Dad had a plane, and we’d fly over the Mojave Desert to have lunch in Las Vegas. I was always fascinated by the topography. As you can see in the map there are sandy plains with rocky outcroppings, which appear to be the foothills of the mountains to the north. The thing that struck me was the way they rise out of the flat sand in the same way that islands stick out of the sea.
When I was young, and from an altitude much lower than the linked satellite image where these mountains appeared more isolated, I imagined failed volcanoes that cooled before reaching the surface and that were being slowly uncovered by the shifting sands. Now of course I can see that they are part of a system. I’m not a geologist (though that would be fun), but I know there’s rhyolite in the Mojave. (I’ve always liked ghost towns, and there’s one in Nevada called Rhyolite. Haven’t been there though.) Rhyolite is a volcanic rock. But I’m sure the mountains are not volcanic and that they came to be from the collision of tectonic plates. The sand was mostly laid down by ancient seas.
So these mountains have a base under all of the sand, right? How far down is it? That is, if the sand is analogous to a water sea, where’s the ‘sea floor’?
Google “basin and range” or see Wikipedia Basin and Range - Wikipedia
“The basins are down-fallen blocks of crust and the ranges are relatively uplifted blocks, many of which tilt slightly eastward at their tops. The normal arrangement in the basin and range system is that each valley (i.e., basin) is bounded on each side by one or more normal faults that are oriented along or sub-parallel to the range front.”
The sand filling the space betweeen the hills is material that has been eroded from the hills.
The geology of the Mojave between the 15 and the 10 is a mix of sedimentary, volcanic and metamorphic rocks shaped by Basin and Range tectonics of crustal extension. However the area is somewhat distorted by the convergence of the San Andreas Fault and the Garlock Fault which is causing a clockwise crustal rotation in the area. The convergence is near where the little icon for the 5 is, it’s that light brown triangle that “points” to the west.
The sands in the area blow around and form dunes along the mountain fronts. When driving on 15 south, you can see sand dunes that literally go up the sides of mountains. The sands are likely from the Pleistocene or Pliocene lakes (appx. 5.3 - 0.01 million years ago) as well as from erosion of the surrounding mountains. Very similar to the Kelso Dunes in the Mojave Desert Preserve.
As far as how deep is the basin fill alluvium in this area of the Mojave, I don’t know actual numbers, but I would guess that it ranges from hundreds to thousands of feet thick (and with a bit of research I could probably find that out). For example, in parts of the Las Vegas Valley , alluvial thickness ranges from a few hundred feet on the west side of the valley to a little more than 13,000 feet thick on the east side of the valley. Sometimes I wonder about how the desert valleys would look if you could take all the alluvium out.
The valleys in the Mojave looked very similar to Death Valley when they were younger*. Death Valley is one of the youngest valleys in the Basin and Range system.
The mountains that poke out of the alluvial sands are called inselbergs. The mountains are being buried by alluvium at a faster* rate than they are eroding.
I am kind of tired and I feel like I am not any making sense. Hope this helps you out. I could give a better description of the geology, but I don’t know where my field books from my geologic mapping days in school are at right now( :o )
*young and fast are relative terms to us weirdo geologists. We think 30,000 years ago was recent
I, also, enjoy the material from LVgeogeek, as I grew up just north of the Garlock fault, in Ridgecrest. I’ve driven across that desert so many times, and always marvel at the varied geology it presents.
Wouldn’t it be cool if we could be sand-burrowing critters that could swim from valley to valley under the sands??
I spend quite a bit of time mapping around the Garlock Fault and up in San Bernardino County a number of years back, and spent a fair amount of time in Ridgecrest… Let’s just say I was glad to get back to Vegas.
That’s why I love being a geologist. There is always something new and fascinating to learn.
Do they have an explanation for WHY the Great Basin has those long parallel ranges of tipped crust remnants? If you’ve ever driven across Nevada on US-50 (and I’m fairly certain you probably have!), you certainly get the idea that they act like giant church pews or something…
It’s known as the Basin & Range province, as described in the link in Yeah’s post.
The “church pews” are normal fault-block mountain ranges resulting from crustal extension. The valleys are the down-dropped fault block (graben) and the mountains are the “pushed-up” fault block (horst).
In a lame/gross analogy- they are like geological stretch marks.
