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timstich
Dec 4, 2003, 1:27 PM
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I've spent a fair amount of time wondering what kind of rock I'm climbing on at an area I know little about. If you are like me, you wonder just how those rocks became the way they did when you are climbing on them from time to time. I just talked to a geologist at the Golden Brewery brewpub and we talked a bit about the Clear Creek to Bouder Canyon area. It appears that it was all once granite that had cooled a bit faster than the say the Texas E-Rock batholiths, which cooled slow enough for relatively large crystals to form. Crystals in Boulder Canyon are much smaller, which I find makes for more enjoyable climbing. The white spots in the picture below are quartz, the black is mica, and the pink is feldspar.
http://www.geo.umn.edu/...virt_egg/granite.jpg
Then came a long period of weathering and uplifting. The sediments from the weathered granite depostied in valleys where the Flatirons and Eldorado now are. On top of different layers of worn granite were deposited even more rock. This compressed and heated the deposits sufficiently to form sandstone. With each deeper layer of compressed sand, more tightly formed sandstone is made. Which is why in Eldorado, as you go deeper in the canyon the rock tends to be better consolidated. Rotten bands are to the right of Wind Tower facing it from the road. Not quite enough heat and pressure. Since there's a mixture of quartz, mica, and feldspar altogether in here, it looks red.
http://www.geo.umn.edu/...rt_egg/sandstone.jpg
With the continual uplifting, the rock beds became deformed and tilted to a 40-45 degree angle. Other igneous intrusions that didn't break the surface also reheated some of the granite and formed gneiss in Clear Creek Canyon. So the feldspar, mica, and quartz that makes up granite can be seen to swirl into layers and patterns. Gneiss is similar in it's feel to some granites. Granite looks more uniform since it was completely molten at one time. Gneiss is a metamorphic rock.
http://www.beavton.k12.or.us/...-01/rocks/gneiss.jpg
Cool, eh? Of course there's more to it than that. Anyone else out there discover some interesting geology about their local crags?
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madman_dan
Dec 4, 2003, 2:16 PM
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I see a booulder problem on the last one :P :roll:
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junaid
Dec 4, 2003, 3:00 PM
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Yeah, my local crag is the type locality for a magnetic reversal known as the Kaena reversal. The crag is actually at Mokuleia, but that was too hard for the guy to say. It is a huge a'a flow that seems to have ponded, good for us, since otherwise it would be too short to climb.
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copperhead
Dec 4, 2003, 11:40 PM
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Absolutely! Nice job, Tim. That’s sweet - a quick little 101 blurb about rocks…
Great pictures for your examples.
Petrology… gotta love it!
Rate of cooling is the predominant controller of grain size in an igneous rock. Generally, basalt is fine-grained or aphanitic (can’t see the grains with naked eye) and granite is medium- to coarse-grained (but may be fine-grained as well) or phaneritic (can see the grains with naked eye) though we can’t forget our pegmatites, and nucleation and diffusion rates…
Cool picture of the banded gneiss. Gneiss is a deformed rock (ductile deformation) that was subjected to extrinsic forces (tectonic stresses, etc…) in addition to an increase in temperature from the surrounding intrusions. We would call this rock an orthogneiss, because it was once granite (or any other type of igneous rock). Heat alone can’t turn granite into gneiss; pressure must be involved. If the rock was simply ‘cooked’, then a variety of things could happen and depending on the temperature increase, metasomatism, partial melting, or complete melting would result, and a recrystallization of the rock would occur; non-foliated textures may result, depending on original structure and degree of melting. If the rock wasn’t heated and the effects of stress took over at lower Ts then the resulting metamorphic rock would be a cataclasite, a sheared rock (brittle deformation), composed of angular fragments cemented in a (usually) dark-colored matrix of tourmaline and micro-grains of the parent rock; the tourmaline is secondary.
When minerals are oriented in a planar (parallel when viewed in two dimensions) structure in a rock, geologists refer to the rock as foliated. Gneissic foliation is referred to as solid-state foliation because the minerals and rock matrix deformed as ductile solids… as opposed to magmatic foliation where minerals are oriented parallel to magma flow during crystallization of a melt (or solution if you are Frank…) by liquid flow. The banded texture in gneiss occurs because of the difference in competence between the mica (biotite) and the quartz and feldspar (orthoclase or microcline, sanidine being the high-T feldspar found in volcanic rocks). On the Moho scale of hardness (1-10), mica is 3, feldspar is 6, and quartz is 7; diamond is 10. If the granitic protolith (rock before it was metamorphosed) was porphyritic, meaning that it contained large (in relation to the grains in the groundmass) feldspar crystals, or phenocrysts (megacrysts if they are huge), then the resulting gneiss would be called an augen gneiss, in reference to the phenocrysts. If we were dealing with schist (another type of metamorphic rock with a more pronounced foliation and preferred mineral orientation), then we would call the augens porphyroclasts, because of grain-size reduction. If the feldspars were garnet, staurolite, or kyanite (in a schist), then we would call them porphyroblasts (or metacrysts, not to be confused with megacrysts…), because of the mineral growth.
Ummmmm……. I think I got lost….. Oh well.
Did I use enough parentheses to make it all seem like one giant math equation?
In reply to: Anyone else out there discover some interesting geology about their local crags?
http://www.supertopo.com/...ybel_clouds_law1.jpg
Yup, the Big Stone. The rock of El Capitan is some of the only ‘true granite’ in the park, though some of it may grade to granodiorite. Most of El Cap is composed of El Capitan Granite, dated at 104 million years old, based on the radioactive decay of uranium to lead (once it passes thorium). The mineral zircon (ZrSiO4) is found in trace amounts in plutonic (intrusive) rocks. Uranium can substitute for Zirconium because of their similar atomic radii. Once grains of zircon are sorted and prepared, isotopes of uranium and lead are measured in ppm and then their ratios are used to calculate an age, based on the half-life of the radioactive isotope, whether it be 238U to 206Pb (4.468 billion years) or 235U to 207Pb (704 million years). I suppose we should include 232Th to 208Pb (14.01 billion years) to complete the list of lead isotopes.
Why is the base of the Nose area of El Cap like a skating rink while the upper dihedrals are what they are… Dihedrals…? Uh, the upper portion of the Nose and Mescalito and the Shield headwall, etc. are composed of Taft Granite, dated at 103 million years old. The difference in fracture behavior is due to the difference in rock type; Taft is more competent. From the upper portion of the Muir, Nose, Tribal, WOEML, Space, and Mescalito… you can see the difference in jointing by the huge corners that cap the proudest part of the Big Stone. Taft granite is finer-grained than El Cap Granite and contains fewer ‘dark’ minerals (i.e. hornblende and biotite); it is thought to be a partial melt product of El Capitan Granite, generated by the intrusion of hotter mafic magma (abundance of dark, Fe- and Mg-bearing minerals, as mentioned above, and plagioclase feldspar), such as the North America Diorite. El Capitan Granite is also thought to be a partial melt product of a mafic source. These hypotheses are derived from samarium and neodymium isotope data that tell us how much of the magma is of a primitive mantle (the plastic layer of the Earth that underlies the crust) source versus a partial melt of an existing source, be it a plutonic unit or a sedimentary unit, or metawhodawhatta for that matter… Mafic minerals melt at higher Ts than felsic minerals so quartz and feldspar will melt before hornblende and biotite. Bowen’s Reaction Series… but he was an old dude and maybe we should start thinking of new ideas, considering no one has been able to solidly solve some of the ‘granite riddles’ for over a hundred years… What gives?
Anyways, just west of El Cap lies a large mafic body called the Diorite of the Rockslides; this unit has also been dated at 104 Ma. (see above). The North America Diorite has been dated at 103 Ma. The Diorite of the Rockslides may be the mafic source (of heat) for the generation of the El Cap Granite and the North America Diorite may be the source of heat for the generation of the Taft Granite. These mafic bodies should be thought of as high-level dikes that have ascended to higher levels than their main source – remnant fingers of the main body of mafic magma. It seems as though both of these mafic intrusions chronologically followed their granitic counterparts, the North America Diorite being a late-stage dike, striking roughly NW with a near-vertical dip. Umm. So then we have two mafic sources, each with its own resultant granitic unit and fortunately, we have El Cap at the upper edge of the whole sequence to show us what’s going on. It’s really a beautiful window into plutonic behavior, if I may say so. We just need to figure it out…
And… to yibber yabber on… light gray dikes of hybrid tonalite strike (apparent) approximately 45 degrees down and to the left and are cut by the North America Diorite. These older mafic dikes are probably hybrid dikes that result from the mixing of El Capitan Granite with the mafic source (Diorite of the Rockslides) to produce an intermediate magma composition. We must remember that we are dealing with both a melt and a skeletal crystalline framework. The movement, crystallization of, and development of structures spans hundreds to thousands of years… maybe a million or so for the entire pluton to cool. Mineral melting temperatures tell us that mafic magmas are much hotter and will melt felsic rocks whereas felsic melts have lower melting temperatures and will not melt (very little) mafic rocks. If we bring a fifth rock type into the story, the white aplite dikes that cut the entire wall, then we have a good example of the intrusive characteristics of dikes of mafic versus felsic magmas. The edges of the diorite dikes are irregular and wavy yet the edges of the aplite dikes are, for the most part, straight as an arrow aside from the occasional bend. This difference in the nature of the contacts also has to do with occurrence in time, as in geologic time… The first set of mafic dikes were probably intruded at a time when the El Capitan Granite had not completely crystallized; this enabled the mafic magma to more readily mix with the existing granite and granitic melt. The late-stage, horizontal aplite dikes (though some may be (sub)vertical) most-likely follow planes of cooling fractures, similar to those found at surface levels, which are referred to as exfoliation joints. Another question remains and that is of the origin of the aplite dikes that cut El Cap. Are they from the Intrusive Suite of Yosemite Valley or are they younger dikes that stem from the eastern edge of the Valley, possibly from the Half Dome Granodiorite of the Tuolumne Intrusive Suite? If only sufficient minerals occured in these felsic dikes to give us a direction...
So, as a generalization, can we consider mafic, dioritic dikes to be roughly vertical in orientation and felsic, aplitic dikes to be roughly horizontal in orientation? If we’re talkin’ original intrusion, then we might as well be thinking about shear zones and mechanisms of emplacement but if we’re talkin’ horizontal aplite dikes, then we’re talking cooling fractures that have been later exploited by remaining felsic melts. Epidote- and chlorite-covered joint surfaces can tell us for sure…
The angular blocks of granite that appear in ‘Mexico’ have always fascinated me… are they floating in the diorite dike or are they fingers that are poking through from the back wall? THAT is the question! Stoping or diking? How does magma emplace itself and how about that ‘room problem’…? Will we ever agree?
Sorry to be a tangential pinball machine and for my lack of English creativity… time for another beer…
http://www.supertopo.com/...bel_streaks_law2.jpg
The coolest part of the base of the Big Stone. The gray finger on the left is a hybrid dike containing enclaves of more mafic material. Foliation can be seen (in the real outcrop) and is parallel to the (apparent) sides/walls of the dike, indicating magmatic flow. I don’t know if I can properly use the analogy of droplets of oil in water but it is something like that. The enclaves are blebs or remaining mafic magma that have not mixed with the surrounding hybrid/granite mush. The rust streaks to the left of Zenyatta result from the weathering of iron-rich minerals such as hematite and magnetite that are found in pegmatites (in this case, small blobs). Two larger blobs of pegmatite exist above but are out of view. ‘The Brain’ on Shortest Straw and ‘The Cauliflower’ on Ned’s Excellent Adventure are both pegmatite blobs – masses of very coarse-grained quartz, potassium feldspar (pink or tan), and plagioclase feldspar (white) that crystallize late in the magmatic sequence from the remaining felsic melt that may contain a high water content. Colorful streaks care of the Holocene and soluble minerals that are basically insignificant to real geology. Dribble dribble.
There is a plethora of wild patterns presented by the Big Stone, should you spend the time and let your mind wander. El Cap is a canvas with many answers, more than we know. At times, you may be gripped out of your mind with nothing more on your mind than your life… but don’t forget about the stone that you climb on, the stone that exists for your fun. It is a complicated piece of stone and be thankful that you are seeing a good bit that many others only wish they could see. Always, always, always, remember to have fun!!!
Anyone care to explain this riddle?
http://www.rockclimbing.com/...mp.cgi?Detailed=9558
I don’t get much stimulation at the dirt bike shop…
And I love rocks and can’t help but pass up an opportunity to yap/think about them.
Thanks for getting me to flip a bunch of pages…
Geology is cool.
Learning is fun.
Rocks are forever.
….Beavis says… “Duh duh, duh duh duh, dana nana nana, dah dann nah nah…”
Ozzy Rules.
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bluesky
Dec 5, 2003, 12:56 AM
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Wow!
First off nice post tim - especially since I climb in that area a bit.
Now - copperhead - that was amazing. I actually read your whole post. I think, just like journal articles, I'll have to read it several times to get a complete understanding! :) and nice choice of stone to describe!
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thomasribiere
Dec 5, 2003, 3:51 AM
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the cliffs I climb on in the Jura are all made of limestone. But I should say limestones as they are not all identical : some of them are full of fossiles and others lack them, some are compact and some are made of visible layers... Plus, some of the cliffs were formed when the Alps erected, so the Jura formed at teh same time with a successions of plateaus separated with abrupt slopes, some of them so abrupt we can climb on them. But other crags were formed later, when the rivers running down from a plateau to another one or when the underground rivers (limestone > karst > water runs underground) carved the ground to make canyons on edges of which we can now climb.
So different limestones formed at different periods, lately modified again by tectonic or erosion...
And now we can climb on those great limestone cliffs. :D
I could talk about geology really longer, but not in English as the terminology is not always "international".
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timstich
Dec 5, 2003, 9:17 AM
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Hey, great post there copperhead. That's exactly what I was looking for. I only took a few geology classes in college, and that was over 13 years ago. As for the limestone Thomas, one thing I've noticed comparing places like the Austin area, Potrero Chico, and the Eiger is that things seem to work out best for climbing when you have a nice strata of limestone tilted almost vertically and all of those sub-standard layers are underneath. In the Potrero the huge anticline that deformed the beds pushed very thick, quality layers of rock almost vertical. Then when the river cut a path through the circle of stone and normal weathering made some pockets, you ended up with enormous, straight faces of good rock. So even if a layer is say 50 ft. thick, you can climb it for as long as it goes before it broke off at the summit.
In Austin the beds are horizontal, so if you are in a quality layer of rock that is soft enough to erode some nice pockets yet hard enough to resist crumbling, you can only climb that layer for as thick as it is deposited. Usually you have chert nodules (flint) mixed in between layers. These can be pretty sharp and painful, but they resist wear. On the Eiger, you have horizontal layers as well, or at least as far as I could tell from photos and road cuts I walked through. There were some absolutely horrible layers of rock that had the color and consistency of wet tree bark. I'm not sure what it was, but encountering that up on that imposing face would be quite frightening. Many of the beds on the Eiger are only a few feet thick as well, and fracture off in fist size blocks easily.
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dingus
Dec 5, 2003, 10:39 AM
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Nice work guys!
Some of my favorite granite in Yosemite NPS has to be the Cathedral Peak rock. Huge crystals. Is this part of the entire Tuolumne pluton or is it it's own animal?
On more of a peak bagging plane I hiked Pyramid Peak in the Funeral Mtns east of Death Valley last week. Interesting rocks to be sure and I reckon much older than most of anything we find farther west.
I've read some of the geology of the area and one of the interesting concepts I have seen is inverted depositional layers, recumbant folds and the like. Basically, older rocks overlay younger rocks, layer after layer.
I believe Pyramid Pk represents this... looking north and west from the summit, toward Schwuab Peak I noted that the layers upon which I had climbed where represented as a continuous band. But interestingly, erosion and uplift produced different results in the two peaks.
To describe what I saw (I deleted the freaking pics from my camera before I could upload them):
The various rock units were layed down, accreted on the edge of N America or welled up from beneath. They all cooled down years before we were born (like 200 million years). Most of this occured before California existed. Before the bulk of the dinosaurs existed.
Something caused the basin and range country to "puff up", expand and crack. This resulted in the creation various crustal blocks, large and small, as well as drop down basins in between. The entire region from the San Andreas Fault system east to the front range of the Wasatch seems to be in the process of being stretched to the north west. We have all heard about how California is moving to Alaska. There is an active question of just what is sliding north and just what is staying behind. Nevada may be in for a rude awakening in some 30 million years!
Some theroize a new fault zone east of the San Andreas is at work, the Walker Lane, stretching from Las Vegas north toward Reno and that this line will represent the point of departure.
Because of this faulting, the floor of Death Valley is sinking and the Panamints and the Armagosa Ranges are rising on either side. These mountain blocks are tilting with what some believe are inordantly "short" roots. Geologists picture mountain blocks as icebergs... large roots are required to hold the peak in the sky. Short roots, say from listric faults that chop the mountain blocks off at their knees, create top heavy units. They rotate and slide with each earthquake, trying to find equalibrium just like a melting iceberg flipping in the warm sun.
The Funeral Range of Pyramid Peak is a subunit of the Armagosa range. They are rising on the west and sinking on the east. So the front range facing Death Valley above Badwater and Furnace Creek, is rising too.
As they rise, these ancient layers are raised into the sky and exposed for our view. In a linear progression we could expect to see the youngest laters exposed first. If we had a crustal unit, deformed or not, broke it into a descrete chunk and rotated it in the crust like a log in the water, if the mass is rising on the west and sinking on the east, then the layer at the top of the peak should be the youngest and the layers farther west should be progressively older.
That is the pattern I reckoned I saw on Schwuab Peak. Yet I could follow those same layers, due to the sparse vegation, to Pyramid Peak. I was startled to note that the top layer on Schwuab correlates to the very first layer I climbed as I came up the SE Ridge. The older layers to the west of Schwuab peak lined up with the summit of Pyramid.
That means that on Pyramid Pk, if my observations are correct, the oldest layers are on the summit and as one descends to the SE, newer and newer layers are crossed. I could "map" each of these layers to Schwuab till I dropped low enough to lose the view. Upended layers, a recumbant fold? No, I think in this case its just the way the block broke, unevenly.
Lastly, as I descended the fan back to my car I wondered, as I had done the previous evening from the summit of Dantes Pk, how alluvial fans move.
I understand flash flooding. There is also the mechanism of the originating mountains are rising and the floor of the desert is droppiung. This causes the angle of the fan to increase and new water channels are cut through previous depositional layers. OK, I understand these movements. But is there something larger at work too?
The Sierra has lots of glaciers that most of us won't recognize as such. They are called 'rock glaciers.' Their essence is that remanant glacial ice is still beneath some of these talus slopes and is still forming, moving and transporting debris downslope. Rock glaciers appear to be incredibly active talus slopes and the whole slope is moving enmass.
Do fans do this as well? Take Emmigrant Wash, a 4000 foot tall fan leading from the Panamints down to Stovepipe Wells. As the Panamints rise and Death Valley continues to sink, is the whole wash sliding enmass into the ditch? Can this happen in large quakes for example.
Closing thought: Two weeks ago I was on the Lost Coast of California. The King Range are among the youngest non-volcanic mountains in California, in the whole country. They are rising at an astonishing rate and are eroding just as fast. Completely different geology and beyond scope. But a hike north from Black Rock Beach takes you right across several active fan washes dropping debris right into the ocean.
These fans are incredibly active and raw looking. They appear as though the rocks came to rest just a few minutes before you approach them. Heavy rainfall (200 inches annually on the east flank of the King Range - Mattole River drainage, among the wettst places in Cali too) and high erosion rates due to landsliding means there is lots of raw material and a very active transport mechanism to move debris down the fans and presumably onto the larger fans beneath the water.
Comparing the active King Range fans to the more settled Death Valley fans leaves me wondering...
How do they move?
DMT
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jeffers_mz
Dec 5, 2003, 12:04 PM
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"Some theroize a new fault zone east of the San Andreas is at work, the Walker Lane, stretching from Las Vegas north toward Reno and that this line will
represent the point of departure.
Because of this faulting, the floor of Death Valley is sinking and the Panamints and the Armagosa Ranges are rising on either side. These mountain blocks are tilting
with what some believe are inordantly "short" roots."
Sounds just like the description I saw on Nova last week, about Kilimanjaro, explaining the processes forming the Great Rift Valley.
"The Sierra has lots of glaciers that most of us won't recognize as such. They are called 'rock glaciers.' Their essence is that remanant glacial ice
is still beneath some of these talus slopes and is still forming, moving and transporting debris downslope. Rock glaciers appear to be incredibly
active talus slopes and the whole slope is moving enmass.
Do fans do this as well? Take Emmigrant Wash, a 4000 foot tall fan leading from the Panamints down to Stovepipe Wells. As the Panamints
rise and Death Valley continues to sink, is the whole wash sliding enmass into the ditch?"
Why does water necessarily have to be involved? An ice glacier is essentially small particles, (smoothed ice crystals) "flowing" under the force of gravity as more weight is added to its upper reaches. When you take a bird's eye view of a rock glacier, the individual blouders are small enough to act as a plastic or liquid aren't they?
Glaciers also have water running under them in some cases, I think if you look at a phase diagram you can get liquid from great pressure even at below freezing temperatures. In that case it is different than water being entrained in the glacier itself, a lubricant under the main mass as opposed to a lubricant mixed in with the mass.
If that water assists the movement of the lower layers of the glacier, where the frictive coefficients are higher than ice-ice interfaces, who's to say the same effect can't occur where boulders at the bottom of a rock glacier have to slide across jagged rock or soft mud to move? Where is the water most likely to be found besides the lowest point below particulates and above impermeables?
Thanks for some good reads, guys. I have to go now, but if I get time, I'll try to work up something on the San Juans. Interesting stuff, possibly the largest volcanic eruptions in earth's history, and a mountain range that once extended above thirty thousand feet.
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dingus
Dec 5, 2003, 12:23 PM
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In reply to: Sounds just like the description I saw on Nova last week, about Kilimanjaro, explaining the processes forming the Great Rift Valley.
Related tectonic forces at work, though there are spreading centers forming in Ethiopian Gulf of Aden. The Rift Valley is directly related to this spreading. And perhaps the Walker Lane will eventually widen and spew forth as a spreading center too.
In reply to: Why does water necessarily have to be involved? An ice glacier is essentially small particles, (smoothed ice crystals) "flowing" under the force of gravity as more weight is added to its upper reaches. When you take a bird's eye view of a rock glacier, the individual blouders are small enough to act as a plastic or liquid aren't they?
Hmm. The word alluvial simply requires water, for starters. And water is a primary downslope motor, be it flash floods, the slow trickles of eternity, or the frozen medium of ice.
In reply to: Glaciers also have water running under them in some cases, I think if you look at a phase diagram you can get liquid from great pressure even at below freezing temperatures. In that case it is different than water being entrained in the glacier itself, a lubricant under the main mass as opposed to a lubricant mixed in with the mass.
Great point. And this lubrication may be minimal or absent in a fan entirely. Yet, also in Death Valley, are formations called Slickenslides. They are massive faulted planar surfaces geologists theorize represent the "point of departure" where active faulting is occuring. Were a slickenslide to coincide with a fan that might provide the reduced friction to send the whole mass down. There is evidence of just such a slide... 5000 feet worth, south of Copper Canyon in the Black Mtns above Badwater. From the air you can actually see how all the rocks on top piled up at the bottom of the slope.
I envisioned a slower sort of creep though, induced from the weight of the fan itself. But I can see friction as being a huge component, so I'm guessing not. A friend of a friend, a petroleum geologist by trade, studied the fans of Death Valley extensively in college. I need to track him down.
DMT
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copperhead
Dec 5, 2003, 6:06 PM
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With respect to the comments that I made about felsic dikes having sharp boundaries and mafic dikes having irregular and wavy boundaries, I should mention that this is not a rule. It is quite common to see mafic or basaltic dikes that are straight as an arrow and have sharp boundaries. This just means that the magma must have been intruded into an existing fracture after the host rock had completely crystallized and that the mafic magma was relatively cool (cool enough to prevent melting of the host rock).
In reply to: Some of my favorite granite in Yosemite NPS has to be the Cathedral Peak rock. Huge crystals. Is this part of the entire Tuolumne pluton or is it it's own animal?
Cathedral Peak Granodiorite (~ 86-88 Ma.) is definitely one of the coolest rock units in the Sierra. The picture that I posted above is a ladder dike in Cathedral Peak Granodiorite and can be found along the north side of the Tuolumne River, about an hour from the beginning of the trail to Glen Aulin. The “huge crystals” that you speak of are obvious in the picture; they are orthoclase ((KAlSi3O8) feldspar) crystals and we call them phenocrysts or megacrysts (as mentioned above) because of their large size. We still don’t know for sure why these crystals grow to such sizes while the rest of the mineral groundmass remains typical in size. Some geologists believe that the megacrysts grew to such sizes during late-stage, post-crystallization metasomatism (i.e. chemical replacement and re-ordering in the solid state). The rest of us believe that they are magmatic in origin; we just need to figure out how and why…
Here is a basic map of the Tuolumne Intrusive Suite that I found on the web. The eastern end of the Valley is about where the letter A is and Tuolumne Meadows is approximately on the northern end of the Johnson Granite. Tioga Pass lies about where the dashed line extends from the edge of the suite, up and left of the letter B.
http://www.virtualexplorer.com.au/2002/Schellart/Dietl/paper1.html
http://www.virtualexplorer.com.au/...t/Dietl/img/Fig3.jpg
As you can see, the Tuolumne Intrusive Suite consists of four different plutonic units, the youngest one being in the center and the oldest one at the outer edge. Kuna Crest is subdivided into Tonalite of Glacier Point, Tonalite of Glen Aulin, Granodiorite of Grayling Lake, and Granodiorite of Kuna Crest, based on location. Half Dome Granodiorite is subdivided into an equigranular facies and a porphyritic facies. The equigranular facies contains large hornblende crystals (black rectangles) and the porphyritic facies contains orthoclase phenocrysts that become larger towards the contact with Cathedral Peak. Contacts between plutonic units in the Tuolumne are both gradational and sharp. These units are grouped into a suite because they are all related, both chemically and in intrusive history. The plutonic units of El Cap that I discussed above are grouped into a separate suite, the Intrusive Suite of Yosemite Valley.
Hey, cool stuff about Death Valley, Dingus! Makes me want to go back down there… Have you been through Titus canyon? Super cool breccia in the marble…
In reply to: The Sierra has lots of glaciers that most of us won't recognize as such. They are called 'rock glaciers.' Their essence is that remanant glacial ice is still beneath some of these talus slopes and is still forming, moving and transporting debris downslope. Rock glaciers appear to be incredibly active talus slopes and the whole slope is moving enmass.
I wasn’t aware that remnant glacial ice still existed beneath these rock glaciers.
In reply to: Do fans do this as well? Take Emmigrant Wash, a 4000 foot tall fan leading from the Panamints down to Stovepipe Wells. As the Panamints rise and Death Valley continues to sink, is the whole wash sliding enmass into the ditch? Can this happen in large quakes for example.
I don’t think so. I think they are mainly fluvial. If you look at the walls of an active wash and look at the cobbles and boulders, they are surrounded by gravel, sand, and silt, right? The slope of an alluvial fan is usually fairly slight. Rock glaciers, on the other hand, are composed of rockfall-generated angular cobbles and boulders but generally lack the smaller particle sizes, meaning that there is a lot of empty space in the voids between the cobbles and boulders, providing room for shifting and creep. Rock glaciers also occur on steeper slopes than alluvial fans. I would think that angle of repose and slope stability (cohesion) would be the key factors involved in rock glacier movement while alluvial fans are predominately shaped by water flow and faulting, as you mentioned.
In reply to: Two weeks ago I was on the Lost Coast of California. The King Range are among the youngest non-volcanic mountains in California, in the whole country. They are rising at an astonishing rate and are eroding just as fast. Completely different geology and beyond scope. But a hike north from Black Rock Beach takes you right across several active fan washes dropping debris right into the ocean.
This must be north of the Mendocino Fracture Zone, where subduction is still taking place. Your comment about debris washing into the sea reminds me of something that one of my profs told us in class one time. We were talking about turbidites (underwater debris flows that flow down the continental slope toward the abyssal plain) and he said that they could reach speeds nearing 200 miles per hour. I was like, no way; how can something driven by gravity that flows underwater exceed the terminal velocity of an object falling through air? What gives?
In reply to: Yet, also in Death Valley, are formations called Slickenslides. They are massive faulted planar surfaces geologists theorize represent the "point of departure" where active faulting is occuring. Were a slickenslide to coincide with a fan that might provide the reduced friction to send the whole mass down. There is evidence of just such a slide... 5000 feet worth, south of Copper Canyon in the Black Mtns above Badwater. From the air you can actually see how all the rocks on top piled up at the bottom of the slope.
Do you mean slickensides, the surfaces of a fault plane that have been polished and smoothed by the repeated sliding of rock on rock? Slickensides are a linear feature that form parallel to the direction of fault movement. You wouldn’t happen to be looking at a picture on pages 40 and 41 in a book by Michael Collier, would you? If so, or something similar, this extensional structure is called a turtleback and is a low-angle detachment fault. The “pile” of rock at the base of the slope is what is left of the upper block or hanging wall and the slope to the right is the lower block, or footwall (old mining terms). Movement along this fault occurred over thousands of years; the pile of rock at the base of the slope did not slide down in one catastrophic event.
Compaction of the sediments in an alluvial fan might produce a slight amount of movement, otherwise it seems to me that any lateral movement of material would take place at the surface of the fan. That’s my guess.
Hey Dingus, have you been to the Racetrack Playa?
http://www.rockclimbing.com/...p.cgi?Detailed=21784 http://www.rockclimbing.com/...p.cgi?Detailed=21785
How do they move?
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dingus
Dec 5, 2003, 6:27 PM
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I'm digging this thread.
Yup, guilty on the Collier geology picture book Copperhead. Light on the technical, but rich in feeling. I like it and it has lots of local detail. I've seen that slickenside (sorry for the consistent mispelling, I was saying it that way too) in that picture and didn't know what it was at the time.
Are you familiar with James G. Moore's Exploring the Highest Sierra? Seems right up your alley. Anyway, he has a good dicussion about the rock glaciers in Kings Canyon NP. This book is the absolute best thing I've ever seen on the Sierra. It has the history of exploration and map making, as well as a complete textbook on Sierra geology. It's outstanding.
Was going to take the ride down Titus this trip but I got a flat earlier in the same day trying to bag Grapevine Peak. I spent a half hour poking around Rhyolite and then actually started up toward Titus. Bagged it within a mile.
Ended up on Dantes Peak watching the sun set behind the Panamints. Sat there for two hours and just let time flow immortal. It was a nice "also ran."
King Range is just south of where the San Andreas strikes the Mendicino Fracture Zone. Rising like 4 inches a century or something. The nearby Scotia Bluffs have risen 13 feet in the last 1000 years. The Eel River carries the highest sediment load of any river in the country, in a flood it exceeds the load of the Mississippi! Future's rocks going out to sea...
Damn I love this geology crap. It's so interesting to look at this vast landscape and have an inkling of how it might have got this way.
Cheers
DMT
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slcliffdiver
Dec 5, 2003, 7:23 PM
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Cillia? Uneven heating of the bumpy thing do to angle of the sun/shadow from the rock? Am I right? Do I get a prize?
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copperhead
Dec 5, 2003, 8:21 PM
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Yeah, the Collier book is cool, eh? Nice pictures. I really like the pic on page 34 of the chevron folds and faults – way cool.
In reply to: Are you familiar with James G. Moore's Exploring the Highest Sierra?
Yeah, I’ve got a copy around here somewhere but haven’t read all of it. I’ll have to take a look for the rock glaciers.
In reply to: King Range is just south of where the San Andreas strikes the Mendicino Fracture Zone. Rising like 4 inches a century or something. The nearby Scotia Bluffs have risen 13 feet in the last 1000 years. The Eel River carries the highest sediment load of any river in the country, in a flood it exceeds the load of the Mississippi! Future's rocks going out to sea...
Oh, ok. I just took a look at my Cali fault map poster. That makes sense; the San Andreas terminates at the Mendocino Fracture Zone. How does a huge rigid block turn in a corner? I bet things are pretty complicated up there. You’ve got the Pacific plate sliding past the North America plate and then slamming into the edge of the Juan de Fuca plate, changing direction to the west, and then sliding past the Juan de Fuca plate. Maybe the mountains that you mention are part of a reverse flower structure or something like that – when there is a transpressional jog (bend) in a strike-slip fault, the crust that is caught at the apex of the bend has nowhere to go but up. As a result, the mountains are forced upwards by a series of thrust faults. The Garlock Fault in the northern Mojave stems from the bend in the San Andreas in Southern California and may be a result of the compressive stresses at the bend. I guess the two ideas are rather similar.
As far as the Walker Lane and basin and range extension, some people think that the extension and high heat-flow are due to the subduction of the spreading center that once fed the trench and fueled the magmatism that generated the Sierra Nevada Batholith. About 30 million years ago, this spreading center was subducted beneath North America; at this time, the San Andreas Fault formed and the North America plate margin changed from subduction to right-lateral (or dextral) strike-slip. North America basically overrode the spreading center but sections of it are still active, north of the Mendocino Fracture Zone and in the Gulf of California. Maybe the spreading center is still active underneath eastern California and the Great Basin; this would provide a weakness in the crust and a reason for the Walker Lane to form. I dunno… too many details and it starts to get confusing.
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jeffers_mz
Dec 6, 2003, 4:47 AM
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Dingus wrote: "
Quote:
Why does water necessarily have to be involved? An ice glacier is essentially small particles, (smoothed ice crystals) "flowing" under the force of
gravity as more weight is added to its upper reaches. When you take a bird's eye view of a rock glacier, the individual boulders are small enough
to act as a plastic or liquid aren't they?
Hmm. The word alluvial simply requires water, for starters. And water is a primary downslope motor, be it flash floods, the slow trickles of eternity, or the frozen
medium of ice. "
I was in a bit of a hurry when I posted this, and was referring to rock glaciers more than alluvial fans, which I'm not very familiar with.
It seems to me that as long as you have some sort of slope angle, and either uniform particle size, rounded particles, or particles of discrete sizes that won't lock together with movement, then nothing more than diurnal heating and cooling will be needed to set the mass in motion.
I don't know if you could have a rock glacier in a desert environment or not, but I know of at least a couple that move year around in climates too cold for there to be ice under them. One sits on the SW face of Engineer Mountain and the other sits on the SE face of Blackwall Mountain, both in the San Juan range in Colorado.
Consider this...glacier ice is pretty hard stuff, but under the accumulating weight it becomes plastic and flows. Water has a specific gravity of 1, and ice floats in water, so just the mass of the accumulating talus at the base of an eroding face has to add up to a considerable weight. Most likely not enough to plasticize the rock itself, but when you look at the aggregate body of loose particles, in that sense it already has plastic properties.
"I deleted the freaking pics from my camera before I could upload them..."
...and some folks think the primary risk from smoking that stuff is that it leads to harder substance abuse....
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dingus
Dec 6, 2003, 7:45 AM
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In reply to: ...and some folks think the primary risk from smoking that stuff is that it leads to harder substance abuse....
Dude, its all gateway. In my case the gate led to a life of abuse of my body and mind, repeatedly and purposely putting myself in harms way just "for the thrill of it."I should be ashamed.
DMT
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dingus
Dec 6, 2003, 8:19 AM
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dmt
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roughster
Dec 6, 2003, 10:05 AM
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There is a full fledged geologist at my work that asks me to bring him back pieces of rock from where I climb for him to take a look at. I wish I had been having him send me emails instead of just talking about it over the water color. He had some great info about much of the local rock around California.
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copperhead
Dec 7, 2003, 8:23 PM
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In reply to: The big question and one I haven't seen any clearly convincing arguments, is what happened to and what will continue to happen with the spreading center that went down under California, just as you outline.
We wish we knew. It’s probably still spreading and will continue to spread until the Gulf of California reaches Reno and beyond. Nevada had beachfront property in the past... looks like it'll happen again. It’s pretty complicated and no one knows for sure, though some neat research is being done on the subject.
In reply to: Anyway, those large scale transforms and frature zones, of which I guess the Garlock may be one? I can't remember the name of the thing, but isn't there a fracture zone off the coast of SoCal that lines up with the Garlock, Santa Monica fracture zone or something like that? And doesn't the Garlock, after seriously bucking the northwest trend of Cali bu cutting straight east/west across the knot of Frazier Mtn, eventually curve north too and join the Death Valley transform?
The Garlock is a left-lateral (sinistral) transform, by definition, but is different from the transforms found in oceanic crust that accommodate opposite directions of movement of oceanic crust between offset segments of a spreading ridge. I can’t seem to find anything about a Santa Monica fracture zone, but I do see a Santa Monica fault on one of my maps – it’s pretty short, similar in strike to the surrounding structures and is a ways south of the Garlock. Everything (fault structures) off of the coast seems to roughly parallel the coastline and the right-lateral (dextral) San Andreas. I think that the E-W change in trend of the coastline is just a reflection of the trend of the San Andreas and the big bend – they are roughly parallel. Another thing to keep in mind is that rocks and structures on either side of the San Andreas don’t directly correlate – there has been somewhere around 300 km of offset along the fault. There is, however, the Big Pine fault, a left-lateral fault on the Pacific plate, adjacent, and parallel to the Garlock fault but it is older than the Garlock. I don’t think that either of these two faults are the result of reactivation of an oceanic transform that underlies the continental crust. Oceanic transforms are relatively straight and the trace of the Garlock is curved, concave to the south.
Ok, if I may babble off the top of my head some more...
(who knows what it’s worth... but I do have a large pile of open textbooks on my floor and am having fun reading them... geek... :shock: )
The Garlock fault formed in continental crust due to compressive stresses at the bend in the San Andreas fault. For example, if we envision a linear strike-slip fault and we put a bend into it, we will have an area of compression, or tension, depending on the orientation of the bend and whether the fault is left-lateral or right-lateral. The Death Valley - Furnace Creek fault zone is also a right-lateral fault and also has a bend in it. Because this bend is the opposite of that of the San Andreas, an area of tension (extension) or a pull-apart basin forms. Badwater is elev. –282, the lowest in the 48, right? And the Transverse Range (San Andreas bend) is a range of mountains, right?
Anyways, back to the Garlock. Just to the north of the big bend in the San Andreas lies the Sierra Nevada Batholith; this huge mass of solid granitic rock behaves as one rigid crustal block. The Western Mojave block is pinched in between the southern tip of the batholith and the bent segment of the San Andreas; there is nowhere for the block to go but east. It seems to me like the Garlock has formed between the Western Mojave block and the batholith to allow eastward (relative) movement of the block. Several short strike-slip faults exist in the Western Mojave block, strike NW-SE, and are right-lateral; they are parallel to the normal strike of the San Andreas (sub-parallel to the bent segment). Are these faults a result of the pinching of the Western Mojave block?
The Panimant Valley fault zone and the Death Valley fault zone are both right-lateral faults and both intersect the eastern portion of the Garlock fault at sub-perpendicular angles. The Garlock does not join the Death Valley fault zone; their directions of movement are opposite. Instead, the Garlock terminates to the east of the DVFZ in a thrust fault in the Soda and Avawatz Mountains. The PVFZ and the DVFZ may be accommodating the bend in the San Andreas further inland, or the San Andreas itself, or they may just be part of the crazy tectonic mess that separates the Pacific plate from the Colorado Plateau. I dunno.
I think that’s about all I should say for now; the more I think about this, the more complicated it becomes and the more confused I get. It’s pretty crazy.
In reply to: Spreading center rides up under the continent. Sits there for 20 or 30 million years, slowly building up heat and pressure, all the while the San Andreas transform and seemingly the forces exerted on the entire great basin, are ripping everything in a northwesterly direction. Perhaps the two oceanic blocks adjecent to Cali simply have to move far enough to split open the new center, which is really an old spreading center moving into a new phase?
Yup, pretty much. The Juan de Fuca plate will someday be subducted beneath North America too. Then what?
In reply to: And on a much smaller scale, at the local mountain building level, aren't the forces and processes involved similar, only smaller? Sort of like a mandlebrot graph, patterns in geology repeat from the infintessimal to the infinite, Mississippi Delta alluvial patterns and braided streams finding small echos in the fans and deltas leading into or out of a mud puddle in a dirt road.
Totally. Isn’t that cool? I guess it’s just how things work – repetition over a broad scale. Geology is a window into physics and chemistry and the natural laws that govern the Universe; you can see how everything works. Not only are processes and patterns repeated on different size scales but in different environments at similar size scales – like the similarity of cross-bedding patterns found in eolian or fluvial sandstones and the cross-cutting, arcuate schlieren patterns found in granitic rocks – similar processes, different mediums?
Hey, nice work on describing the Racetrack rocks! That’s a lot better than I originally did when I submitted the photo. There is a good journal article (which I had to find and reread) from ’95 on the subject.
Reference:
Reid, John B., Bucklin, Edward P., Copenagle, Lily, Kidder, Jon, Pack, Sean M., Polissar, Pratigya J., and Williams, Michael L., 1995, Sliding rocks at the Racetrack, Death Valley: What makes them move? Geology, v. 23, p. 819-822.
This is a short article that I found that talks about two opposing theories.
http://www.larryo.net/RaceTrack.html
So, what next? The Chicxulub crater and the 65 Ma K-T boundary? Cause of extinction or not?
Dingus, have you read any John McPhee?
Anyone else out there…?
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thegreytradster
Dec 8, 2003, 6:15 PM
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Looks like I've finaly run across some one that might know :)
How does the Southern California Batholith fit into all of this?
It evidently goes all the way from Cabo to Riverside, making it larger than even the Sierra Nevada Batholith. Which plate is it on? San Jacinto has always reminded me of a giant broaching submarine as it moves north rising up and creating the largest escarpment in the lower 48 from White Water to the peak (8500/9000 ft). Tahquitz (on the north side of the San Jacinto uplift) is tonalite. From what I understand this is a rock that forms very deeply in the crust. But much of the uplift is very recent. the San Jacinto fault on the west side is also one of the most active and distructive in So Cal.
Years ago I found what I presumed was Olivine at the base of Rubixoux, (near the river) That lines up as the very north end of the Batholith. Is that possible?
Is it really going to spread up the Gulf of Ca, or is the So Cal Bathyolith driving north compressing the Garlock fault and everything in between, (LA basin).
The Landers Quake shifted north 27 ft and only marginaly E/W. Have a great picture of my kid standing in the fracture when he was about 10 or so.
Could the E/W change in the coastline be driven by bending and compression from the So Cal Baytholith moving north?
Any ideas from someone that knows what they are talking about?
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dingus
Dec 8, 2003, 7:11 PM
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Copperhead - Murray Fracture Zone is the one I was trying to think of, a lesser cousin of the Mendocino?
DMT
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