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Tork
climber
Yosemite
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Topic Author's Original Post - May 17, 2010 - 01:39pm PT
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Tell me about this, in granite?
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Dapper Dan
Trad climber
Menlo Park
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May 17, 2010 - 01:40pm PT
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where
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Minerals
Social climber
The Deli
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May 17, 2010 - 01:52pm PT
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Weren’t these previously posted in a Wawona Dome thread? Hmmmm… You must have just read my post on Power Dome… :)
Before I comment, do you have any close-up photos of the rock on the walls of those “tubes” that show the individual minerals and their color?
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Tork
climber
Yosemite
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Topic Author's Reply - May 17, 2010 - 02:27pm PT
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Yep, read your Power Dome blah, blah , blah.
Was hoping for some blah, blah, blah about Wawona Dome. I kinda like reading that sh#t even if I don't understand it.
Greg Stock said he had never seen anything like it in Granite.
I don't have any close up photos. If you check out Nanook's Wawona Dome site he has a mega high res shot that zooms in close.
The pics are of a tunnel through found on the third pitch of Fatty Can't Fit on Wawona Dome (Aka Flanders fits). One of the coolest routes I have ever climbed. To bad an hour approach is to much for most folks. Although, on the other hand, there will never be a line for any of the great routes there.
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msiddens
Trad climber
Mountain View
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May 17, 2010 - 05:21pm PT
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I'd hike that in a second.....so damn cool looking.
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Mungeclimber
Trad climber
sorry, just posting out loud.
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May 17, 2010 - 10:29pm PT
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caused by worms
take some flagyl, be right as rain in a couple weeks, er years.
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ß Î Ø T Ç H
Boulder climber
from the Leastside
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May 17, 2010 - 10:31pm PT
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Ultimately , anything molten could have bubbles / venting I'm guessing .
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Minerals
Social climber
The Deli
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May 18, 2010 - 02:40pm PT
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OK, sorry this took so long…
Hmmmmm, without seeing the outcrop up close, I’m simply making an educated “blaw” guess here, so if anyone would like to chime in to correct me, or to help out, feel free, please.
Wawona Dome is composed of 103-million-year-old (103 Ma) El Capitan granite. This rather large pluton is exposed from roughly the northern boundary to the southern boundary of Yosemite National Park (see link to Park map below). The erosion feature pictured above appears to be much different than the typical “solution pockets” (or series of solution pockets) that form on gently to moderately inclined granitic surfaces due to water runoff, pooling of water, and the resulting physical/chemical weathering. In this regard, I don’t see how surface weathering could create a “tube” in a mass of (relatively) homogenous and quite solid granite.
As far as the Valley goes, I don’t recall ever seeing any features like this in El Cap granite, however such features are somewhat common in both Half Dome granodiorite and Cathedral Peak granodiorite of the ~93–85 Ma Tuolumne Intrusive Suite. There are several of these holes on the South Face of Half Dome. Take a look at the Reid big wall guide and the topos for the routes on the South Face (I don’t have a copy in front of me now…). There are at least a few holes on the Harding route, some of which are named. The notation “Bivy Hole” appears on the Lost Again topo; Klaus said that you could crawl inside the hole and it was so deep that he couldn’t see the far end. On Cataclysmic Megasheer, Klaus made some wicked free and aid moves to climb out of the “Courtney Love Hole.”
Although it lacks tube-like geometry, “The Pod” (as I call it) is a similar example of such weathering features and can be seen while climbing Cathedral Peak, in Tuolumne. “The Pod” is located down and (climber’s) left of “The Chimney” and is labeled as “alcove” in the SuperTopo guide. There is a hole on the upper section of Dumpster Evangelist on Dozier Dome; I wedged myself in the hole in order to drill and place the next bolt.
So, what’s the deal with these holes and how do they form? If you look closely at the mineralogy in the rock surrounding the hole in the examples that I listed above, you will notice that the black minerals (biotite and hornblende) are absent and have been replaced, usually by two green minerals – epidote and chlorite. You may also notice that the rock overall appears more white or lighter in color than normal; this is because calcium has been chemically removed from the plagioclase feldspar, shifting the plagioclase composition towards albite (see plagioclase link below).
This difference in mineralogy is the result of hydrothermal alteration of the granitic rock which occurs at depth, after the rock has formed and is fully crystallized, but long before the rock is uplifted and exposed at the surface.
Hydrothermal = hot fluids.
Alteration = chemical changes in rock whereby new minerals are formed at the expense of old minerals (no kids for me, thank you…).
Hydrothermal fluids may percolate through certain areas of granitic bedrock, as a finger-like network, altering the existing granitic rock along fluid pathways. When the rock is eventually exposed at the surface (or lies near the surface), it is exposed to surface weathering, which begins to break down the rock. The secondary minerals that form during the alteration of granitic rock are less resistant to chemical and physical weathering and thus, altered zones weather at a faster rate, leaving depressions or holes in the rock.
We discussed epidote and hydrothermal alteration in the Geology Quiz thread a little while back:
http://www.supertopo.com/climbing/thread.php?topic_id=1048215&msg=1058381#msg1058381
Epidote:
http://en.wikipedia.org/wiki/Epidote
Chlorite:
http://en.wikipedia.org/wiki/Chlorite_group
Plagioclase feldspar:
http://en.wikipedia.org/wiki/Plagioclase
I can’t be sure that the crazy feature/hole that you have pictured above has formed by the weathering of altered granite, but considering that it is not in a major drainage, I don’t know what else would cause this to form. If this feature is halfway up a route, in the middle of a cliff, then it seems doubtful that it could be a pothole of sorts, formed by the erosive action of sediment-laden flowing water (i.e. creek/river or sub-glacial creek/river). Does the hole have a name?
What is interesting is how the section in your first photo appears to be so cylindrical – this may reflect an initial, roughly cylinder-shaped hydrothermal fluid flow pathway and/or the continued rounding of the walls and widening of the hole by subsequent weathering/erosion at the Earth’s surface. The multiple openings at the bottom of the feature may be caused by the branching of hydrothermal fluid pathways or simply by additional weathering. If the mineralogy of the rock on the walls of the hole is the same as that of the surrounding rock, then it is possible that all of the altered granite has weathered away, leaving “normal” looking granite.
So, why don’t we see holes on the front side of Half Dome if we see them on the South Face? The answer is they never really have a chance to develop.
The hydrothermal alteration that contributed to the formation of the holes on the South Face most likely propagates through the entire rock mass of Half Dome as well as the Upper Tenaya Canyon/Tuolumne Meadows areas, although it has influenced only a very, very minor percentage of the total rock mass of the area (almost insignificant). The Northwest Face of Half Dome is quite steep and is often characterized by climbers as having the feel of a series of expanding sheets or shells of rock that are barely stuck to the wall.
Half Dome lost its dome on its Valley side because of the major joint system that cuts right through the monolith. Glaciers alone cannot turn the side of a dome into a sheer, vertical plane of rock – joint systems provide huge weaknesses in solid rock for freeze/thaw cycles and “bulldozers of ice” to take advantage of. Loose rock is carried away by the conveyor belt, which later disappears, leaving all of us to look up (and down) in awe.
As opposed to its counterpart face that continually sheds its skin to renew, the South Face of Half Dome has an armored skin that has been exposed to the tests of time and has endured the weathering of thousands of years of Sierra climate, winter and summer alike. With the exception of the joints that form the two major left-arching features, the South Face is nothing less than the epitome of an impenetrable fortress of monstrous proportions – Sierra granodiorite (or any rock, for that matter) at its very finest.
But not every square foot of this dreamy face is as golden as can be, and altered zones of Half Dome granodiorite have been exposed to the elements on this lower-angle face for much longer periods of time than the more-temporary sheets of rock on the Northwest Face. Because it is lower in angle, the South Face is subjected to much more runoff and pooling of water over its surface than does the steep Northwest Face. This greater amount of water runoff accelerates surface weathering, which is accentuated where the granodiorite has been altered, forming the wild bivy holes.
Those who have traveled the Bushido Gully route to the top of the Diving Board/Porcelain Wall may have noticed that the rock in the bottom of the gully is not only full of joints (parallel cracks), but is much more pink in color; the pink color is due to the alteration of potassium feldspar (orthoclase). The major joint system that cuts through Half Dome served as a pathway for hydrothermal fluid flow that altered the rock that surrounds the joints.
Here are a few examples of hydrothermal alteration in granodiorite in the Tuolumne area:
A weathered-out pod in a section of hydrothermally altered Half Dome granodiorite provides a small haven for vegetation. This slab exposes a cross section of a cylindrical hydrothermal fluid pathway; the accelerated weathering observed here is due to the presence of secondary alteration minerals that are less resistant to surface weathering. This outcrop is located a little ways downstream of Tenaya Lake. Note juice bottle for scale.
Hydrothermal alteration in Cathedral Peak granodiorite (red arrows). This outcrop is located a little ways up the hill, to the NW of Micro Dome. Note pocketknife at base of outcrop on far right for scale.
Closer view of alteration on right side of above photo. Unaltered rock consists of orthoclase feldspar megacrysts in a coarse-grained matrix of plagioclase, orthoclase, quartz, and biotite (plus what we can’t see…). Altered rock contains epidote, chlorite, and albite-rich plagioclase and lacks biotite.
Closer view of the above photo – the light-green is epidote and the large dark-green patches are chlorite (pardon the poor 4 megapixel photo quality).
Hydrothermal alteration concentrated along an aplite/pegmatite dike in Cathedral Peak granodiorite at the base of Dozier Dome, on its far right side. Notice darker patches of epidote and chlorite within altered zone and acceleration of weathering as shown by the concave shape of the altered zone. Note pocketknife for scale.
Multiple weathered alteration pods, centered along an aplite/pegmatite dike in Cathedral Peak granodiorite. This slab is located just north of the drainage of Lower Cathedral Lake and can be seen while descending Dozier Dome to the south. As seen in the above photo, these alteration pods will often form along the plane of felsic dikes; this is most likely due to the structural planar weakness that exists at the boundary between the dike and the host rock, which provides a path of least resistance for hydrothermal fluids to flow. Alteration affects both dike rock and host rock. As opposed to a series of solution pockets in a granitic surface that form a chain of pods that is oriented parallel to the slope of a slab, these pods follow the dike, which is oriented at an oblique angle to the slope of the slab (notice the faint blacks steaks that are oriented parallel to the slope that form from water runoff).
A pothole in Cathedral Peak granodiorite, a couple hundred feet above the Tuolumne River and upstream of Glen Aulin. This pothole formed during sub-glacial water flow, rather than within an ancestral Tuolumne River drainage. Potholes should not be confused with weathered alteration holes and pods and are not the result of hydrothermal alteration; potholes result from the physical scouring of bedrock by solid rock fragments (of varying size) that are entrained in flowing water.
If one stops to pay attention, evidence of hydrothermal alteration can be seen here and there in the granitic rock of Yosemite and elsewhere. I remember explaining some of this stuff to fellow climbers at the base of East Cottage Dome, down and left, by the route Flintstone. There is at least one good example of hydrothermal alteration of Cathedral Peak granodiorite there on the slab at the base. Take a look around next time you are there. The slabs above the road at Olmsted Point are also hiding another good example that is not difficult to access, if you don’t mind walking for a few minutes.
Well… You said “talk to me”… so I got’a “blaw” ramblin’… Hope this helps and isn’t too much.
My comments and explanation above aren’t terribly scientific, considering that I haven’t referenced much in the way of sources… (Wiki and geologic maps linked below don’t really count). Although I did do a quick Google search that turned up stuff that would not be understandable if posted (links) and is full of jargon, what I have written above has been pretty much based on my understanding of what I have seen in the field, and discussions with professional geologists and professors in the past. Any other interpretations of Tork’s photos are gladly welcomed!!! What do you think, Greg Stock? Others?
For more info on El Capitan granite and the rocks of the Wawona area, see the following:
Yosemite National Park Geologic Map:
http://geomaps.geosci.unc.edu/parks/crops/YNP.jpg (cropped version)
http://geomaps.geosci.unc.edu/parks/fulls/Yosemite%20National%20Park.jpg (full version)
Yosemite Geologic Quad Map:
http://geomaps.geosci.unc.edu/quads/crops/Yosemite%20c.jpg (cropped version)
http://geomaps.geosci.unc.edu/quads/fulls/Yosemite.jpg (full version)
Ratajeski, K., Glazner, A.F., and Miller, B.V., 2001, Geology and geochemistry of mafic to felsic plutonic rocks in the Cretaceous intrusive suite of Yosemite Valley, California: Geological Society of America Bulletin, v. 113, p. 1486-1502.
http://bulletin.geoscienceworld.org/cgi/content/abstract/113/11/1486
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Minerals
Social climber
The Deli
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May 18, 2010 - 02:40pm PT
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“Ultimately , anything molten could have bubbles / venting I'm guessing .”
For the most part, yes, that is correct. The most common form of “bubbly” rock is referred to as vesicular basalt – the “bubbles” are called vesicles. Vesicles form when confining pressure is released, as magma ascends from depth in the Earth’s crust and volatiles (gas) within the magma expand. “Bubbles” or gas pockets may form in felsic granitic rock and are referred to as miarolitic cavities. I have not seen any miarolitic cavities in El Cap granite, however they are quite common in Johnson granite (the youngest plutonic unit in the Tuolumne Intrusive Suite). Miarolitic cavities are usually lined with well-developed crystals that are larger in size than that of the host rock. The tube in Tork’s photo does not appear to be a miarolitic cavity. It also does not appear to be related to a pegmatite pod within El Cap granite.
We discussed miarolitic cavities a bit in this thread:
http://www.supertopo.com/climbing/thread.php?topic_id=731799&msg=745947#msg745947
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mike m
Trad climber
black hills
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May 18, 2010 - 02:52pm PT
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Bubbles?
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Nate D
climber
San Francisco
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May 18, 2010 - 03:01pm PT
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Cool stuff. Thanks for taking the time to share, Minerals. The explanation for the differences in the rock on the S. vs. N. Face of HD, seems to apply to many other Sierra domes. Shuteye domes and crags illustrate much of this - a prime example being High Eagle Dome.
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Minerals
Social climber
The Deli
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May 18, 2010 - 03:14pm PT
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Mike, the pools atop Aquarium Rock appear to be simple solution pockets, which are quite common, especially in coarse-grained granitic rock in areas like JT and the Buttermilks.
Solution pockets in coarse-grained desert granite, Nevada. These solution pockets are formed by chemical and physical weathering of the surface of the granite, in areas of additional water runoff/pooling.
Nate, I’d love to check out Shuteye and the Southern Yosemite areas! Looks like wonderful stone.
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Dickbob
climber
Colorado
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May 18, 2010 - 03:38pm PT
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Once again, one of the reasons I do the Taco. learned a lot. My break is over. Thanks for that minerals.
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Mighty Hiker
climber
Vancouver, B.C.
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May 18, 2010 - 04:03pm PT
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Groovy stuff on the Apron at Squamish. This is nearly chimney-sized - you can get inside it in places and do armbars and heel-toes, or gastons if you're modern. Most just do high-steps, which is why some call it the elephant steps. Overall the feature runs for over 100 m, deeper at bulges, shallower on open flat slabs. There are some similar features elsewhere on the Apron, but nothing so long or deep.
Edit: It's graded "beyond category" - there's nothing else like it. Essentially water falling on the slabs above is funneled into this area, collecting stuff en route, and then shoots down. 10,000 years later, and voila.
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Minerals
Social climber
The Deli
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May 18, 2010 - 04:32pm PT
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That’s cool, Anders!
Anyone have a good photo or two of a climber on the Water Cracks on Lembert Dome, Tuolumne?
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MisterE
Social climber
Across Town From Easy Street
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May 18, 2010 - 09:44pm PT
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From Mountain Project:
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Tork
climber
Yosemite
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Topic Author's Reply - May 18, 2010 - 10:20pm PT
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Minerals, thanks for all the time you put into that, nice read!
Come up and climb on Wawona dome this summer.
Jeff
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hooblie
climber
from where the anecdotes roam
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May 18, 2010 - 10:32pm PT
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great contribution minerals, a lot of work and much appreciated
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Technogeekery
Trad climber
Tokyo, Japan
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May 18, 2010 - 10:32pm PT
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Minerals, thanks for posting, interesting stuff
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Fritz
Trad climber
Hagerman, ID
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May 18, 2010 - 10:46pm PT
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Minerals: Thank you for taking the considerable time to do a decently researched and very informative post!
You the man!
So-------thinking of hydrothermal alteration in granitic rocks: makes me think of a strange arch at City of Rocks Idaho.
As I recall: most arches are caused by erosion from wind or water, and/or exfoliation of the surface layers. However, an area of weakness from hydrothermal alteration seems like it could start bizarre formations such as this one.
Of course when you compare it to a limestone arch in Idaho: that was almost certainly caused by wind and water erosion------they are not that different.
Geology: The more I learn, the less I know.
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