Tuesday, February 23, 2016

Finding a Thesis: A Cinder Cone Aside

Just north of Silver Peak on S.R. 265 and just downhill from Mineral Ridge, one of Nevada's most accessible basaltic cinder cones rises out of the low alluvial slopes. The photo, taken from the northeast side of Mineral Ridge, where we made our last stop, shows the basalt cinder cone known as "The Crater" sitting in front of a southern bit of the Weepah Hills, the more distant and linear Paymaster Ridge, and the Clayton Valley playa, with its pale turquoise lithium brine fields.
A Google Earth view of The Crater and associated basalt flow rock. 
The cinder cone is assymetrical, as are many, with a breached east-facing side, where basalt lava flows erupted and spread downslope toward the Clayton Valley playa.
The cone is right beside S.R. 265, just north of Milepost 4, between Silver Peak and Blair Junction (see the map below).
Access to the cone includes a two-track dirt road on the south side and a pullout on the northwest side.
Click to enlarge this location map, the spliced and labeled Tonopah and Silver Peak 30x60 sheets, courtesy USGS. 
I pulled into a flat area on the northwest side of the cinder cone and walked around a little bit, stopping briefly while on my way to elsewhere: In 2010, I was on a loop from Tonopah to the Klondyke mining district to Silver Peak and Mineral Ridge and then back to Tonopah; in 1976, I was leaving Mineral Ridge to go to my next stop in California. Both trips took me past the cinder cone; both trips resulted in my next immediate destination being Blair Junction.
The Crater, looking southeast from its northwestern flank.
The cinder cone—at least the small chunk I saw—is composed dominantly of basalt blocks and bombs and smaller, lapilli-sized fragments.
A basic grain-size classification scheme for tephra, pyroclastic fragments that were thrown out of volcanoes, courtesy USGS.
Almost all the info on the web about this extinct volcano is contained in this one article by the Nevada Bureau of Mines and Geology (NBMG). The most detailed geologic map of the cone and area is here, in Figure 3B, along with a good description of the cone on page 9. This latter citation (Hulen, 2008) has the best reference to the only age date I could find:
“...the basalt has been K/Ar-dated at ~400,000 years (J. Witter, pers. comm., 2008), but this age is at odds with the cone’s barely-disturbed morphology. Accordingly, Witter has submitted a sample of basalt flow rock, clearly sourced from the cinder-cone vent (Figure 3B) for more reliable 40Ar/39Ar age-dating.”
All the other references I found state that the cone is about 390,000 years old without providing credit to an original source.

After parking and getting out of the truck, I walked around for a little while, trying to find some good examples of volcanic bombs. Rather than collecting any hand samples, I took a few photos.
Vesicular basalt.
This hand specimen from The Crater doesn’t show any indications that it was molten or semi-molten when it was thrown out of the vent or when it landed on the cone’s slopes—it is therefore considered a block and not a bomb—but it does show the average character of the basalt: dark-colored and with lots of small to medium-sized vesicles. The basalt is reported to contain phenocrysts of feldspar, hornblende, pyroxene, olivine, and magnetite (Price and Price, no date). I readily identified the small, light greenish olivine crystals in the basalt, though I didn’t get any pictures of the phenocrysts.
Volcanic bomb with a ropy or fluted texture.
The bomb is elongate in shape, and one end is wider than the other. Possibly the narrow end was upward when the bomb fell, still viscous, through the sky.
This could be a bit of flow or cooling texture in a basalt lapilli, but I'm not sure.
A large block of basalt with ropy and vesicular texture.
Am I seeing some aerodynamic shaping in this block, and perhaps some slight breadcrusting? Is this a bomb?
And here's a closer shot.
Here's a bomb with fluted texture, approaching spindly.
I probably should have looked harder for better examples of bombs, and maybe next time I visit, I’ll get around to the far side of the cone—or even up to the top!
A spiny bush and a prickly pear cactus growing on a desert pavement of basalt lapilli, small blocks, and bombs.
The bush looks like some kind of Atriplex or generic "saltbush," but I'm not sure which kind. Any ideas?
We can see the Weepah Hills and bluish Lone Mountain off to the northeast, beyond the dark, lower slopes of The Crater.
Next in this series, we'll head over to California.

Selected References:
Albers, J.P., and Stewart, J.H, 1972, Geology and mineral deposits of Esmeralda County, Nevada [available for sale only]: NBMG Bulletin 78, 80 p.

Hulen, J.B., 2008, Geology and Conceptual Modeling of theSilver Peak Geothermal Prospect,Esmeralda County, Nevada: unpublished report for Sierra Geothermal Power Corporation, 23p.

Price, J.G., and Price, E.M., no date, Cinder cone in Clayton Valley, Esmeralda County, Nevada: NBMG, Nevada EarthCaches webpage (with .kmz downloads available), accessed 18Feb2016.

Spurr, J.E., 1906, Ore Deposits of the Silver Peak Quadrangle, Nevada: U.S. Geological Survey Professional Paper 55, 174 p.

Related Posts:
Thesis: Finding an Area
Finding a Thesis: Battle Mountain to Austin to Gabbs
Finding a Thesis: Pole Line Road
Finding a Thesis: Pole Line to Belmont
Finding a Thesis: Klondyke District
Finding a Thesis: A Joshua Tree Aside
Finding a Thesis: Into the Palmetto Mountains
Finding a Thesis: Farther into the Palmetto Mountains
Finding a Thesis: A Bit O' Geology in the Palmetto Mountains
Finding a Thesis: Future Stories from the Palmetto Mountains
Lida Summit Roadcut
Finding a Thesis: Next Stop, Silver Peak!
Finding a Thesis: Coming into Clayton Valley
Finding a Thesis: On the Southern Route to Mineral Ridge
Finding a Thesis: The Northern Route onto Mineral Ridge and a Little Geology
Finding a Thesis: Up to the Millsite and Back
Finding a Thesis: Views and Geography and ... Oh, What's That?

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