From the Road: A Joshua Tree Lunch Stop with Volcanic Rocks

It wasn't long after I stopped to make coffee on the Turtle Mountains road, that it was time for lunch.

(Actually, before I found a place for lunch, I mistakenly pulled off on the paved road to McLaughlin, thinking that I could get a better picture of the Castle Mountains, a place I had worked long before, only to find that the road had no legitimate turnarounds until nearly reaching the Colorado River. This unforeseen escapade surely added an extra hour to my driving time!)

North of Searchlight, I found a suitable dirt road and drove away from the four-lane highway on blessed dirt. (Highway 95 wasn't four-laned in this part of the world the last time I drove on it; road widening began sometime after publication of the 2002-03 Nevada road map and was complete by the publication date of the 2005-06 road map.)

Before making lunch (and then more coffee), I wandered around and took a few pictures, including this one of an alive but bent over and half-burned Joshua tree. (I don't know if the tree was hit by lightning or lit on fire by an obnoxious passerby. The other Joshua trees nearby showed no signs of fire, so I don't suspect an old wildfire.)

What's that in the distance beyond the Joshua tree?

It's a hill of light-colored rocks covered by darker rocks (part of the Highland Range, Clark County, NV) .

My idea of the geology while out there was simple, and turned out to be fairly incorrect. I did get the type of rock right: Yes, it's (primarily) volcanic. I was thinking of a fairly thick ash-flow tuff sheet, with a thick poorly welded zone (in buff or pale orange), a very thin to intermittent vitrophyre (the thin black bed on the far hill), all overlain by a strongly welded zone (the sloping dark brownish hill-capping formation). Wrong. And wrong.

I found the detailed geology in this M.S. thesis (Colombini, 2009); the basics of her geology was largely from a map by Faulds et al (2002), but with more detail than I want to report. The larger picture consists of hill-capping basaltic andesite (this section might commonly be identified as basalt in the field) overlying a thin, dark, glassy rhyolite to rhyodacite flow (I guess I was partly correct on this!), overlying pale orange non-welded tuffs of mixed sedimentary and volcanic origin (and partly correct on this). Some flow-banded rhyolitic flows are mostly hidden at the base of the orange hills, barely sticking up high enough to be seen in the photo.

These rocks are all mid-Miocene in age, and they have all been tilted to the west (away from us) by widespread extreme extension related to detachment faulting within the Colorado River extensional corridor. The structure of the area is complicated by being within or near the Black Mountains accommodation zone (Faulds et al, 2002; abstract only, the rest is behind a paywall).