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nature
climber
Flagstaff, AZ
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Topic Author's Reply - Mar 14, 2005 - 10:18pm PT
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yeah... neither do i. Guess that should be monzodiorite. I've made the correction. thanks for catching that.
(though hmm... I typed it twice. Either I wrote it wrong after reading the paper or? - i gonna have to look at the paper again tomorrow).
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WBraun
climber
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Mar 15, 2005 - 01:23am PT
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I love this theoritical side of geology, much more so than identifying the rocks we pick up on the ground.
Why is that Dingus?
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Haggis
Trad climber
Scotland
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Mar 15, 2005 - 10:36am PT
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Re: the rocks being older in the west thing:
Older rocks are representations of smaller degrees of meting therefore are saturated in elements that do not like being in rocks. Silica likes being in rocks and things like CaTs and dolomite (yes it can be produced by igneous processes) does not thus really low melt fractions (with some CO2) will produce ultabasic rocks like nephilinites and weirder.
Thus as age increases melting increases magma changes composition to more silica rich when melting increase producing more quartz.
this also accounts for the colour.
just a theroy
Rob
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nature
climber
Flagstaff, AZ
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Topic Author's Reply - Mar 15, 2005 - 11:26am PT
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"I love this theoritical side of geology, much more so than identifying the rocks we pick up on the ground"
I'm with DMT on that statement. Identifying rocks is basic geology. There isn't a ton of research science to do identifying rocks when compared to say the area of paleoclimate. Arm waving about peleoclimate shifts or plate rafting is fun stuff.
In the right circles, if you put for the "plate rafting" theory some folks will hit the roof. The debates will begin (as well as more beer consumption). I've seen it first hand. I was lucky enough at the time to not know enough to get involved (probably still don't when it comes to tectonic/ig-pet stuff). But I love to be in the middle of the paleoclimate discussion.
I should have masters in geology but I'm now beyond the term limitation on getting my thesis signed. My undergrad work was in NRPI (Natural Resources Planning and Interpretatioon) with emphasis in Interp (relating complex scientific ideas in laymans terms). My focus for my masters was soils geomorphology applied to paleoclimate studies in and around the Sierra. With folks like DMT participating I see no reason not to continue various Geo 419 threads.
thanks for the effort you put in to that post, DMT.
Haggis: "Silica likes being in rocks and things like CaTs and dolomite (yes it can be produced by igneous processes)" - please provide a citation for this statement regarding dolomite. Thanks.
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Minerals
Social climber
The Deli
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Mar 15, 2005 - 02:20pm PT
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Oh man, I don’t know where to begin. Lots of cool stuff here… my favorite… pluton emplacement… I guess I’ll start by getting some breakfast.
Rocks, rocks, rocks…
Hey, what happened to blueschist???
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Minerals
Social climber
The Deli
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Mar 15, 2005 - 06:42pm PT
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Intrusion, diking, magma mixing and mingling, jointing/fracturing, climbing.
How about this one?
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quickhiker
Trad climber
Seattle
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Mar 15, 2005 - 09:57pm PT
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Hmmm… a lot of cool stuff, and I always like the rocks…
A couple of points:
1) the map of Joshua Tree cannot be directly compared to the ternary diagram that looks like it was scanned out of Compton’s ‘Geology in the Field.’ This is because the names in the ternary classification shown in the original post postdate the 1961 map. To keep things extra confusing, some of the names were recycled- including quartz monzonite. The previous classification allows QM to have more quartz. Now it would be considered a granite or granodiorite based on modal assemblages.
2) Simple fractionation from a basaltic ‘parent’ magma to produce the granites of the sierras (or Cordillera Blanca, J Tree, K2, Cascades, etc…) isn’t really the concept in favor at this point (though, arguably, there are those that would still defend it as the holy grail and it still shows up in many 101 textbooks). In most cases the supposed ‘parent’ magma pointed to in the field has a different isotopic value than the granite, which is inconsistent with fractionation. Does fractionation happen? Of course. Is it the only game in town? No. Magma mixing, assimilation of host rocks deeper in the crust and other processes are important processes contributing to the compositional evolution of granites, etc. This stuff would show up in igneous petrology courses (or at least it should if your instructor is up to the times).
3) The age relations in the Sierras span a large chunk of time (40 million years between ellery lake and olmstead point alone). Part of this is likely due to changes in the angle at which the Farallon Plate was subducting below North America. Part of it is likely due to more local tectonics. Many of the sierran plutons were emplaced along transcurrent faults (mixed extension and strike-slip motion) so their locations are moderately controlled by fault geometries (there are some great examples of fault control on pluton emplacement in Death and Panamint Valleys, albeit in a different tectonic environment and 11-15 million years old). If you look at the age relationships in the Tuolumne Intrusive suite (TIS), there is a bullseye pattern, which is roughly commensurate with increasing silica and shift in Strontium isotopic values. This is a subject of ongoing research. These ‘nested’ plutonic complexes are common throughout the sierras and the world. Another good example is around Mt. Whitney, for those who venture beyond the Valley neighborhood. Regardless, the ages pulled from the TIS they span ~10 million years. Contact relationships indicate that most of the units were partially molten at the same time as their outer neighbor and that they were exchanging material. So, this provides a neat window into the timescales over which a magmatic system is ‘viable.’ Keep in mind that there were probably many generations of volcanic expression at the surface over those 10 my as most volcanic centers, per se, last about 0.5 to 1 million years (rough, of course).
4) Yep, you can find good quality sierra granites in Santa Cruz that were transported north along the san andreas system of faults. I say system because plate boundaries are very messy places and the SA fault has evolved considerably over its slightly more than 20 million years of activity.
I’ll stop there before I get too carried away. I’d be happy to answer any specific questions on granite. I spent enough years of my life studying it and writing about it that it’s nice to put some of it to work now that I’ve moved on to other ventures.
Oh yeah… Minerals… Is the lower picture from the upper part of Tenaya Canyon, maybe a half mile from the Sunrise Trail as the crow flies? Composite dike with basaltic andesite, surrounded by aplite surrounded by host granodiorite, with its own set of mafic inclusions. Pretty cool outcrops. I spent an afternoon documenting those a few years ago and have always thought it would be fun to map them out a bit more. Order of operations:1) crystallization of granodiorite to at least about 80% crystals/ 20% melt (or less) along with some mixing and stirring for the mafic inclusions; 2) intrusion of aplite dike, possibly related to the large aplite body further down Tenaya Canyon (making the inner gorge gorgeous); 3) basaltic andesite dike follows path of least resistance and follows mushy aplite. This partially remelts and partially disaggregates the aplite; 4)Basaltic andesite quenches preserving some pretty neat mixing and assimilation textures.
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Lambone
Ice climber
Ashland, Or
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Mar 15, 2005 - 11:17pm PT
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whoa...now there's someone who knows his rocks!
I look at those pics and see climbing lines, holds and friction smears...someone else see's "mafic intrusions"...nice!
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WBraun
climber
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Mar 15, 2005 - 11:32pm PT
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The one mind blower for me has always been that the continent of North America has been in diorite on El Cap. How does one explain that?, especially since it’s in such good detail.
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nature
climber
Flagstaff, AZ
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Topic Author's Reply - Mar 16, 2005 - 12:11am PT
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yes yes yes! minerals is here! oh man! and quickhiker too! thanks for sorta saving my ass from further making a fool. As stated, this area isn't my specialty so I'm so happy ya'll found this post and participated. yes! (though I will say I sorta hoped I'd bait BL into this :-)
quickhiker - yup it was from compton's. I should have cited that. Can you expand on the statement or provide another classification to defend " Joshua Tree cannot be directly compared to the ternary diagram"? Hey... what's your background/education? Obviously you know your stuff so I'm just wondering where you learned it and how far you took it. In fact, as I re-read your stuff it's clear it's packed and pretty thick!
quickhiker (QH) - do you know John Longshore?
QH - 20 million on the SA activiation. I've always had 40 M stuck in my head (subduction of the rift and eastern section of the Pacific plate thus activating the SA). Can you cite to correct/help my understanding?
Werner - it's shape is 100% random. The fractionation(?) to allow the shape I'm sure is best left to try to be explained by our experts that just joined this fun.
QH - STOP? NO! don't stop!
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golsen
Social climber
kennewick, wa
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Mar 16, 2005 - 12:13am PT
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Really cool stuff. Nice pics...I bagged geology in school to be an engineer and I need some of you guys to help with a question concerning basalt. Not looking for a dissertation here just a laymans explanation. Devils tower, smith rock gorge, Vantage in Washington is all what I call basalt (I am sure they are different types) but they all seem to form columns that are pentagon shaped. My understanding is that this was due to the crystal structure and how it cooled? Is this even remotely correct?
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Jaybro
Social climber
The West
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Mar 16, 2005 - 12:16am PT
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Dingus said it all. If I may combine some bites;
"I love this theoritical side of geology, much more so than identifying the rocks we pick up on the ground. - Professional geologists ... have to know it all and have to know how to find the oil, or .... "
I was drawn to geology, esp. paleontology, just for this detective-like peicing together of ideas and theories. I was briefly a well core anaylising oil pimp and it soured me on the profession. Now, as an amatuer, a climber and roadcut driver by-er , I have much more latitude, I find there is no limit to things to think about. I like being part of the john Mcphee school more than being a tool for pig oil.
I wonder about what made the topography the way it is. Why is the glue in Maple canyon strong enough to hold those river stones solidly enough that we can climb on them when most similar formations are choss?
I'm tempted to think of the black Diorite 'North America," inclusion on El cap as an anomolus coincidence, but, as Werner suggested, it's too perfect. Is it's shape, and that of the continent most of us here live on some sor tof naturally ( inevitably) recurring fractle pattern? If so, how does that effect a physical, metaphysical interpretation?
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WBraun
climber
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Mar 16, 2005 - 12:16am PT
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I ain’t buyin the ”random” shape of the North American continent in diorite on El Cap. It’s just too coincidental for me to say it’s random. It’s definitely a mind blower.
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nature
climber
Flagstaff, AZ
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Topic Author's Reply - Mar 16, 2005 - 12:17am PT
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remotely correct it is (and I'm pretty sure all the places you refer to are basalts). The jointing is also part an artifact of the cooling and degassing of the basalt.
The columnar jointing is also what suckers people into thinking Owens River Gorge is Basalt - it's not. It's the 763,000 year old (paleomag reversed) Bishop Tuff. My jaw stats flappin' when folks call it basalt. It's the youngest paleo-mag reversed rock unit that has been radiometrically dated. It's the big player in nailing down the timing of the Brunhes-Matuyama magnetic flip.
Werner - I sorta didn't think you'd bye the random part of that but oh well. You'll be hard pressed to find a better "explaination". Think of it this way. If you added 500 feet to the section of the wall - 500 feet thicker - would it look the same? I doubt it.
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WBraun
climber
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Mar 16, 2005 - 12:49am PT
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That means that this universe is just a random event that occurred for no apparent cause whatsoever. Why does the Earth move around the Sun? Is it for no apparent reason? Why does the grass grow at a specific temperature?
It cannot be for no apparent reason.
There is no such thing as randomness since there is a cause to this universe. If random things happen, then that means they have no cause.
Yes Dingus
The captain has a nose an eye and a heart. And it's in North America.....
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WBraun
climber
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Mar 16, 2005 - 12:53am PT
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Yes
And where comes the source of beauty
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WBraun
climber
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Mar 16, 2005 - 01:05am PT
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Ok, very nice, you are happy toulomne*. I hope you get some nice outside time soon.
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quickhiker
Trad climber
Seattle
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Mar 16, 2005 - 01:16am PT
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Columnar jointing is a cooling phenomenon, rather than being related to crystal structure. The scaling is different. The columns result from fracture intersections at 120 degrees (times three is 360). Do that a few times and propagate them, and you get a column. The 120 comes from energy minimization as the material shrinks during cooling. Since cooling is along a gradient (cooler at the surface, for example), the shrinking is the greatest where coldest and thus there is a gradient in stress. Therefore, it isn't too big a stretch to see that the cracks are perpendicular to the cooling front and propagate with time/cooling into the rock. The regularity of the columns is a function of cooling rate and order in the cooling front (planar or curvy). If you look at examples of lavas cooled against glaciers, you get some pretty neat patterns.
As to how far I went in school... let's just say my family is glad to see me finally have a job.
As to north america wall... the mafic material is actually a tabular body in three dimensions that obliquely intersects the east face of el cap. give it a few meters of weathering and it won't look like NA anymore. The human mind is designed to look for patterns... take a look around whatever space you are in and see how many things you can see that look like human faces. Does this make anything less beautiful or worthy of respect? I would hope quite the contrary.
I may have spoken out of context... the rocks at J Tree are perfectly classifiable using the IUGS ternary diagram. No Problemo. The classifications from the 1961 map are apples and oragnes in terms of names. Similar vintage rock classifications include terms like 'alaskite', which would simply be a high K (and Al) granite in the IUGS scheme. Purely semantic jargon crap, but kinda useful if you go back into some of the older literature (there is some great stuff from the 1930s-50s on the sierras).
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Lambone
Ice climber
Ashland, Or
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Mar 16, 2005 - 01:22am PT
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"I ain’t buyin the ”random” shape of the North American continent in diorite on El Cap. It’s just too coincidental for me to say it’s random. It’s definitely a mind blower."
I concur.
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quickhiker
Trad climber
Seattle
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Mar 16, 2005 - 01:32am PT
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Anyways, I'm definitely stopping for a few days while I'm in the field, in the sun, and away from the 'puter.
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