Discussion Topic |
|
This thread has been locked |
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Original Post - Mar 10, 2005 - 12:05pm PT
|
On geological rock types...
What is Quart Monzonite? What is Granite? Both are igneous rocks and both fall under the same classification scheme. They look the same because they formed under similar physical (high pressure/temp) and chemical regimes. The chemical composition at the time the magma crystalized determined the rock classification.
Igneous rocks (and plutonic rocks in general) such as these are classified on a ternary diagram (below). At the top is quartz, on the point bottom-right is plagioclase and on the left is orthoclase (K-spare).
In the rocks of Yosemite and Joshua Tree quartz is the clearest of the grains, plag is the opaque white grain, and k-spare typically takes on a flesh to pinkish color. Color, however, is not diagnostic of a particular variety of feldspar but it'll serve our purpose in Yosemite and Josh. Striations on cleavage faces are a better diagnostic of plagioclase.
Classifying the rock is pretty simple. A handlense is helpful. Looking at your rock you blank out all the dark minerals (they don't count unless >10%). This includes the mica's, pyroxene and hornblend. First determine what percentage of the light minerals is quartz. Then determine the ratio (in percentage) of plag to k-spar. After you figure that out you look for that spot on the diagram and you have your rock type.
In Yosemite your classification exercise might go:
Quartz: 45%
Feldspars: 55% (85% plag, 15% k-spar)
Turns out this makes most of the granitic looking rock in Yosemite a Granodiorite.
In Joshua Tree your classification might be:
Quartz: 15%
Feldspars: 85% (70% plag, 30% k-spar)
This classifies the rock as a Monzodiorite
OR :
Quartz: 15%
Feldspars: 85% (50% plag, 50% k-spar)
Classifying it as a Quartz Monzonite
I have a couple papers that I usually carry around with me that folks find of interest. Both are from California Geology. One covers some of the metamorphic aspects of Yosemite as well as the glacial aspects, the other is on the geomorphic evolution of Joshua Tree. It details the distribution of rock types in JTree. It also covers the tectonic evolution - explaining the higher elevations. If anyone is interested in finding these I can dig up the ref.
This about covers our Geology 419 topic for the day. Minerals has been missing from the supertopo geo seminar circuit. Figured I'd throw-down in his place.
For our next topic (we might get):
a) why is JTree rock so damn grainy?!?
b) What's up with all that desert varnish?
c) what the hell are those big black knobs at sugarloaf all about?
d) what nitwit sucked Owens Lake dry?
e) is there really cheese on the moon?
f) what makes the bishop tuff so special?
g) what's an exotic blue schist knocker?
h) why is there so much more quartz in the Yosemite rock?
i) why does orogeny lead to subduction?
j) what's a roof pendant and should i buy my wife/gf one?
k) desert pavement - what? they paved the desert?
One minute to go. gotta run. Enjoy!
Editors note: inspired by the discussion in the JTree/RRocks post. Most of this stuff isn't something covered in Geology 110 courses (geology for physical science majors). You'd learn this stuff in Minerology and Petrology courses - upper level shiztnitz.
|
|
up2top
Big Wall climber
Phoenix, AZ
|
|
|
Mar 10, 2005 - 02:18pm PT
|
Cool. And here I thought it was all just granite. What about kniess (or is it kneiss)?
Ed
|
|
Edge
Trad climber
New Durham, NH
|
|
|
Mar 10, 2005 - 02:29pm PT
|
Ed, it's "gneiss."
Me likey the gneiss....
|
|
Donny... the OHHH!- Riginal
Sport climber
Boald'r Effin See Oh
|
|
|
Mar 10, 2005 - 02:33pm PT
|
Great Nature....how 'bout some NON-geological rock types?
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 10, 2005 - 02:36pm PT
|
Ed - the prerequisite for this seminar was geology 110. In case you were out climbing the day they covered metamorphic rocks here's a url to get you caught up.
http://www.rocks-rock.com/gneiss.html
yeah... but what's up with gneiss? I've never climbed in an area where the attraction was the climbs on gneiss. You see it in the bands above the igneous rocks - like when hiking up to the Queen Mountain area in JTree. Where are areas where you climb on the stuff?
I'm sorry Donny, you'll have to enroll in "Busting Donny's moms' chop's 101" to get the graph to differentiate crack-cocaine and crystal meth.
|
|
jfk
climber
Santa Barbara, CA
|
|
|
Mar 10, 2005 - 02:51pm PT
|
nature,
if you could cite the refs for those papers that would be cool.
thanks,
john
|
|
Edge
Trad climber
New Durham, NH
|
|
|
Mar 10, 2005 - 02:52pm PT
|
Nature,
Some of the "gneissest" climbing in the country is in Western Mass at Farley, Great Barrington, and Happy Valley/the Hideaway. These are primarily bouldering destinations, although the cliffline at Farley is awesome as well. The stuff out here is awesome to climb on.
A close relative is schist, which is what you find at Rumney. Very sharp layers of stone, with some amazing features. I don't have a pic, but there are a few places where you see the strata twisted all pretzel-like. Truly amazing.
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 10, 2005 - 03:24pm PT
|
Yosemite ref is:
California Geology December 1991
Geology of The Joshua Tree National Monument - California Geology, April 1984
|
|
Donny... the OHHH!- Riginal
Sport climber
Boald'r Effin See Oh
|
|
|
Mar 10, 2005 - 03:28pm PT
|
I was hopin' you'd come up with somethin' like Buddy Holly....and maybe Whitesnake.
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 10, 2005 - 05:22pm PT
|
I'm going to correct myself before someone else does. This will also add to the confusion. I found the paper on JTree Geology. I took a good hard look at the geological map.
Here's how they classified the rock types in various areas we all know:
Hidden Valley Campground, Wonderland or Rocks, Jumbo, Ryan all get mapped as:
Kwt - Cretaceous White Tank monzodiorite.
Split rock is mapped as:
Kgp Cretaceous Gold Park diorite
Queen Mountian is mapped as:
Kqm - Cretaceous Queen Mountian monzogranite
Around Indian Cove it's mosttly Kwt but there are pockets of Jtp - Jurassic Twntynine Palms porhyritic quartz monzonite.
Looks like there is very little Quartz Monzonite and no monzonite (edited that part out above) in JTree. Still, the idea is the same.
Geology after Dibblee, 1967 and 1968, Rogers 1961.
|
|
jfk
climber
Santa Barbara, CA
|
|
|
Mar 10, 2005 - 05:57pm PT
|
thanks,
should be fun to hit up the library before i hit the road for a little post-climb fire-side education.
john
|
|
up2top
Big Wall climber
Phoenix, AZ
|
|
|
Mar 10, 2005 - 06:08pm PT
|
Mt. Lemmon is all gneiss.
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 10, 2005 - 06:36pm PT
|
Thanks Ed. So it is gneiss. In fact it's 1.5 Billion year old Catalina gneiss. I've never climbed there. Tragedy really but the nothern part of this state is so loaded I rarely find a reason to drive the four hours to get there. Will have to check it out for sure now.
|
|
Lambone
Ice climber
Ashland, Or
|
|
|
Mar 10, 2005 - 10:50pm PT
|
this all seems like Geology 101 to me...???
|
|
troutboy
Trad climber
Newark, DE
|
|
|
Mar 11, 2005 - 11:32am PT
|
"Geology after Dibble, 1967 and 1968, Rogers 1961."
Just guessing (I don't have the cited pub) but would this be Dibblee, not Dibble ???
TS
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 11, 2005 - 11:33am PT
|
Where did they give you the ternary diagagram and monzonite in 101?
The typical ingneous rock series taught in your first geo course is:
Granite, Diorite, Gabbro, Peridotite for the Intrusive rocks
Rhyolite, Andesite, Basalt for the extrusive.
Additionally you might also have to identify Obsidian, Tuff (or Breccia Tuff), Pumice and Scoria in your lab for the intrusives. I'd be surprised if Monzodiorite is lab test question in any into geo course.
Petrology
troutboy - yup, thanks for the catch. i fixed it
|
|
Josh Higgins
Trad climber
San Diego
|
|
|
Mar 11, 2005 - 08:18pm PT
|
I heard a touron in J-Tree say, "See all the sand on the ground. This is all sandstone." That was classic. Great post.
|
|
Haggis
Trad climber
Scotland
|
|
|
Mar 12, 2005 - 01:52pm PT
|
Ok
Ignore the ternary diagram for the moment and consider what you are doing to form these rocks.
Quartz Monzonite and Granites are evolved rocks. They start from a similar silca saturated magma sitting in two magma chambers, the quartz Monzonite system has more K, Ca, Na, Al and Fe etc in the melt than the granite system.
Ok start to cool the systems down.
Step one: both systems form Olivine then Plag’ and mag’ then Augite in this order as you cool. These sink to the bottom on the chamber and/or partly erupted out.
Hence the magma in both chambers has been reduced in K, Ca, Na, Al and Fe etc
Step two. Since your liquid line of descent is a Calcalkiline setting (has H2O in the Magma) therefore the next suit of minerals to for are Amphiboles and Biotites in that order hence further reducing the K, Ca, Na, Al and Fe etc: SiO2 Ratio. You plag compositions move down from Ca rich to Na rich
So after you have done all this what are you left with?
A lot of SiO2 which forms qtz, and some K which forms feldspars and a bit of Biotites that got left behind thus you have a granite. HOWEVER your quartz Monzonite system started with a great volume of K, Ca, Na, Al and Fe etc relative to silica. So there is more of this k, Ca, Na, Al and Fe etc kicking around that forms a lager quaintly of K feldspars and Na plag’.
Thus
Due to the starting composition of the magma. There is more aulimius material in the melt which remains through evolution and gets used up in the end when it all freezes in K feldspars and Plag.
So what is granite.
It is course grained evoloved silica rich igneous rock
What is quartz monoztie
It’s a course grained evolved peralikaline granite or Alkali rhyolite in the UK.
The only difference is the amount of K, Ca, Na, Al and Fe etc at the start.
Rob
|
|
nature
climber
Flagstaff, AZ
|
|
|
Topic Author's Reply - Mar 14, 2005 - 09:22pm PT
|
dingus... i'm taking that one on. I'm doing what any good geologist worth his weight in tuff would do - i'm doing some research/reading. I'm really stoked on the last two posts (dmt and Haggis) but need to give both some thought.
right now I'm leaning towards thinking that your/our understanding of the intrusive systems and how they should work are simply off-base.
Garlock faulting and some weirdness (whacko rotation) going on south breaks that model (basin and range extension) down easily. More complexity is going on towards the south than is easily explained from a tectonic perspective than the north.
Heck, explain this one. We're on the leading edge of a massive subduction zone. Why the Basin and Range (extensional)? Where did the compression go? (I bet DMT has a pretty good idea)
|
|
bobh
climber
Bishop, California
|
|
|
Mar 14, 2005 - 09:55pm PT
|
Nature wrote:
"In Joshua Tree your classification might be:
Quartz: 15%
Feldspars: 85% (70% plag, 30% k-spar)
This classifies the rock as a Monzogranite"
I don't see monzogranite anywhere on your ternary diagram.
|
|
|
SuperTopo on the Web
|
Let us know!