Geological History of Earth - Granite?

Search
Go

Discussion Topic

Return to Forum List
This thread has been locked
Messages 1 - 85 of total 85 in this topic
JuanDeFuca

Big Wall climber
Stoney Point
Topic Author's Original Post - Nov 26, 2008 - 02:08pm PT
I was watching this show on Nat Geo and they talked about how in the history of the earth all these granite bubbles drifted to the surface at about the same time?

Does that mean all granite is about the same age.

You Hubble servant Juan De Fuca
Norton

Social climber
the Wastelands
Nov 26, 2008 - 02:15pm PT
How old is the earth, Juan ? 6000 years?
72hw

Trad climber
Hollyweird, CA
Nov 26, 2008 - 02:18pm PT
It's my understanding that most of the granite we can see on the surface of the earth's crust was formed in the pre-cambrian, but not all granatic intrusions are of the same date. Granite has been, and still is, forming throughout all geologic time.

I am not a student of geology, nor am I entirely sure my theory holds water (pun intended here), but I would assume that the majority of granite currently on the surface of the planet being the same age is somehow related to erosion?

I've also been told that the granite in Death Valley is the same stuff as what you see in Yosemite, just a sh#t ton older, hence the choss. Maybe speaking in geologic time, the granite bubbles did indeed rise to the surface around the same time, just that time period may be millions of years.

Anyhow, that's all I got...
Minerals

Social climber
The Deli
Nov 26, 2008 - 04:06pm PT
Wow… Something that’s actually worth posting to!

Where to begin? Well, first of all, there are three main rock types – igneous, sedimentary, and metamorphic. Igneous rock forms from the cooling and crystallization of molten magma. Sedimentary rocks form from the deposition of… yes, sediments, although the only thing to know about sedimentary rocks is the first and only Law of Sedimentology – sh#t flows down hill! Metamorphic rocks are igneous or sedimentary rocks that have been physically and/or chemically changed by the effects of temperature and/or pressure.

Granite is an igneous rock, as is basalt, the difference being that granite is an intrusive rock (plutonic - forms at depth in the crust, well below the surface of the Earth - Yosemite) and basalt is an extrusive rock (volcanic - forms at the surface of the Earth - Hawaii). Intrusive rocks are coarser-grained than extrusive rocks mainly because of cooling time. Since intrusive magma cools very slowly, the mineral grains (crystals) have more time to grow to larger sizes whereas extrusive magma is quenched at the surface and the small grains are “frozen” before they have a chance to grow to larger sizes.

It is still debated today whether or not large magma “chambers” exist and later cool to form plutons, but “granite bubbles” do not drift to the surface. The closest idea would be that of intrusive diapirs of magma (still more debate) which rise through the crust at incredibly slow rates, due to buoyancy; this is similar, in effect, to the way in which salt domes form.

Since granite forms well below the surface, all of the granite that we see and climb on today has been exposed by a combination of erosion and isostatic adjustment of the crust. Isostacy is the principle by which the crust of the Earth floats on the mantle. The crust is thicker in mountainous regions and as material is removed through erosion, there is less overlying mass on the crust and thus, it rebounds upward. An easy way to think of this is to imagine several people in a small boat – as each person exits the boat, it rides slightly higher in the water. Another example would be Hudson Bay in Canada. This shallow bay is an artifact of the last major ice age; the mass of the continental ice sheet was great enough to depress the crust below sea level. Now that the ice is gone, the crust is rebounding, but at a very slow rate.

Ages of granites may vary widely across the globe. As Dingus mentioned, the youngest granitic rocks in the Sierra lie along the Sierra Crest although the eastern portion of the range contains some of the oldest granitic rock in the Sierra. The granite of Lee Vining Canyon is about 210 million years old and the Tungsten Hills granite (??? not sure of the exact name of this unit off the top of my head… Buttermilks area), which is part of the Scheelite Intrusive Suite, is about 220 million years old and is the oldest “granite” in the Sierra.

Granitic rock quality is not necessarily a function of age; it is mostly dependent on geologic environment and weathering. Granitic rock that is found in the desert is generally more “chossy” because it has been exposed to surface and near-surface weathering for much longer than say, the bullet-proof domes of Tuolumne that have been scoured down to solid rock by recent glaciation(s). It would not be at all uncommon to find old granite that is bomber and much younger granite that is chossy – you just won’t find them in the same area.


Next up… not all granite is granite!
UncleDoug

Social climber
Nov 26, 2008 - 04:23pm PT
Minerals,

What about regions that do have solid granite and 30 feet away is good ol DG?
Lots of examples around the Tahoe area.
Would it be due to variances in mineral/crystalization?
Hardly Visible

climber
Port Angeles
Nov 26, 2008 - 04:26pm PT
Juan,
I too am just a layman when it comes to this, however I do know that granite is formed at depth and is generally thought of as being the roots of old volcanoes. Presumably anywhere volcanoes exist today something that most of us would call granite is being formed at depth. Uplift and erosion are what brings it to the surface for us to appreciate. Since uplift and erosion take time we don’t see new granite on the surface of the earth today. In areas where uplift has occurred rather rapidly like the North Cascades there are sizeable batholiths as young as 20 million years old which is the youngest exposed granite that I know of. At the other end of the spectrum you have areas like the Tetons and Veedawoo where the rock is 2-3 billion years old.
I see Minerals has jumped into the conversation since I’ve been typing this so I’ll let him elaborated on it further in a way that I never could.
With a name like Juan de Fuca I would have figured you would know all this.
Minerals

Social climber
The Deli
Nov 26, 2008 - 05:02pm PT
UncleDoug,

Good point. It’s easy to space things when I get rambling with my fingers…

There are examples of what you speak of in many areas. The difference between solid rock and DG is due to weathering rates, not the initial mineral composition of the rock. I would guess that in the examples that you have seen, the mineral composition/texture is the same throughout the rock. Some sections of an outcrop may be more fractured or sheared than others, which allows fluids to pass through sections of the rock mass and increase chemical weathering rates in these sections. Outcrops that exhibit spheroidal weathering contain rounded masses of solid rock that are surrounded by weathered choss or DG. A good example of this can be seen in the road cut on the west side of 395/frontage road between Washoe Valley and Carson City. There are probably more in the Tahoe area. DG is referred to as saprolite and the coarse-grained granular sand that forms from its decomposition is called grus.

Thanks Tami!
Mike Bolte

Trad climber
Planet Earth
Nov 26, 2008 - 06:01pm PT
Thanks Minerals!
Gary

climber
Desolation Basin, Calif.
Nov 26, 2008 - 06:04pm PT
It's tuff to be gneiss to a schist.
Ricardo Cabeza

climber
Meyers,CA
Nov 26, 2008 - 06:06pm PT
OK Minerals,
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?

This thread rocks. I love geology! Thanks man.
Mighty Hiker

Social climber
Vancouver, B.C.
Nov 26, 2008 - 06:46pm PT
Bryan is a lot of fun to go climbing with - you learn lots about the natural history of the place, at least the geological side of it.

"I'll take the wax JDF doll out of the pentagram."

Better keep a few pins at hand, just in case.
Karl Baba

Trad climber
Yosemite, Ca
Nov 26, 2008 - 08:45pm PT
Thats a bunch of schist Minerals, pure superstition from somebody who doesn't believe in the spaghetti monster. But thanks for typing,


now here is some historical fact that you can take to the bank.







Sherlock Holmes was heading the Fisher Towers to tick off a route with Watson.



Watson was horrified at the base of the route and said, "This rock bullshit Homey! What type is it?"




Holmes didn't bat an eyelash. "Sedimentary my dear Watson!"


Perhaps you can take that to the sperm bank if the regular bank won't buy it?!

:=)

karl


Jaybro

Social climber
wuz real!
Nov 26, 2008 - 09:17pm PT
Quartz Monzonite? Cabeza?

This granite is old

precambrian.

While this sierran orogeny stuff, is just


batholith come lately.
Minerals

Social climber
The Deli
Nov 27, 2008 - 02:26pm PT
We climbers refer to the rock in Yosemite, etc. as granite, but quite often it would be more appropriate to refer to it as “granitic rock” or a “granitoid.” After all, there is more granodiorite in the Sierra Nevada Batholith than there is granite. And there is diorite too, right? So, what’s the difference?

Plutonic rock types are differentiated, based on their mineralogy. The classification system currently used is based on the relative percentages of the three primary minerals that compose granitic rock – quartz, potassium feldspar, and plagioclase feldspar. The rest of the minerals that are commonly found in granitic rock are referred to as accessory minerals; these include biotite, hornblende, titanite, magnetite, apatite, garnet, muscovite, ilmenite, monazite, zircon, etc. The diagram below outlines how plutonic rock types are classified and is referred to as a Q-A-P diagram. Q = quartz, A = alkali/potassium feldspar, P = plagioclase feldspar.




For example, if we have a sample of rock that is 50% quartz and 50% feldspar (excluding accessory minerals), with potassium feldspar making up 65% of the total feldspar in the rock and plagioclase feldspar making up 35%, we would have a sample of good ol’ granite. If we have a sample of rock that is 30% quartz and 70% feldspar, with potassium feldspar making up 20% of the total feldspar and plagioclase feldspar making up 80%, we would have a sample of granodiorite. If we have a sample of rock that is 10% quartz and 90% feldspar, with potassium feldspar making up 5% of the total feldspar and plagioclase feldspar making up 95%, we would have a sample of quartz diorite. Make sense?
apogee

climber
Nov 27, 2008 - 03:15pm PT
Minerals- this is awesome. Thank you for the time to share it.

And timely, too- I was having a conversation about this with a co-worker about this very topic a few days ago. We were both wondering about the origins of much of the quartz monzonite in Joshua Tree, and it's relative age in relation to ranges north and south of there. My understanding is that the larger crystals are due to a much slower cooling process- would that mean that the JT batholith was deeper and remained submerged for a longer period? How does that batholith differ from those in the Sierra in terms of age or content?
Greg Barnes

climber
Nov 27, 2008 - 03:32pm PT
Cool Minerals!

An early climbing partner of mine was from Sweden and in a geophysics PhD program at Stanford. He said that he climbed on 2 billion year old granite in Sweden, some of the oldest granite on earth. So, if you know, what's the oldest granite that we climb on in the US (and Squamish), versus the oldest on the planet? Or maybe just the age break down of the CA climbing areas - like Tahquitz versus the Valley?

Thanks!
Mighty Hiker

Social climber
Vancouver, B.C.
Nov 27, 2008 - 04:52pm PT
Bryan, sometimes people say that "true" granite can be known because it's pinkish in colour. We have some at Lighthouse Park, near Vancouver. Does pink = granite? If so, what makes it pink?

I think the Chamonix Aigulles are also pinkish, and maybe other places. Though I guess weathering may have something to do with it, too.

Squamish is I believe mostly granodiorite, with basalt and some aplite dykes. But it's probably more complicated than that.
kpinwalla2

Social climber
WA
Nov 27, 2008 - 06:26pm PT
The City of Rocks has two "granites". One is 30 million years old (Oligocene) and the other is 2500 million year old (Archean). I wrote a book about all this granite stuff, how it weathers, how the landforms are created, etc., etc. It's published by the Idaho Geological Survey and Called "Etched in Stone, the Geology of City of Rocks National Reserve". If you're really turned on by granites, and especially by JT-like landforms, I suggest you check it out - it was purely a labor of love thing - no royalties for me. Minerals - that's good stuff you write....
Minerals

Social climber
The Deli
Nov 27, 2008 - 08:24pm PT
Holy cripes! What have I gotten myself into? Just kidding…

The only references that I have in front of me are a small field guide to rocks and minerals and a dictionary of geological terms; all of my other stuff is in storage. Unfortunately, there isn’t much in the way of good technical info on the web, unless you pay for a membership to specific websites, etc. I’m also not on my computer so I can’t post any of my photos or reference ages of specific rock units… but hopefully soon. If anyone else has info, go ahead and help out!

Apogee - I’ve only been to JT once and have almost no knowledge of the area. I believe that the granitic units in So Cal are relatively similar in age and composition to that of the Sierra Nevada Batholith. Crystal size is mainly due to cooling rate although this wouldn’t necessarily mean that the pluton was emplaced at a deeper level in the crust than a finer-grained rock (smaller crystals). The entire formation of a pluton, including late-stage alteration, occurs at depth (on the order of several kilometers or more). Pluton emplacement/formation and cooling may take anywhere from less than a million years to several million years; it generally takes several million to tens of millions of years for a pluton to become exposed at the surface of the Earth. Regional tectonic uplift (which I forgot to mention earlier) as well as erosion and isostatic adjustment work together to expose plutons. To keep things simple, I’ll leave it at that for now.

Here’s a simplified geologic map of JT. Click on the colored boxes below that are labeled with geologic time units to see photo examples and basic descriptions of the rock types in the area.

http://www.geology.iupui.edu/research/rocklab/JTNP/geologic_map.htm


Here’s a good USGS article on the geology of So Cal – it covers some of the basics on the plutonic units in the area and it also has a geologic time scale which is a useful reference when dealing with ages of rocks.

http://geomaps.wr.usgs.gov/socal/geology/geologic_history/index.html


GregB – Whew… trying to make me work hard, eh? I don’t know what the oldest granite in the US or on the planet is; I’d have to look it up… if I had something to look at… Kpinwalla2 mentions above that there are Archean granitics at the City of Rocks – that’s pretty darn old. Never been to Tahquitz and it would probably take me a minute to even find it on a map. I’ll try to post dates for the rock units in Yosemite at some point.

Still typing…
Mighty Hiker

Social climber
Vancouver, B.C.
Nov 27, 2008 - 08:30pm PT
I believe some of the oldest rocks on Earth are in Greenland, the Canadian Shield, and Australia. There are garnets that are over 4 billion years old, but actual rocks are a different matter. Beyond that is beyond me.

It would be interesting to know what the oldest rocks are that people usually climb on?
gstock

climber
Yosemite Valley
Nov 27, 2008 - 08:39pm PT
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
Minerals

Social climber
The Deli
Nov 27, 2008 - 11:18pm PT
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!
apogee

climber
Nov 28, 2008 - 01:24am PT
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'
Minerals

Social climber
The Deli
Dec 1, 2008 - 02:03pm PT
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???
TwistedCrank

climber
Ideeho-dee-do-dah-day
Dec 1, 2008 - 02:09pm PT
Pff...

Anything that's not Archean is just overburdon. Just ask them Montana boys.
Captain...or Skully

Social climber
Where are YOU from?
Dec 1, 2008 - 02:16pm PT
ToasterS??

Must be close to snacktime......Rock Rocks.!!
Jaybro

Social climber
wuz real!
Dec 1, 2008 - 02:21pm PT
Sherman Granite (Vedauwoo) is noted for it's pink K-spars.
Captain...or Skully

Social climber
Where are YOU from?
Dec 1, 2008 - 02:24pm PT
Really cool stone in that Sherman stuff.That's fairly old, ain't it?
stevep

Boulder climber
Salt Lake, UT
Dec 1, 2008 - 02:27pm PT
I thought Sherman granite was noted mostly for its plethora of V-Grades.
taorock

Trad climber
Okanogan, WA
Dec 1, 2008 - 02:37pm PT
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.
Jaybro

Social climber
wuz real!
Dec 1, 2008 - 02:43pm PT
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?
scuffy b

climber
On the dock in the dark
Dec 1, 2008 - 03:15pm PT
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?
BASE104

climber
An Oil Field
Dec 1, 2008 - 03:42pm PT
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.
yossarian

climber
WA
Dec 2, 2008 - 11:30am PT
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).
scuffy b

climber
On the dock in the dark
Dec 2, 2008 - 11:44am PT
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.
GOclimb

Trad climber
Boston, MA
Dec 2, 2008 - 12:56pm PT
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
scuffy b

climber
On the dock in the dark
Dec 2, 2008 - 01:01pm PT
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)
mojede

Trad climber
Butte, America
Dec 2, 2008 - 01:07pm PT
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.

GOclimb

Trad climber
Boston, MA
Dec 2, 2008 - 01:17pm PT
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
mojede

Trad climber
Butte, America
Dec 2, 2008 - 01:21pm PT
Jaybro, it looks like the Bighorn Mountains in Wyoming are almost as old as the Beartooths--give or take a half-billion years.
Jaybro

Social climber
wuz real!
Dec 2, 2008 - 01:28pm PT
They just don't make rocks like that anymore...
scuffy b

climber
On the dock in the dark
Dec 2, 2008 - 01:44pm PT
well, sometimes my dumbness surprises me.
Where's the doggone K in that chart?
scuffy b

climber
On the dock in the dark
Dec 2, 2008 - 02:32pm PT
okey-dokey, thanks, Wes.
Mighty Hiker

Social climber
Vancouver, B.C.
Dec 2, 2008 - 02:33pm PT
There's a good article in the science section of today's New York Times about the formation of the Earth's surface, the earliest rocks, zircons, the Late Heavy Bombardment, and other fun stuff.
http://www.nytimes.com/2008/12/02/science/02eart.html?_r=1&ref=science
GOclimb

Trad climber
Boston, MA
Dec 2, 2008 - 03:00pm PT
Wes - sounds like I should go back to my earlier explanation, that the Acadia Granite around Otter and Precipice Cliffs gets its deep pink (maybe verging on purple when weathered) from all the Alkali/K Feldspar - the pink stuff in your photo. In other words, it is the bottom left stuff like I thought, not bottom right.


GO
drunkenmaster

Social climber
santa rosa
Dec 2, 2008 - 03:22pm PT
Gneiss Schist Minerals! Thanks. More inforumative posts like this are much needed - yes.
Minerals

Social climber
The Deli
Dec 9, 2008 - 02:34pm PT
Good stuff! Nice to see a few more of you chiming in.


Don’t know about that, Jaybro. My knowledge of granites is pretty much limited to the Sierra Nevada and Northern Nevada.

Wes, I’ve never really thought about the patina at the Buttermilks but that is an interesting question. What’s the Dorn story? Your thoughts?

There is granite in Utah…?

GOclimb, you’ve got it right; the rock that you climb on lies in the lower left side of the diagram. From your last photo (nice joints, btw) it appears that there are a few lighter spots – this is most likely quartz but could also be plagioclase. So, the rock is probably granite, but could be an alkali feldspar granite, or even something closer to the syenite field. Looks like nice stuff.


Well, we might as well throw in a little mineralogy so that we have a few building blocks to make granitic rock…


Common minerals found in granitic rock:


 Quartz – SiO2

 Potassium feldspar (Orthoclase) – KAlSi3O8

 Plagioclase feldspar (Albite – Anorthite) – NaAlSi3O8 – CaAlSi3O8

 Biotite – K(Mg,Fe)3(Al,Fe)SiO10(OH,F)2

 Hornblende – (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(Al,Si)8O22(OH)2

 Titanite – CaTiSiO5

 Magnetite – Fe3O4

 Apatite – Ca5(PO4,CO3)3(F,OH,Cl)

 Garnet (Almandine) – Fe3Al2(SiO4)3

 Muscovite – KAl2(AlSi3)O10(OH)2

 Ilmenite – FeTiO3

 Monazite – (Ce,La,Nd,Th)PO4

 Zircon – ZrSiO4

 Tourmaline – (Na,Ca)(Mg,Fe+2,Fe+3,Al,Mn,Li)3Al6(BO3)3(Si6O18)(OH,F)4



Wes mentioned earlier that plagioclase feldspar is a solid solution, with albite and anorthite as the end members. Labradorite is one of the intermediate varieties of plagioclase and is commonly seen as the iridescent blue-colored mineral found in some granitic countertops. Where’s our countertop man, Skully?


Time to dig up some photo examples…
Minerals

Social climber
The Deli
Dec 9, 2008 - 04:47pm PT
Ok, I have a question for you guys. While out in the desert last week, on the east side of Winnemucca dry lake, I stumbled across this pothole in what I think is bedrock (although it is possible that it is a boulder). It threw me for a loop! The inside of the pothole is rounded and the surface is polished smooth, just like a pothole that you would see in a river or glaciated areas, like Tuolumne. There is obviously no water in the area now, however this outcrop would have been submerged during the high-stand of Lake Lahontan approximately 13,000 years ago. The outcrop does not lie in a drainage, but on more of a slight ridge on a slope below the main outcrop of bedrock that forms the mountainside.


The pothole. The polished areas are rust-brown in color; in some areas, such as behind the hammer handle, the polish has weathered away. There are two rounded scoops on the top rim, one at the back (where the darker patches of rock are) and the other on the right side. The light-colored section on the left is part of a pegmatite dike.


Another view – the outcrop looks more like bedrock than a detached boulder. No?


The pothole is located on the short slope below the obvious bedrock, just left of center in the photo. Also note series of felsic dikes.


What do you think? Gstock?
scuffy b

climber
On the dock in the dark
Dec 9, 2008 - 04:55pm PT
Is there more of that polish (regardless of shape) on any of
the bedrock around there?
Have you ever seen features like that which you could attribute
to wave-related erosion in a lake?
Minerals

Social climber
The Deli
Dec 9, 2008 - 05:32pm PT
Scuffy, I didn’t notice any other polished surfaces; the rock surfaces are pretty rough in this area. That’s the thing… I generally think of potholes as forming from the abrasive action of cobbles swirling around in rapidly moving water – in a river. How would this be possible in a shoreline environment? I dunno.


Interesting idea, Wes. Keep us posted on your findings. What was the original Dorn idea?
Jaybro

Social climber
wuz real!
Dec 9, 2008 - 05:53pm PT
Isn't a lot of (funky) black Patina in desert places, IC, the Swell, Redrocks, volcanic tablelands, etc, thought to maybe be a product of a sort of evaporative/leaching/constituent mineral/etc migration with possible bio agent contribution kinda deal? Clearly moisture dependent, but media (almost) agnostic?
scuffy b

climber
On the dock in the dark
Dec 9, 2008 - 06:04pm PT
It seems so unlikely, Minerals.
Just thought I'd have to put it out there.
Maybe in several-to-many miles of shoreline,
a spot like that might develop?
Lahontan had a pretty big shoreline. Maybe you found
a tiny jackpot. Check the next couple hundred miles?
Jaybro

Social climber
wuz real!
Dec 9, 2008 - 07:22pm PT
Misnomers abound.
the Phx Rock guide with the ape on the cover refers to the "Solution pockets" in the Queen creek, volcanic rock.
Minerals

Social climber
The Deli
Dec 12, 2008 - 12:53pm PT
Thanks for the Dorn info, Wes. That sounds like some weird stuff… with bacteria and all…

Yeah, Scuffy. Maybe that pothole is a rarity. Who knows… I just thought that it would be fun to throw it out here. It’s definitely not a solution pocket – I’ve seen many of them and this pothole is polished on the inside – too smooth and rounded to be a solution pocket. This thing sure looks like it formed in a fluvial environment to me… But how???
Minerals

Social climber
The Deli
Dec 12, 2008 - 12:55pm PT
Here’s an easier one…

Name the four basic rock types in this outcrop, listing them in relative order, from oldest to youngest.


scuffy b

climber
On the dock in the dark
Dec 12, 2008 - 02:09pm PT
upper left volcanic youngest,
pale center,
dark lower center,
grey lower left oldest?
KyleO

Ice climber
Calgary, AB
Dec 13, 2008 - 02:42am PT
Youngest
Erosion creating surrounding clasts
Injection of light colored igneous intrusion
Folding event (axial trace 45* up to right)
Dark metamorphic rock (amphibolite)
Greenschist metamorphic rock
Oldest

Sweeet outcrop. Not easy at all!!

Dr. Rock

Ice climber
http://tinyurl.com/4oa5br
Dec 14, 2008 - 01:11pm PT
You ever heard of "The Lisping Geologist?"

It's a little routine I worked up out of boredome one day while sitting in on Dr Ramsey's Geo class at Sac City with the little lady.
We were drinkin 10 AM wine coolers in the back row as usual (co-dep), use the sport bottles, anyway, this guy had a slight lisp, so anything with an S.

Observe:

"Ssscchhist, Ssscccchlate and Shhhherrerpintine were formed from ssstalagmite intrusions of ssssedimentary rock.
Ssssshubduction zones formed by sssshhpheroidal weathering were studied using ssshheismic tomography have been known to contain elements of ssshhilicon oxygen tetrahedron.

Any quesssssshhtionssshhons?"

You get the point.
Dr. Rock

Ice climber
http://tinyurl.com/4oa5br
Dec 14, 2008 - 03:35pm PT
Applications:

Pink Granite is used in the machine shop as a level surface.

Amorphous Silica has just recently been used in high end audio transformers, see search Lundahl.
Dr. Rock

Ice climber
http://tinyurl.com/4oa5br
Dec 14, 2008 - 03:35pm PT
Applications:

Pink Granite is used in the machine shop as a level surface.

Amorphous Silica has just recently been used in high end audio transformer laminations, search Lundahl.
Minerals

Social climber
The Deli
Dec 15, 2008 - 09:38pm PT
Glad to see that this thread is still going and that some of you took a stab at the rock types in the outcrop photo. Cool.

I wasn’t looking for anything fancy, just the basic rock types. And yeah, maybe it’s not so easy if you don’t have the actual rock in front of you!

Here’s what I was thinking:

Oldest

 Metamorphic rock (phyllite/schist), the dark-gray rock, lower center, and the greenish-colored rock in the upper right.
 Intrusive igneous rock (granodiorite), medium-gray/tan-colored, left side of outcrop.
 Intrusive igneous rock (aplite/pegmatite dike), light-colored band of rock that cuts the older rock types.
 Sedimentary rock (tufa – leave it to the hydro guys…), tan-colored surface choss on the far upper left that is sharper than schist!

Youngest



Back to basic mineralogy…

Here’s a photo of the surface of a boulder of granodiorite from the good ol’ Nevada desert. This rock is similar in age to the granitic rocks of the Sierra Nevada Batholith (mostly Cretaceous) and has good examples of some of the common accessory minerals found in granitic rock.

H = hornblende
B = biotie (mica)
T = titanite

Hornblende is easily recognized by its rectangular shape and black color. The largest hornblende crystal in this photo (lower section of photo) contains smaller crystals of feldspar. A mineral that contains smaller grains of another mineral is said to have a poikilitic texture. The smaller mineral grains within are referred to as poikilitic inclusions. Biotite is also black but is much softer than hornblende (can be scratched with a knife) and occurs in stacks or clots of platy hexagonal sheets. Biotite often weathers from black to a dark-greenish-color. The titanite grains in this rock are yellowish-brown, however the titanite grains seen in Yosemite are more brownish in color, sometimes resembling a “root beer” brown color.

The small white grains are plagioclase, the small tan grains with a tinge of pink are potassium feldspar, and the small light-gray grains are quartz.







Tami, I haven’t forgotten about your question – just need to finish typing…
Darwin

Trad climber
Seattle, WA
Dec 16, 2008 - 01:26am PT
With little or no basis whatsoever, I've always thought of columnar basalt
as granite that cooled much more quickly. I'm sure that's wrong, but:
can someone give examples of distinct rock types that have similar/same composition,
but differ in cooling rates at the time of formation?

Darwin
Minerals

Social climber
The Deli
Dec 16, 2008 - 07:53pm PT
Good stuff, Wes.
Increase in H20 content decreases magma viscosity.
Minerals

Social climber
The Deli
Dec 16, 2008 - 07:54pm PT
Well, I hope this all makes sense. I may not have described some things in the best way possible but it’s an attempt. Please correct me if you notice any mistakes/errors.



Tami wrote:
“Have you seen The Stawmus Chief ? It's got an almighty basalt dyke ( the Black Dyke ) splitting the entire cliff. Better still, this basalt dyke on the face of the Chief is only a visible piece of a massive basalt dyke - you can see where it comes out of the sea , crosses through the Malemute cliff, crosses the highway, cuts through the main face of the Chief , over to the backside and so on ( I guess ! ) . It's an astonishing geologic feature.

How does it get there as a "dyke" of rock in the larger granite whole ?”



Tami, I’ve never been up there but that Black Dike sounds really neat! A pretty major feature, eh? Based on its extent, maybe it is some sort of feeder dike (system?) that fueled volcanic activity far above the granitic rock that it cut(s) through. I dunno. Are there other basaltic dikes nearby or just the one?


Hey BASE104, lots of good info in your post! Thanks for explaining how dikes form, and yeah, the “Wild Dikes” on the East Ledges descent are wicked cool. I need to read that section of the McPhee book on zircons and the craton, but it’s wrapped up and stored under lock and key. Thanks again for that!


Hmmmm… Well, I’ll give this a shot…

As far as water’s involvement in the formation of granite, Dingus is on the right track. Water (H2O) content affects the melting temperature of the upper mantle and the sediments riding on a subducting slab are saturated with seawater. As a slab dives downward, water is added to upper mantle material and magma is generated. The magma is very mafic in composition and is representative of its primitive mantle source. Compositionally, the magma is far from granitic.

As stated earlier, mafic minerals generally have a higher melting temperature than felsic minerals and thus, mafic magmas are generally hotter than felsic magmas. As mafic magma rises into the crust, it may partially or completely melt and digest crustal rock (assimilation), which changes its overall composition, making the magma more felsic. If the magma reaches the surface after little interaction with crustal material, then basaltic (mafic) volcanism results. If the magma (or a partial melt byproduct of the magma) reaches the surface after significant interaction with, and assimilation of crustal material, then rhyolitic (felsic) volcanism results. If magma does not make it to the surface, it cools and solidifies to form a pluton. Separate magmas of differing composition may mix to create hybrid magmas of intermediate composition. Felsic magma may also be produced by the partial melting of older plutons (mafic to felsic) during the emplacement of younger mafic magma.

Terminology side note:
Melt = molten rock in liquid form.
Magma = molten rock in liquid form that may or may not contain solid crystals in suspension.



Below is a series of simplified conceptual diagrams showing the development of a magmatic arc, from a passive continental margin to a convergent margin, and subduction of oceanic lithosphere beneath continental lithosphere. The dip angle of a subducting slab may vary greatly, from a shallow angle to a steeper angle as shown in the diagram. The modern-day Atlantic coast is an example of a passive margin and the modern-day Cascades in the Pacific Northwest and Andes in South America are examples of oceanic-continental plate convergence. The upper mantle is called the aesthenosphere and is shown in orange on the diagram. Magma is shown in red and the third stage diagram shows a cooled pluton in gray, beneath an inactive volcano.


(Image pirated from the net…)





Here are a few photo examples of dikes:



While not exactly the clean granitic rock of Squamish, this photo shows two basaltic (mafic) dikes cutting through Cretaceous granodiorite. These dikes are significantly younger than the granodiorite (probably Tertiary in age) and formed after the granodiorite cooled and crystallized completely. As BASE104 mentioned above, magma forces its way along fractures in solidified and (in this case) much cooler host rock, separating the two sides of the fracture, forming a roughly planar sheet of magma. This magma may cool and solidify in place or may be transported through the fracture opening to another location, either within the crust (intrusive) or at the surface (extrusive). The pre-existing fractures in the host granodiorite may result from internal cooling of the pluton (shrinkage) or from external localized or regional stress (tectonics). Fractures may develop in mostly-crystalline magma with melt still present; i.e. magma does not have to be completely solidified in order to fracture. (I forget the minimum percentage range of melt to crystals in which magma becomes rigid enough to deform in a brittle manner (shearing in a skeletal framework.))



This is an example of how rock types of differing composition weather at different rates. The rounded boulders in the foreground are pieces of granodiorite and the line of boulders of lighter-colored rock that looks like an old rock wall is what’s left of an aplite dike. Aplite is a fine-grained felsic rock that is composed mostly of quartz, potassium feldspar, and some plagioclase feldspar, which makes it more resistant to weathering than the host granodiorite; hence the “rock wall.”



Here’s a boulder of granodiorite that includes a section of an aplite dike, showing the 3-dimensional nature of a dike. The dike can be seen on the backside of the boulder as well, giving the appearance of a layer of white sandwiched between two chunks of darker rock. (The black spots in the granodiorite are hornblende and biotite.) When viewed along the plane of a dike, it appears as a straight line or linear band. When viewed from a sub-perpendicular to perpendicular orientation, a dike may appear as an irregular shape, such as the patch of aplite on the top left of the boulder. The slabs at the base of Upper Yosemite Falls are great for viewing aplite/pegmatite dikes in three dimensions.



Pegmatite is generally similar in composition to aplite but is very coarse-grained. Pegmatite may contain more accessory minerals than aplite. This photo of the planar surface of a pegmatite dike shows (in this case) the difference in color between potassium feldspar (pink) and plagioclase feldspar (white). The very light-gray mineral is quartz and the thin black lines are sheets of biotite (mica). I’m not totally sure, but think that the yellow spots that are mainly seen between the edge of the pegmatite and the granodiorite are due to iron staining, mostly on/in quartz.



In addition to quartz and feldspar, the aplite/pegmatite in this example contains muscovite (mica – the fine- to medium-grained light-gray lines), garnet (the obvious red mineral), and a band of fine-grained magnetite (black) that stains the rock a rusty-yellow color as it weathers. The thing at the bottom of the photo with the shadow is a dead piece of cheat grass.



This is a section of the pegmatite dike shown in the “4-rock-outcrop” photo in my earlier post. The pegmatite is composed mostly of potassium feldspar and quartz. The small red dots are garnet and the jet-black mineral is tourmaline; there might also be a little muscovite in there as well. My finger points to a “graphic” texture of a quartz-tourmaline intergrowth (kinda neat…).


Aplite and pegmatite may also occur in the form of pods and irregular masses of varying size, in addition to dikes, as seen in some sections of the SE face of El Cap (“The Brain” and “The Cauliflower” etc.). Aplitic magma is the remaining fraction of a granitic magma that is left over after all/most of the mafic minerals have crystallized. Aplitic magma crystallizes last (the youngest unit) in the crystallization sequence of a pluton and aplite/pegmatite dikes can often be seen cutting all other rock types in an outcrop.



Here’s a texture that forms in dikes that is not terribly common. This texture is referred to as comb layering, because of the alignment of prismatic minerals (in this case mainly hornblende) that grow perpendicular to the walls of the dike. I have only seen this texture in outcrop in two locations – west of Lake Tahoe (pictured) and in Tuolumne.



Photo by Greg Stock, YNP Geologist
This is the spot on the East Ledges descent of El Cap that is labeled “Wild Dikes” in the Reid guide, the spot that BASE104 asked about. Note vegetation for scale.



Photo by Greg Stock, YNP Geologist
A closer view of this amazing feature shows the intricate geometric relations between highly fractured diorite/gabbro (dark-gray) and aplite/pegmatite (white). My guess (with emphasis on the word guess!) is that this batch of hot mafic magma was quickly emplaced into the surrounding El Cap or Taft granite, which was much, much cooler but still may have had melt present. The mafic intrusion may have been large enough in volume to contain the thermal energy necessary to partially melt the surrounding granite, but small enough in volume such that it quickly lost its thermal energy and was rapidly “quenched” within the granite. This rapid cooling may have caused the batch of mafic magma to quickly solidify and basically shatter. As an extreme example, think of it this way; if you slowly heat up a beer bottle in a campfire until it is red-hot and then toss it into a bucket of cold water, what happens? The partial melt from the granite (very felsic) was remobilized and injected into all of the weaknesses (fractures) in the diorite/gabbro, creating the angular mosaic of separated puzzle pieces that we see today. It appears that there were at least two stages of felsic diking, given the fact that some of the dikes cut other dikes.



This example of diking is similar to the relations and patterns seen in the Wild Dikes photos. This is (was) a boulder at the base of Lower Cathedral Rock, in the Mecca area, that has since been decimated by more-recent rockfall. The coarse-grained diorite/gabbro was shattered and intruded by felsic magma, creating a “breccia-like” texture. The rust-colored stains result from the weathering of pyrite (FeS2) to hematite (Fe2O3).



What happened here? This aplite dike has been offset by post-magmatic(?) left-lateral displacement along brittle joints in the granitic host rock. (left-lateral = sinistral; right-lateral = dextral.) One thing to remember when looking at a structural feature in two dimensions is that the structure may also be displacement in the 3rd-dimension (upward or downward in this case). Displacement that is seen in two dimensions is referred to as apparent offset. (Note blue pen for scale.)



More deformed dikes… This granitic dike was emplaced into metamorphic rock (phyllite/schist) while the metamorphic rock was undergoing tectonic-related ductile (plastic) deformation. Sections of the dike have been squished apart, parallel to the foliation (planar alignment of minerals and structural weakness) in the metamorphic rock, and now appear as discontinuous ellipsoidal blobs of granitic material. This “pinch and swell” structure is referred to as boudinage and the individual blobs are called boudins. Boudinage forms due to the difference in competence between separate layers of rock when deformed under stress.



Uggg… That’s enough for now. I have other photos to post - maybe more at some point… Magma mixing?
Dr. Rock

Ice climber
http://tinyurl.com/4oa5br
Dec 16, 2008 - 09:58pm PT
Awesome!

Howabout some Au intrusion models?
Minerals

Social climber
The Deli
Dec 17, 2008 - 04:59pm PT
Thanks, Dr.

Don’t know anything about gold. Never studied ore deposits. You?

The gold polish in Tuolumne is pretty nice...
UncleDoug

Social climber
Dec 17, 2008 - 07:49pm PT
Minerals,

I'm an avid crystal hunter and in regards to pegmatites I have a couple of questions.
It seems that most pegmatites in granite originate as an intrusive element, except for the case of "gas-pocket" pegmatites.
How common are GPP's and is there any rhyme or reason to their formation?
And if you know much about GPP's could you elaborate on them a bit?

I've pulled some stupidly-sized crystals from GPP's in N. Nevada and want to learn more abut what I've found.

Thanks much!

mongrel

Trad climber
Truckee, CA
Dec 17, 2008 - 10:40pm PT
Minerals, I expect I speak for a legion of tacoists in thanking you for fantastic, informative posts on this thread. One of the huge joys in rock climbing is interacting with geology closely, which is enhanced greatly by learning about it. Thanks!

As for your pothole, one thought to consider is a spring, as superficially unlikely as that may seem. This fits with the Fe-staining; O2-depleted groundwater conducting more-soluble ferrous then it comes out of solution when oxidized to ferric. These little springs come and go a good deal in the Great Basin, I've seen ones that winked out due to no external cause other than maybe a slight near-surface fault shift, within the past 10 or 20 yrs. Another one in S. Tahoe area that has recently substantially increased in flow, drowning a whole big patch of pine forest. Again, virtually 100 percent certainty it's seismic. Admittedly, this doesn't seem like enough flow to account for the amount of erosion of the boulder seen in your photo, but it's an idea.
Dr. Rock

Ice climber
http://tinyurl.com/4oa5br
Dec 17, 2008 - 11:48pm PT
re:AU

all i got is a cooling and cracking of rock, then water hitting molten rock, creating a steam engine to drive the heavy gold into the fissures.
quartz is the host rock.

happened during geo period of Ca. that had the rivers running N to S, like the Sac.

Then sierra uplifting forced the rivers east to west.
where the east to west rivers bisect the north to south tertiary rivers/deposits, you have a burned out gold town.


or is it primary deposits?

primary in the N to S rivers, Tertiary in the E to W river biescts.
grey/blue gravel is a good sign.

Hwy 80 runs thru a red rock open pit gold mine near Cisco.

Flat rocks in the river means good gold hunting.

low pressure inside bends.

freeze your ass for nothing, don't go there.

unless hit hits 3000 a Oz that is...
Double D

climber
Dec 18, 2008 - 06:07pm PT
It's important to understand dikes. There are regular dikes...






















































and then there are bull-dikes

Double D

climber
Dec 18, 2008 - 06:10pm PT
Sandstone dikes?


Minerals

Social climber
The Deli
Dec 18, 2008 - 10:49pm PT
This thread is now officially climbing-related, complete with historical significance. Nice.
Minerals

Social climber
The Deli
Dec 18, 2008 - 10:58pm PT
There is actually more info on the web than I thought… it just takes time to find.


Here’s a neat bit about intrusion-related gold deposits in the Yukon:
http://www.loganresources.ca/i/pdf/2006-6-tombstone.pdf

And a much more comprehensive discussion:
http://gsc.nrcan.gc.ca/mindep/synth_dep/gold/rirgs/index_e.php

And another:
http://www.ga.gov.au/image_cache/GA7241.pdf



UncleDoug, I’m guessing that the gas pockets that you refer to are miarolitic cavities. Here are a few links to articles on miarolitic cavities, which explain formation in more depth than I can. Miarolitic cavities are somewhat common in the youngest unit in the Tuolumne Intrusive Suite, the Johnson granite. Some of the landscape boulders around the Tuolumne Store parking lot contain small miarolitic cavities that are lined with smoky quartz. The smoky color in quartz comes from crystal damage due to the radioactive decay of traces of Uranium contained within the crystal.

What kinds of minerals have you been finding in your hunts?


Definition of miarolitic cavity:
http://en.wikipedia.org/wiki/Miarolitic_cavities

Short and understandable article:
http://www.geocities.com/oklahomamgs/London/Pegmatite2.html

A more comprehensive article:
http://www.minsocam.org/ammin/AM71/AM71_396.pdf

Pegmatite mining San Diego County abstract:
http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_79683.htm



Hey, thanks Mongrel. This has been a really good exercise for me – a refresher, a chance to learn more, and something to keep me busy out in the desert (without wasting ammo). If it helps climbers to learn more about what they are climbing on, then that’s cool. Geologists would be so psyched to be able to see what we climbers see.

Back to the pothole… I thought about a spring but figured that it wouldn’t produce enough flow over time to scour out such a void, as you say. There’s probably a typical normal fault running right along the base of the hill, though. Braintwister…
Minerals

Social climber
The Deli
Dec 18, 2008 - 11:15pm PT
Holy ptygmatic folding! Meta-volcanic, Wes? Way cool.

Comb layering in your first image? Where? That one’s got a bit of a twist too…

Let’s see more!
MisterE

Trad climber
Raising Arizona
Dec 18, 2008 - 11:17pm PT
Such a wealth of great information - bookmarked for further digestion.

Thanks, Erik
Minerals

Social climber
The Deli
Dec 18, 2008 - 11:32pm PT
That sounds like fun. Where?





Toasters?

Old windows?

That reminds me... Lucho and I each have a cooking stone stashed at one of the bivies... Can't beat cooking on a slab of granite over an open fire.
MisterE

Trad climber
Raising Arizona
Dec 19, 2008 - 02:12am PT
The high-country stone-cooked wrap is a taste of near-heaven, I can tell you.

Miss you guys,

Erik
Jaybro

Social climber
wuz real!
Dec 19, 2008 - 02:34am PT
Man, you hard rocks guys are in a world of your own.

Am I the only Paleontologist (Quasi-emeritus) climber around?
UncleDoug

Social climber
Dec 19, 2008 - 10:32am PT
Minerals,

Thanks for the info!
I've been mainly going after smoky quartz.
There is an article in Rocks & Minerals, Sept/Oct 1993 regarding "Nevada Smoky Quartz" that caught my eye and I've been hooked ever since.

The largest pieces I've managed to cary/drag back home are 120 & 147 lbs. respectively.
The 120 is fully formed and gradates from jet black smoky at the tip end to crystal clear near the base.
The 147 is oddly shaped with only 3 truly faceted sides. It sits on the ground like an anvil. I have found several miniature( about 1 oz.) versions of this expression in the same pit.
These are from gas-pocket pegs.

Got some crazy smoky-amethyst-citrine-rootbeer turkey heads and sceptres from the Petersen Mtn./Haleluja Jnctn. area. Nothing obscenely large but very beautiful.

Don't have any photos to post but will work on it.

Thanks again for your knowledge release!
scuffy b

climber
heading slowly NNW
Apr 13, 2012 - 02:58pm PT
overburden bump
ydpl8s

Trad climber
Santa Monica, California
Apr 13, 2012 - 03:32pm PT
Am I the only Paleontologist (Quasi-emeritus) climber around?

Ha! Paleontologist = Frustrated Biologist

But then I'm a geophysicist so, flame away!
Klimmer

Mountain climber
San Diego
Apr 13, 2012 - 07:21pm PT
Good thread. Don't think I've seen it before.

Bump to get back to it later.
Kalimon

Trad climber
Ridgway, CO
Apr 15, 2012 - 01:58am PT
An excellent and informative thread . . . I noticed that JDF's only post was the very first. He sure knew how to get the ball rolling . . . He was before my time here in STland . . . what a sad loss.

Peace brother Juan.
NutAgain!

Trad climber
South Pasadena, CA
Oct 23, 2014 - 11:10am PT
Great thread... thanks to Ed for resurfacing it in this other thread:
http://www.supertopo.com/climbing/thread.php?topic_id=2515339

Would be nice to have seen what Wes had shared that is now deleted.
pyro

Big Wall climber
Calabasas
Oct 23, 2014 - 03:40pm PT
Thank-you Juan for starting this thread!
Messages 1 - 85 of total 85 in this topic
Return to Forum List
 
Our Guidebooks
Check 'em out!
SuperTopo Guidebooks


Try a free sample topo!

 
SuperTopo on the Web

Recent Route Beta