
Photo: Jarbidge mine dumps and fireweed.
On our long trip to and from Oregon, we dedicated our only full day in Jarbidge to hiking, although we also walked around town and up canyon in the evening. Overall, it was a good day, with clouds providing some relief from the hot sun, while also contributing to an unusual dampish humidity.

Photo: Jarbidge Rhyolite in the the road.
The layering in the slabby bedrock sticking out of the road is probably flow layering; the rock is the Jarbidge Rhyolite — a crystal-rich rhyolite, often hydrothermally altered, which hosts the gold-bearing quartz-adularia veins of the Jarbidge mining district. The Jarbidge Rhyolite is about 15.5 to 17 Ma (from two dates by Coats, 1987, from elsewhere in northeastern Nevada).
Notice the rocks scattered over the road. The rhyolite weathers to a coarse grus-like material that made going uphill interesting and going back downhill downright slippery.

Photo: Hand sample of the Jarbidge Rhyolite.
This is the culprit! The yellow-brown to reddish-orange to brownish weathering, pink and white, crystal-rich rock weathers into sub-rounded chunks, creating a road surface remindful of a bunch of ball bearings.










Photo: Partly devitrified, flow- or compaction-foliated rhyolite.
As we approached the saddle, the soil became dark, apparently from weathering of a dark gray to brown, glassy, flow-banded rhyolite, the Cougar Point Tuff, which is about 10.5 to 13 Ma. The strongly flow-banded rhyolite was variably devitrified, locally forming gray, subrounded balls similar to Apache tears, but dull and much less glassy in character. The banding or foliation could be the result of compaction or flow or both.

Photo: Folded flow-foliation in the Cougar Point Tuff near the top of the saddle.

Photo: Flow lineations in the flow-banded rhyolite.
Flow lineations can be seen above my boot and below the whitish-looking lichen. This kind of flow foliation in extremely welded or high grade ash-flow tuff is common in high-silica to peraluminous welded tuff formations, making it difficult to distinguish these types of tuffs from rhyolite flows in the field.

Photo: Brownish outcrop of Cougar Point Tuff.
After walking to the saddle, in hopes of seeing a grand view to the north or west but instead finding only another canyon, we sat down and rested against some brown-weathering flow-banded rhyolite.

Elevation gain: about 850 feet. Distance total: about 5 miles. Time from start to finish: about 3 hours. Both gain and distance include walking to and from the motel. Other plants found besides those mentioned: sagebrush, Woods' rose, antelope bitterbrush, alder or water birch., and many, many unidentified or unmentioned others.
Trip report to be continued...
Some References:
Bonnichsen, Bill, 1991, Geology of Scenic Jarbidge Canyon Near Murphy Hot Springs, Idaho: Idaho Geological Survey, GeoNote 16, 2 p.
Cathey, Henrietta E, and Nash, Barbara P., 2004, The Cougar Point Tuff: Implications for Thermochemical Zonation and Longevity of High-Temperature, Large-Volume Silicic Magmas of the Miocene Yellowstone Hotspot: Journal of Petrology, v. 45, no. 1, p. 27-58.
Coats, R. R., 1987, Geology of Elko County, Nevada: Nevada Bur. Mines and Geology, Bulletin 101, 112 pp (links to sales info).
Jewell, P. W., Rahn, T. A., and Bowman, J. R., 1994, Hydrology and Chemistry of Thermal Waters Near Wells, Nevada: Ground Water, v. 32, no. 4, p. 657 - 665 (links to abstract).
Flow banding and rheomorphic deformation at Magma Cum Laude, January, 2011.
3 comments:
Such great contorted shapes with this Rhyolite. I remember walking on pumice into the crater that was like marbles 100s of feet deep. Yet the views and flowers are stunning, and I'm sure the shade much appreciated.
Fantastic walk - I can see why people compare the northwest USA with parts of the UK.
I think your unidentified pink flower is a Geranium species, but I'm not that good on stinkin' flowers, so that's as far as I'll go!
Julia, thanks for the clue on Geranium. I think it's probably either Geranium richardsonii or Geranium viscosissimum.
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