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gstock
climber
Yosemite Valley
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Nov 27, 2008 - 08:39pm PT
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Great stuff, Bryan. You've got the knack for presenting this in a way that is understandable. John McPhee should tag along with you in Tuolumne for a few days next summer!
Greg
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Minerals
Social climber
The Deli
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Nov 27, 2008 - 11:18pm PT
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You guys are too nice to me. McPhee rules! I’m still trying to figure out the difference between obsidian and my beer bottle…
Ho man, I just got sucked into a mega Google search and found some really cool stuff… Need… to… go… back… to… school… My head is spinning, and it’s not the SNPA!
That’s rock?!?!
Image found here:
http://www.earth.ox.ac.uk/~oesis/micro/index.html#igneous
Yeah, Malemute, you’ve got it. The pink color in granitic rock is potassium feldspar. The pink color doesn’t necessarily reflect true granite; the potassium feldspar is just pink in color, as opposed to light tan or white. The rock in the Bushido Gully at the base of the right side of Half Dome is pinkish; this results from localized alteration of Half Dome granodiorite by hydrothermal fluid flow along the main joint system (series of tightly-spaced fractures and sheared areas – brittle deformation) that forms the face of Half Dome. Much later, glaciers acted as bulldozers and pushed away the other half of the dome.
Mica IS an accessory mineral in granitic rocks; I failed to mention that “mica” is a group of minerals and biotite and muscovite are two minerals that belong to the mica group.
Hey look!… a black Ibanez and a Marshall practice amp! (House sitting for Miller)
TIME OUT!
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apogee
climber
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Nov 28, 2008 - 01:24am PT
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John McPhee rocks! (No pun intended or actually occurred.)
Must-reads for geologic wannabes:
'Basin & Range'
'Assembling California'
For any self-respecting conservationist:
'Encounters with the Archdruid'
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Minerals
Social climber
The Deli
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5th or 6th page…?!?!? This discussion is far from over!
Hey Kpinwalla2, that sounds like a really cool book! For those who may be interested, where can we find a copy? Are you still a geologist?
Tami, that is a great question! Although dike formation is a relatively simple process, we need to cover a few more basics so that the whole picture makes sense. I also want to address some of Dingus’ comments on magma generation and emplacement.
I’ve got to get going now, and there is no wireless signal out in the desert, but I’ll be back when I get a chance.
Toasters???
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TwistedCrank
climber
Ideeho-dee-do-dah-day
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Pff...
Anything that's not Archean is just overburdon. Just ask them Montana boys.
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Captain...or Skully
Social climber
Where are YOU from?
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ToasterS??
Must be close to snacktime......Rock Rocks.!!
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Jaybro
Social climber
wuz real!
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Sherman Granite (Vedauwoo) is noted for it's pink K-spars.
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Captain...or Skully
Social climber
Where are YOU from?
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Really cool stone in that Sherman stuff.That's fairly old, ain't it?
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stevep
Boulder climber
Salt Lake, UT
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I thought Sherman granite was noted mostly for its plethora of V-Grades.
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taorock
Trad climber
Okanogan, WA
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Good thread. Thanks for your efforts Minerals. As a geologist (formerly practicing - now amateur) I know what it is like to address these topics.
For those interested, pick up a used copy of - Rocks and Rock Minerals by Dietrich and Skinner. It is classic, well written, good in the field and very concise. Written for geologists, yet accessible to the layperson with a real interest.
If you ever see it in a used bookstore pick it up. You'll actually use it.
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Jaybro
Social climber
wuz real!
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1.2 billion y.o. , Cap, Pre-cambrian there's older stuff (~3 billion?)further north. According to Dingus McGee who also sez it was once the edge of the continent (like somewhere between Wheatland and Casper?) i've been to lazy to look it up.
Know anything about that, Bryan?
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scuffy b
climber
On the dock in the dark
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kpinwalla,
wandering through the PreCambrian rock at the City, I'm
puzzled that it doesn't show signs of metamorphosis around the
contact with the Almo pluton.
What gives here?
Am I just not looking closely enough?
Shouldn't there be a zone of gneissic rock?
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BASE104
climber
An Oil Field
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I am stepping out a little here...I have done nothing but sedimentary stratigraphy for 25 years...
I know that dating "granitic" rocks has advanced by leaps and bounds in the last twenty years using inclusions in zircon crystals, which are common in granitic rocks, and unfortunately really rare in basaltic rocks.
It has really revolutionized how the north american craton came to be. There were small blobs of granite here and there, which sort of floated around and bumped together and fused. Felsic rocks are lighter than mafic rocks, so continental crust does generally bob and float on basaltic rocks. You can take granitic outcrops and date them with a very high degree of accuracy. So, since continental crust is more often than not composed of granitic rocks, you can see how the craton was formed by dating in different areas. Some of it is super old. I was a judge at a big poster session last year, and I saw a super good paper on dating an area with zircons. This little blob dates this, and this one dates a little later, blah blah, and you can kind of see how what was once thought of as sort of a boring continental crust is actually really cool as it grew.
Oceanic crust is more basaltic, which is heavier (although thinner) than continental crust, so it gets gobbled up in subduction zones. I think that the oldest oceanic crust is only Cretaceous, but I am sure that will be corrected by someone.
The exception to finding really old oceanic crust is probably in ophiolite suites, essentially a sliver of oceanic crust that kind of splintered off of subducting oceanic crust and was preserved on top of older continental crust. California has quite a few examples of this.
Granitic rocks "float" on basaltic rocks...the granitics really do have lower density than say, a peridotite rich mantle. So yes, the larger craton was put together from much smaller blobs. I am sure that there are published dates for Veedawoo, or practically any place where you have granitic outcrops. The dates are now very accurate. Zircons are almost indestructible, and you can date inclusions using radiometric dating, without the risk of contamination, alteration, whatever. An area like Veedawoo is probably a different age from basement rocks only a hundred miles away.
The batholiths that you see along subduction zones are not the only way granitic rocks are formed and expressed. As far as water goes, I haven't heard much of that other than hydrothermal alteration or dikes. A lot of metals are found there.
And a dike works like this. You have an emplaced granitic rock. Old, cold, and solid..or nearly so. At some later point, a rock from a different melt (if it is really whacko, like gabbro through granite). Follows and expands fractures into a sheet like intrusion. So where it intersects air, it looks linear. One thing you know is that the "country rock" was there before the dike. This is kind of the short story.
Also, I thought that the micas were one of the three main constituents of granitic rocks. If you ever see a pegmatite dike, there are all kinds of really cool super crystals. Muscovite mica chunks as big as your fist.
Have any of you checked out that gigantic round cave like thing while going down the east ledges? It is like a solid tube of gabbro that is totally splintered with white dikes. I really want some schooling on what the hell that thing is...
Minerals is really good at igneous stuff. As far as one area having good rock, like most of the Nose, and then climb through that crappy diorite, some of that has to do with how different minerals weather. An easy way to understand this is if you take the melting temperature of a mineral compared to surface temperature. The higher temp minerals generally weather first. Then down the scale to Quartz, which is essentially bullet proof and just breaks into smaller and smaller pieces (yes, I know about overgrowths), making sand. Even shales (which basically forms from mud) can have a very high silica content.
So crap rock may have been altered at some point, have a more mafic mineral assemblage, been fractured to beat hell, or simply face north in some cases.
And chill, Minerals. We both know that you are drinking from an obsidian bottle.
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yossarian
climber
WA
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The black granite in Westwater Canyon (Utah) is over 1.75 billion years old, which ranks it up there as some of the oldest granite on earth.
Mount Kinabalu (Borneo) is suppose to be the youngest granite (forced up 1 million years ago).
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scuffy b
climber
On the dock in the dark
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I believe the Tetons and the Wind River Range are both
composed of PreCambrian granitic rocks, although the ranges
as topographic features are quite young.
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GOclimb
Trad climber
Boston, MA
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Ricardo Cabeza asked: Now that I'm back here in good old NH, I've been told that what we call granite in our state isn't really that at all. It's pink something or something. Is this just a variant, or is it a different breed altogether?
Minerals wrote: The pink color in granitic rock is potassium feldspar.
I can't shed any light on Ricardo's question in general, but Mineral's comment above, along with his triangular diagram, suggests an answer to something I've wondered for a while: What is the deal with the beautiful granite of Acadia National Park, in Maine. For those of you who haven't been there, it's such a dark pink, it almost looks purple! Here are some pics:
So that means this granite (or whatever) is probably a mineral in the lower left corner of this triangle:
But wait, perhaps not! This website suggests that perhaps the dark purple rock is Gabbro (in the lower _right_ of the diagram) colored red by lots of iron: http://www.us-parks.com/acadia/geology.html
A complex series of events led to the intrusion of several different types of molten, or igneous, rocks. The intrusive rocks cooled beneath the earth’s surface, allowing the crystals of various minerals to form and grow. Each rock type is composed of a unique set of minerals. The first and oldest is a gabbro. This rock is dark in color and is made up of iron-rich minerals.
Interesting! So maybe the beautiful granite splitters and dihedrals of Precipice Cliffs is not granite at all, but Gabbro (i.e. mostly plagioclase feldspar).
GO
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scuffy b
climber
On the dock in the dark
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GoClimb:
I think you've got the right and left mixed up.
The P is for Potassium (why not K, go figure), Orthoclase
Feldspar, aka K-spar.
Plagioclase is really the A (alkaline, soda-lime)
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mojede
Trad climber
Butte, America
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Oldest rocks in the US?
Somebody can check me on this, but the Basement Rock (whatever that is) in the Beartooths is reckoned to be 3.5 billion years old, if my geologic maps are correct.
edit:that "basement rock" is Archean basement complex (mostly quarzofeldspathic gneiss)
There's also some of the Stillwater Complex rock near the Beartooths--defined as: layered mafic-ultramafic intrusive complex, includes anorthosite; associated with hornfels aureole.
The Stillwater Complex is where this country mines Platinum and Palladium, FWIW.
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GOclimb
Trad climber
Boston, MA
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Scuffy wrote: I think you've got the right and left mixed up. The P is for Potassium (why not K, go figure), Orthoclase Feldspar, aka K-spar. Plagioclase is really the A (alkaline, soda-lime)
Not according to Mineral's post, where he said: Q = quartz, A = alkali/potassium feldspar, P = plagioclase feldspar.
Who can be the arbiter of this disagreement? I'm really curious now.
GO
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mojede
Trad climber
Butte, America
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Jaybro, it looks like the Bighorn Mountains in Wyoming are almost as old as the Beartooths--give or take a half-billion years.
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