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Dos XX
Trad climber
Los Angeles, CA
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Apr 27, 2012 - 05:43pm PT
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@greg & others involved in the study: I didn't mean to sound so in your face with my comments -- I got an early start on the Friday Night Posting While Drunk thread, I think ;-) The project sounds really cool. Maybe at some future date there could be a follow-up study that includes structural data collected on the rock itself, rather than remotely as will be done in the present study. You'd just need a few climbers who are handy with a Brunton.
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Minerals
Social climber
The Deli
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Topic Author's Reply - Apr 27, 2012 - 05:49pm PT
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Thanks for all of your comments! Glad that this is of interest to climbers and geology students alike. Right on!
Thanks, Wade! Ah, the Alien Skull… But you know that’s megacrystic granodiorite from another intrusive suite…
Thanks again for the use of your photos, Tom! Way cool.
Spider, aside from the specific ages of the various rock types in the Valley, the geologic maps linked in the first post will answer your question on the rock types of each different formation. Have a look at the maps.
Interesting idea on the use of a remote-control helicopter for taking photos. Thanks. I don’t know if it would be possible to get close enough to the wall to get rock texture photos and it would be tough to see the thing if it were too high up the wall. We might end up picking up a bunch of pieces at the base… Would you want to be stuck at a belay or on lead if I were at the controls…?!? : )
Prod, I think it would be helpful to get more photos from routes where we already have belay photos. If you end up climbing Mescalito, it would be great to get more photos, including mid-pitch and at contacts between different rock types.
There certainly are many structural features on El Cap, and joint systems that repeat in a pattern across the wall; the many dihedrals on the southwest face are a great example. One of the things that has struck me over the years is how the major dihedrals/corner systems on the upper portion of the Muir Wall, The Nose, The Real Nose, the Wall of Early Morning Light, Space, and Mescalito have formed in Taft Granite, yet these major features end roughly at the lower contact of Taft Granite and the band of tonalite below. This must be due to differences in competence and structural behavior of the different rock types.
Another good example of the differences in structural behavior of different rock types can be seen to the left of belay 6 on Tangerine Trip. Notice the series of joints in the tonalite/granodiorite dike on the left side of the photo; this series of joints appears to be oriented roughly perpendicular to the dike and they terminate just past the dike margins.
Photo by Tom Evans.
The red arrow in this photo points to the rightmost joint in a series of at least 7 left-leaning joints, some of which form ramps. Several different climbing routes take advantage of these joints, from Zodiac to Bad to the Bone. Also notice the repetition of right-leaning ramps in the lower portion of the photo, on pitch 5 of The Shortest Straw, the left side of the Black Tower (Zodiac), and the left side of The Devil’s Tower (Born Under a Bad Sign).
How about magmatic faults?
Apparent right-lateral offset of a dike hosted within Taft Granite, with a climber on the first pitch of Zenyatta Mondatta.
Photo by Tom Evans.
Apparent right-lateral offset of a magmatic feature in El Capitan Granite at the base of the southeast face, below Iron Hawk.
Apparent right-lateral (dextral) offset of a tonalite/granodiorite dike by magmatic faults and apparent left-lateral (sinistral) offset by the intrusion of a younger aplite/pegmatite dike on Mescalito (I annotated this photo a while ago). The question is, what has caused the change in direction of apparent offset over time? Is this a function of a change in regional or localized stress fields? There are two separate sets of aplite/pegmatite dikes exposed on El Cap; please refer to the 7th and 8th images in the first post of this thread. The older set appears to offset plutonic features in a left-lateral sense while the younger set appears to offset plutonic features in a right-lateral sense.
Photo by Tom Evans.
View looking up the wall from the base of The Footstool. Notice the large concave depression where the main portion of North America Diorite is exposed on the face. This is due to rock type – the diorite is much more susceptible to fracturing than the granites, which has caused the diorite to fall apart at an accelerated rate. I think the presence of North America Diorite is the main cause for the overall concave shape of the southeast face of the wall. If North America Diorite were not present, I am guessing that the southeast face would be more planar or convex, rather than concave.
Thanksgiving Ledge seems like it would be related more to the Salathe Roof and Chickenhead Ledge if one were to try to connect these sub-horizontal structural features, don’t you think?
Are the major joint systems seen on El Cap and the rest of the Valley regional tectonic and/or uplift features, or are some of these joints older cooling features of the plutons?
Roger and Greg, thanks for your posts! Great stuff!!!
Don’t knock it Jeremy; you are in one of the photos… ; )
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stunewberry
Trad climber
Spokane, WA
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Apr 27, 2012 - 06:05pm PT
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Two pix up, the late aplite has matching walls, and 2m left of the old magic dike, the zig zags appear to match-- which, unless it's a 3D viewing effect, indicates only extension on the late aplite opening, no horizontal translation.
Be careful about right and left lateral when in fact the horizontal surfaces should be called east or west vergent (depending of course which way the wall faces)
Bravo to this effort! More later
"Stu"
Edit --the iPad knows more about what I wanted to say than I did "magic" should be "mafic"
Also -- being asked to participate in "bedrock" ( sorry) science like this is unique. The climbing community is generally intelligent, curious, and observant, as well as skilled in techniques no one else can do. It's like, if they'd existed at the time, the Galapagos Island Chapter of the Audubon Society had been asked by Charles Darwin to measure the sizes and shapes of the finch's beaks on the islands. everyone knows what.a waste of time that data and understanding it allowed turned out to be!
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Dos XX
Trad climber
Los Angeles, CA
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Apr 27, 2012 - 06:15pm PT
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Regarding the accelerated wasting of the black diorite on El Cap's southeast face, here's something that my Geomorphology professor (Larry Lattman, who was taught by Hans Einstein) mentioned regarding dark rock in a lighter coloured matrix: the dark rock would be more susceptible to thermal weathering than the lighter coloured rock and thus waste at a greater rate, leaving localized concavities. Intuitively, that makes sense, but I remember when I heard Prof. Lattman say that, I immediately thought of one of my favourite climbing crags that had countless, black diorite chickenheads protruding from a sea of white monzonite -- just the opposite of what my professor indicated should happen. Geology: it's a wonderful art ;-)
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Minerals
Social climber
The Deli
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Topic Author's Reply - Apr 27, 2012 - 08:48pm PT
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Thanks, “Stu.”
“… the late aplite has matching walls, and 2m left of the old mafic dike, the zig zags appear to match…”
I’m not sure I understand what you mean by this. I used the word “apparent” to indicate that overall offset direction may not be what we see in the rock in two dimensions because we don’t know exactly how the dikes are oriented in three dimensions. In the Zenyatta photo (4 photos up), the arrows are to show that the dike is offset; movement towards or away from the observer could also create this apparent offset, depending on how the dike is oriented in the third dimension. Am I making any sense?
Dos XX, as far as thermal effect of weathering of rock, I asked some questions about that in this thread:
http://www.supertopo.com/climbing/thread.php?topic_id=1784752&msg=1789192#msg1789192
Mafic minerals are less resistant to chemical weathering so we would expect mafic enclaves to weather out faster than surrounding granite, rather than slower, such as is indicated by the “chickenheads” found in the western part of Yosemite Valley and other granite areas. I think this is a function of grain-size, where the finer-grained mafic enclaves (usually diorite) weather slower than the coarse-grained granite host, leaving really cool holds for climbing. In some cases I have seen the opposite, where fine-grained mafic enclaves have weathered out faster than the surrounding granite, leaving a cavity in the rock. I’m not sure why the weathering rates are inconsistent.
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gstock
climber
Yosemite Valley
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Apr 30, 2012 - 06:34pm PT
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I'm not sure either why mafic enclaves sometimes stick out as chickenheads and sometimes erode back as jugs. The most interesting boulder I've seen this play out on is at Turtleback Dome. It's a sloping boulder with parallel sides (the top side is a slab and the underside is an overhang). The mafics on the top stick out, and the ones on the bottom are eroded back. Check it out next time you're there - it's a mind-bender.
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PSP also PP
Trad climber
Berkeley
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Apr 30, 2012 - 08:13pm PT
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Mapping the different types of granite on El Cap sounds like fun especially the climbing and the rapping. Other than that it sounds extremely tedious and just maybe a bit boring. Kind of like identifying all the differnet kinds of oatmeal in the world; which could be fun if it was all on a vertical face.
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213
climber
Where the Froude number often >> 1
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Apr 30, 2012 - 08:33pm PT
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Killer maps Roger! Really pumped for the 'fieldwork' and knowledge enhancement that will go down as a result of your work. Here's my one comment: The maps need a scale bar!!!! (I know you are gonna go 'DOH!' at that one :) hehe).
Send my regards to miz robin and the dawg!
-ben
Edit to add: Those joints on Tangerine Trip belay 6 look a lot like boudins, could they be?
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Bob Harrington
climber
Bishop, California
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Apr 30, 2012 - 08:34pm PT
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Here's my theory on mafic enclave innies and outies: when the rock surface has been exposed for a long time (whatever that is...), the grain size effect that Minerals mentioned is the principal factor in the weathering rate, resulting in chickenheads that stick out from the rock surface. In subsurface weathering in fractures where the soil water has lower pH than precip (from soil gas CO2 and organic acids), chemical effects dominate and the mafics weather faster than the granite. The road cut at Donner has a lot of innies.
I'd like to see that boulder on Turtleback.
Great thread, thanks you guys.
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213
climber
Where the Froude number often >> 1
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Apr 30, 2012 - 09:11pm PT
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the finer-grained mafic enclaves (usually diorite) weather slower than the coarse-grained granite host
That seems a bit counter-intuitive (lol, like many physical processes), finer grains mean greater surface area, so would that point more towards porosity and permeability as driving the rates?
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hairyapeman
Trad climber
Fres-yes
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Apr 30, 2012 - 09:41pm PT
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Very cool! I have always wondered about tangerine and NAW. Interesting! I will be following this!!!
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eeyonkee
Trad climber
Golden, CO
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Apr 30, 2012 - 09:45pm PT
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Big thumbs up!
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Captain...or Skully
climber
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Apr 30, 2012 - 09:52pm PT
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Dig it, Jeremy. Most things are a waste of Time. Some folk even think Wall Climbin' is a waste of Time(Jinkies! Say it ain't so!)
At least it's cool.Even I managed to learn a little Geology while climbin' with the Mineral Boy. The focus is astounding.
Right on, Bryan & co.....I can dig it.
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eeyonkee
Trad climber
Golden, CO
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Apr 30, 2012 - 09:58pm PT
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Is there a route where you stay on the same rock type all of the way up? Seems unlikely...but it would be nice to describe the route as start here and stay on the tonalite. It might look easy, but if its diorite, you're OFF ROUTE!
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RP3
Big Wall climber
Temporarily Chapel Hill
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Apr 30, 2012 - 10:08pm PT
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eeyonkee,
The closest I believe you could come to a route of that nature on the SE face would be the Pacific Ocean Wall. As you climb it, you are basically in El Capitan granite the whole way up. The last couple of pitches appear to run through a tonalite band. However, I believe it is as close as you can come.
Many of the routes on the SW face remain in El Cap granite the whole way (of the routes we have surveyed so far, Lurking Fear, the Aquarian Wall, and the Salathe are all entirely within El Capitan granite.
Good question!
-Roger Putnam
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Bryan: TFPU. What an exciting project. Any thoughts of using 3D visualization software (EarthVision, 3D voxler, Gocad, etc) to generate a 3D model of the intrusive units?
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DonC
climber
CA
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related to tuolumne_tradster's post - in general what technology are you using? How are you processing the lidar and what mapping/display packages are you using?
I'm interested in both the geology problem as well as the technology you are using.
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RP3
Big Wall climber
Temporarily Chapel Hill
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Regarding the previous two posts:
We are using I-Site Studio software developed by Maptek to process the LiDAR data. I have been digitizing contact lines within ArcGis and will georeference it to the LiDAR data once I am finished with the digitizing the contacts between intrusive units.
tuolumne_tradster: I have not personally used any of those programs. However, what you are talking about is the end goal of all of this. Dr. Allen Glazner and Dr. Kent Ratajeski mapped the summit in great detail about a decade ago, and we plan on combining that data with the results of this project to do exactly the modeling you speak of. It is pretty exciting because, as far as we know, a three-dimensional map of a portion of a batholith has never been made.
Thanks for the interest!
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stunewberry
Trad climber
Spokane, WA
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I was going to start a new thread about chickenheads, but the El Cap mapping thread makes a perfect place to introduce this stuff and at the same time bump the thread back to the top of the list. A lot of the pictures Minerals posted in his two main posts in this thread show some of the kinds of relationships I want to talk about.
Most of the darker-colored, fine-grained goobers in granites that climbers call “chicken heads” are called by geologists “mafic inclusions.” There are a lot of ways for errant chunks to end up in a granitic magma as it forms through melting, makes its way upwards, mixes with what it meets along the way, and crystallizes to form what we finally see exposed at the surface. The first is through an unmelted bit of the source area being carried along with the magma as it rises. Inclusions of this sort aren’t common in the high Sierra granites, or if they are they are present they haven’t been recognized.
The second is as chunks of the wall rocks that a magma passes through are ripped from the walls of the conduit and incorporated in the rising magma. These are recognized as being quite different from the host granitic rock, often being angular in outline and being metamorphosed sedimentary or other igneous rocks. Sometimes they are chemically and mineralogically unstable in the granite magma and react to form new metamorphic minerals, like the oblong chunk of green skarn with a coarse grained rim (metamorphosed dirty limestone, note the very coarse grained reaction rim. The brown patches are just dirt on the rock). The second picture is a biotite hornfels (metamorphosed shale) inclusion from the other side of the same block. Both inclusions are in a swarm of mafic inclusions, which are the next type to discuss. When a magma intrudes a solid host, the process may be arrested, with abundant dikes of the invading magma breaking the host up into a “magmatic breccia” along dikes where the angular pieces of the host can still be matched up across the dikes.
A third way, and the one that probably creates most of the chicken heads in Yosemite and elsewhere, is when two different composition magmas physically interact along some kind of interface, they will have different compositions, heat contents, degrees of and kinds of initial crystallinity and mineralogy, viscosity, and other parameters. Different mixing scenarios can be envisioned in which initially small amounts, or globs, of hot, mafic (diorite) magma mix with large volumes of another, usually cooler, felsic (granitic) magma. When this happens the higher temperature mafic magma loses heat quickly to the enveloping granite, and the minerals that form (hornblende, biotite, plagioclase feldspar as the main ones) are usually fine grained. As the degree of crystallinity of the mafic glob increases, its viscosity increases, and the mafic glob becomes partially solid as the growing crystals interfere with one another. At the same time, the heat lost from the mafic magma heats up the cooler granite magma, and may reduce its viscosity and even partially melt some of the crystals it may contain. If enough mafic magma is introduced, eventually the system gets heated up by the earlier batches, and the viscosity of the newly introduced magma may remain low enough that it does not form an effective solid, but instead can mix with the host granitic magma (which by now has heated up and may be partially remelted).
The analogy that I like to draw, and something you can try at home, is to take pre-stirred but still cold vanilla ice cream (the granite magma), into which you introduce increasing amounts of hot fudge sauce (the diorite magma). You have to use real hot fudge sauce, chocolate syrup doesn’t have the appropriate viscosity characteristics. Mix in just a bit of HFS and you get globs of chilled chocolate. Mix in more and eventually the ice cream begins to melt while the HFS doesn’t chill to globs, leading to true mixing between the chocolate and the vanilla, creating a hybrid. Some of the initial globs may remain as globs in the hybrid. All kinds of interesting mish-mashes can result, including multiple-generation mixtures, just like in the real world, as the next pictures show.
Large inclusions may quench such that the rims are fine-grained, while the cores retain their heat longer and thus crystallize more slowly, forming larger crystals in the core. Here is a picture of two adjacent mafic inclusions with fine-grained margins and coarser-grained (and visually lighter-colored) cores, the one on the left later cracked as a brittle solid. Following the cracking, the still-fluid ‘granite’ intruded into the crack (the pen points to the granite-filled crack).
When a lot of inclusions are present, they can travel with the host magma as it moves, or they may gravitationally settle through the host to accumulate as layers of inclusions at the bottom of whatever volume the magma occupies. The first picture in this post shows skarn and hornfels chunks along with some more typical mafic inclusions; the entire pile could only form through some physical (flow or gravity) accumulation of the chunks and globs.
I don’t think anyone has looked systematically at the weathering characteristics of inclusions and their hosts to see what the “inny-outy” controls of weathering of mafic inclusions are. The chicken head (outy) weathering may be related to the tightly-interlocking nature of the fine grained crystals of the inclusions, and/or there may be more microcracks in the host granite which promotes introduction of water and weathering, but those are just speculations.
"Stu"
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Dos XX
Trad climber
Los Angeles, CA
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Excellent material, Stu. Hooray for chickenheads!
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