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Minerals
Social climber
The Deli
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Jan 10, 2012 - 02:33am PT
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Back to weathering of hydrothermally altered granitic rock…
On Saturday, I noticed this small “cave” on the upper far right side of West Farthing Wall (in Tuolumne) when I walked by… Somehow I missed this before on previous descents. It’s pretty cool looking, although warm would probably be more appropriate if one were in desperate need of a sheltered bivy in bad weather, either rain or snow. The cavity would comfortably sleep two, if necessary. Here are a couple of photos, looking in and looking out. I should have taken a photo of the outside from farther away, with something for scale. Ah, good enough for now…
Looking into the small “cave”
Looking out, towards Olmsted Point and the Tioga road, etc.
This “cave” is another example of accelerated weathering of hydrothermally altered granitic rock (Cathedral Peak Granodiorite in this case), and is similar in geologic origin to the Wawona Dome feature posted by Tork.
We have been continuing our research on hydrothermal alteration within the Tuolumne Intrusive Suite and Allen gave a talk on the subject last month. From field observations, it appears that alteration is commonly associated with aplite/pegmatite dikes, which form during the last magmatic stages of pluton formation and final crystallization of residual felsic melt.
Abstract:
http://static.coreapps.net/agu2011/html/V14B-05.html
…found here:
http://m.core-apps.com/agu2011/abstract/575bd9e9b6f37808cc38b4ce53d4b39a
Here are a few more photos of examples of hydrothermal alteration in Half Dome Granodiorite in the Tuolumne area that are exposed on slabs; the material contained in the depressions is loose, coarse-grained sand that has accumulated from the weathering of the rock.
Weathered depression in hydrothermal pipe
Weathered depression in hydrothermal pipe centered along aplite/pegmatite dike
Weathered depression in hydrothermal pipe within aplite/pegmatite dike
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cleo
Social climber
the canyon below the Ditch!!!!
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Jan 10, 2012 - 10:19am PT
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That's weird stuff, Brian, thanks for sharing!
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Tony Bird
climber
Northridge, CA
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Jan 10, 2012 - 10:36am PT
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doesn't look like bubbles to me, looks like scoured-down-by-current, as you see at fossil falls. remember, lots of glaciation history, all kindsa water flow in the past. they didn't want to believe it with the channeled scablands.
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scuffy b
climber
heading slowly NNW
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Jan 10, 2012 - 03:07pm PT
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Awesome, Minerals!!
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Tony Bird
climber
Northridge, CA
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Jan 10, 2012 - 08:05pm PT
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why dingus, all it takes is one uintathere to fall down a tube like that. messes it up somethin' terrible.
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tom Carter
Social climber
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Jan 11, 2012 - 12:31am PT
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Thanks BL
Reminds me of the cave on Sorcerers
Have you been there?
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Minerals
Social climber
The Deli
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Jan 12, 2012 - 12:11am PT
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One thing that I should have been more clear on above… The coarse-grained sand within the depressions is not just from the weathering of the altered rock itself, but more of a collection of material that has weathered from the bulk of the rock and has washed or blown across the slab and was caught in the depressions.
“… much of that chlorite is likely altered hornblende.”
In Half Dome Granodiorite, yes, but Cathedral Peak Granodiorite contains very little hornblende; most of the mafic minerals are biotite. So in the cave photos above, it’s pretty much altered biotite. On a few occasions, I have seen biotite pseudomorphs after hornblende where a clot of fine-grained biotite occurs in the shape of a rectangular hornblende crystal, 1 to 2 cm in length. But these pseudomorphs are most likely due to a magmatic process, if not a metasomatic process, rather than hydrothermal alteration. I don’t understand the exact mechanisms and chemistry behind it and have heard it described as very complicated, chemistry-wise.
It’s interesting how some mafic enclaves and xenoliths weather more quickly than the surrounding rock and thus form a depression or hole whereas others are more resistant and have positive relief. Sometimes, it appears that grain size of the inclusion versus grain size of the host rock plays a more significant role than the difference in composition – finer-grained rocks can be more resistant to weathering. The giant “chickenheads” on routes in the western part of Yosemite Valley (Pat and Jack area, Elephant Rock, etc.) protrude significantly from the surrounding rock, making great climbing holds, even though these mafic enclaves are composed of mafic minerals which are less resistant to weathering than the host El Capitan Granite, which is quite felsic in composition, but coarse-grained.
I’ve got a few plutonic-related geo maps by D. A. John… pretty cool stuff!
Thanks for the paper, Wes. Saved it and will check it out further.
“Reminds me of the cave on Sorcerers
Have you been there?”
Nope. But I’m not badass like you, Tom!
It is shown at the beginning of the last pitch of the route in the Reid/Falkenstein guide. Got any photos of it to share? Do tell! :)
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Jan 12, 2012 - 01:04am PT
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Looking at this thread reminded me of this photo taken of the Trummelbach Falls near Lauterbrunen, Switzerland.
The Trummelbach Falls are a series of subterranean glacier-waterfalls made accessible by tunnel-lift. It drains the Eiger, Mönch, and Jungfrau glaciers (24 km²)and carries 20,200 tons of boulder detritus per year and discharge at 20,000 liters per second.
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cleo
Social climber
the canyon below the Ditch!!!!
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Jan 21, 2012 - 10:56pm PT
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On a little recording excursion to Lower Cathedral Lake, we stopped by to look for the cave.
Turns out to have a great sunset view:
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Minerals
Social climber
The Deli
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Jan 21, 2012 - 11:50pm PT
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That looks wild, Tradster! Limestone?
Nice photos, Cleo! Glad that you found it! :)
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Phil_B
Social climber
CHC, en zed
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Aug 23, 2016 - 03:40pm PT
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Cleo and I were up at West Farthing Wall this weekend. Thanks Minerals for the great routes for us less than bold climbers. We had a kid making the transition from gym to outdoors and he now loves slab.
Dang, but that wall gets a lot of sun. I was cooked!
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