of some of the boulders.  Imagine – in the spaces that we see between the rocks, there once flowed hot magma from deep beneath the earth.  And, think about it – all these rocks were once one huge rock, in a sense – the crust of the earth!

Peek at the Arch

Compared to some arches I’ve seen (at Arches National Park in Utah), these is rather a teeny arch.  However, it does illustrate how the wearing away of rock over the years has created some lovely pieces of natural art.  And, in time, the continued wearing away will result in this arch falling and tumbling to become just some smaller rocks along the path.  Things do change, whether we want them to or not, is the lesson to be learned!

Soil Line

Can you see what you may think of as discolored rocks in this image?  Well, they really aren’t just discolored.  That’s what geologists call “desert varnish”.  If you look toward the bottom of the “varnish” you can see a lighter color to the rock (the rock in the middle of this image seems to illustrate this the best).  That line reflects where the soil once was.  It’s just another way we can see that nature is constantly changing all things around us.  Sometimes, as in the rocks, we can see the change, and at other times, it’s more subtle and we don’t really notice the change.  I find it interesting that the Mojave Desert area was once quite moist…..about 8 million years ago!  Perhaps part of the change in our climate is truly part of a natural cycle.  Worth considering.

 Sculpted Rock

Here’s another piece of Art by Nature!  If you look at the bottom of the rock in this image, you can see that the bottom portion of it has been sort of sculpted out.  This happened naturally via a processed called cavernous weathering and undercutting.  This occurs when water is trapped on the surface of a rock or in the soil long enough to begin to dissolve some of the minerals in the rock.  Running water and wind then wash away the dissolved minerals.  Generally, this type of sculpting is found on the shady side of rocks, where the moisture doesn’t evaporate so quickly.  Nature is just full of amazing things!

Enjoy!