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JuanDeFuca
Big Wall climber
Stoney Point
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Topic Author's Original Post - Nov 26, 2008 - 02:08pm PT
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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
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Norton
Social climber
the Wastelands
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Nov 26, 2008 - 02:15pm PT
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How old is the earth, Juan ? 6000 years?
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72hw
Trad climber
Hollyweird, CA
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Nov 26, 2008 - 02:18pm PT
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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...
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Minerals
Social climber
The Deli
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Nov 26, 2008 - 04:06pm PT
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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!
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UncleDoug
Social climber
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Nov 26, 2008 - 04:23pm PT
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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?
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Hardly Visible
climber
Port Angeles
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Nov 26, 2008 - 04:26pm PT
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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.
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Minerals
Social climber
The Deli
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Nov 26, 2008 - 05:02pm PT
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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!
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Mike Bolte
Trad climber
Planet Earth
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Nov 26, 2008 - 06:01pm PT
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Thanks Minerals!
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Gary
climber
Desolation Basin, Calif.
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Nov 26, 2008 - 06:04pm PT
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It's tuff to be gneiss to a schist.
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Ricardo Cabeza
climber
Meyers,CA
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Nov 26, 2008 - 06:06pm PT
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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.
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Mighty Hiker
Social climber
Vancouver, B.C.
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Nov 26, 2008 - 06:46pm PT
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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.
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Karl Baba
Trad climber
Yosemite, Ca
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Nov 26, 2008 - 08:45pm PT
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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
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Jaybro
Social climber
wuz real!
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Nov 26, 2008 - 09:17pm PT
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Quartz Monzonite? Cabeza?
This granite is old
precambrian.
While this sierran orogeny stuff, is just
batholith come lately.
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Minerals
Social climber
The Deli
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Nov 27, 2008 - 02:26pm PT
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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?
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apogee
climber
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Nov 27, 2008 - 03:15pm PT
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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?
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Greg Barnes
climber
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Nov 27, 2008 - 03:32pm PT
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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!
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Mighty Hiker
Social climber
Vancouver, B.C.
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Nov 27, 2008 - 04:52pm PT
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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.
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kpinwalla2
Social climber
WA
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Nov 27, 2008 - 06:26pm PT
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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....
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Minerals
Social climber
The Deli
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Nov 27, 2008 - 08:24pm PT
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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…
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Mighty Hiker
Social climber
Vancouver, B.C.
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Nov 27, 2008 - 08:30pm PT
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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?
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