granite - teaching hand specimen of typical granite
Granite is an intrusive igneous rock composed primarily of feldspar, quartz and mica. At least 35%of the feldspar is white to pink orthoclase, and quartz is at least 20% of the rock by volume. The mica in this granite is biotite. During the Cretaceous it intruded the Jurassic granodiorite of the Sierra Nevada batholith and forms a light colored mass on the Sierra Nevada eastern slope at Sand Canyon in Kern County, California. Its extrusive equivalents are rhyolite, pumice and tuff.
Granites range in color from white like this one to pink, with the orthoclase feldspar giving it the characteristic shade. The granodiorite of the batholith is much darker, with dacite its extrusive equivalent. True granites are not common. Elementary and secondary students would be correct in calling anything that looks like granite "granitic," and should recognize that an accurate identification is done by experienced geologists and by specialists in petrography. Granitic rocks are a large family and "granitic" is more than acceptable. Students in a geology course who can distinguish between quartz and potassium/sodium feldspars and who can determine the percentages of each in a hand specimen, should be able to say this is a "true" granite.
The photos show the granite boulder pile and a boulder exfoliating. The third outcrop photo shows a darker granodiorite boulder with a dike running through it, typical of the Sierra Nevada batholith. Notice how much darker it is than the granite. Granodiorite is intermediate in composition and color between granite and basalt, is the intrusive equivalent of dacite and close in composition to andesite, intermediate between granite and basalt. The photos were taken just to the north of the mouth of Sand Canyon, Kern County, CA. The access road constructed alongside the Los Angeles Aqueduct crosses the granite intrusion and exposes unweathered granite.
As an igneous rock, granite seems impervious to fluids, but even granite has some tiny porosity. Years ago, in the cemetary next to the Cornell campus in Ithaca, New York, a gardener used an old fashioned oil can to lubricate his mower. You could see where he habitually paced his can on top of a granite headstone. About a third of the way down the face of the marker, a spot of oil eventually emerged. It had worked its way down from the top, but who knows how many years it took.
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