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korntera


Oct 27, 2004, 1:58 PM
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Registered: May 13, 2004
Posts: 422

Geology  (North_America: United_States: Oregon: Willamette_Valley: Flagstone)
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I am taking geology 101 and we just did a few labs on different types of rock. The guidbook says that flagstone is plagioclase andesite, however in my labs the rock at flagstone looks and feels much more like ryolite or a welded ash tuff. Does can anybody confirm what type of rock this is? Andesite and ryolite have the same formation and cooling process, one has just a little more quartz in it than the other. Also, all the ryolite i have seen is greyish and the andesite is often much more black. If anybody knows more about geology than a 101 student please help me out and respond.
-Travis


iching


Dec 10, 2007, 10:04 PM
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Re: [korntera] Geology [In reply to]
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Travis,
I thought the same thing when I visually classified the rock at Flagstone.
I called a climber/geologist from the Eugene area and he varified it as and andesite with light feldspar plagioclase.
It would really take a thin section anaylsis. Definitely let me know what you find out.
This would be a great project for a geo-climber. A thin section analysis of climbing areas in western oregon. My hypothesis which is only touch, site, and a drill is that most of the rock climbing in western oregon is primarly on formations that are dacitic.
I'd love to know what others think.


Tree_wrangler


Dec 11, 2007, 9:18 AM
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Re: [korntera] Geology [In reply to]
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I can also verify that Flagstone is andesite. Andesite is usually light grey in the Oregon Cascades, and is the only stuff on the major peaks solid enough to climb.

Andesite, to the layperson (like me) is complex in that it is rhyolite-like.......In GEO101 (U of O) they presented the two rock-types as if they were one in the same, but apparently (of course) it's more complex than that.

The climbing geology of Western Oregon would have been a good research project for a student like 50 years ago, but the outcrops have all been identified by the USGS long ago. The information is all out there already. Greg Orton's books are highly accurate in regard to geology of the crags.

Rhyo-dacite is the very common "Old Cascades" geology. In some areas it is intermingled with Breccia outcrops (very cool, with pockets, but also has rare plants). As you move up the west-cascade slope, Andesites and Basalts outcrops dominate. Of course, Basalts are by far the most common rock-type in Oregon, so you'll find them everywhere. "Flood Basalts" are the only kind worth climbing (Skinners Butte Columns).

Tuff's aren't uncommon (Petrified pyroclastic flow) anywhere from Smith Rock and to the SW, and of course, the Callahans are made of a coarse grained sandstone.

If you were ever to make it into the SW oregon backcountry, you'd have granites, peridotite, and marbles all within walking distance of each other. Cool stuff!


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