Chris Blackerby

Columbia - Geology 110

January 15'2020

Imagine that you are hiking and stop to look at an outcrop of rock with light-colored, large crystals. Provide the complete history behind that rock, including being at the Earth’s surface. If you wanted a dark-colored, large crystal rock for your kitchen countertop, would you expect to find it deeper beneath the outcrop? Yes or No. Defend your reasoning. An initial post and at least two replies are required. 

According to Dr. Lorence G. Collins, with the Department of Geological Sciences at California State University, Granite is a kind of igneous rock, found on Earth but nowhere else in the Solar System and is usually found in the continental plates of the Earth's crust. Granite is dense and can be cut, carved and shaped. It is resistant to water and pollution. It is formed from hot, molten magma. Its color ranges from Brown to Black, according to the proportions of its minerals (Collins). it forms under the surface of the Earth, where it is forced upwards by tectonic movement. The magma cools and turns slowly into solid stone. As the magma cools, these minerals form crystals (Hobart M. King). Granite has several possible origins, depending upon the processes that operate on the rock systems. Some granites form (1) by magmatic processes, depending upon crystal settling and the order of crystallization of minerals from a magma (2) by melting of sedimentary rocks whose chemical composition is the same as that in granite, (3) by partial melting of rocks in which the first minerals to melt have the composition of granite, and, finally, (4) by chemical replacement processes (Collins). The mineral composition of granite consists of a comparatively coarse-grained igneous rock, it is not a pure substance but is a mixture of several different silicate minerals and oxides. It commonly consists of about one-third quartz, one-third potassium feldspar, one-third plagioclase feldspar, minor amounts of iron- and magnesium bearing biotite (black mica), and traces of various accessory minerals, including zircon. In addition to biotite, other varieties of granite may contain small amounts of other iron- and magnesium-bearing silicates or muscovite mica, but biotite granite is the most common variety. In all granites, however, quartz and feldspars are the dominant mineral species, making the rock white, light cream, or pink, but speckled with one or more of the dark iron-bearing minerals (Collins). The overall color of granite depends largely on the kind of feldspar in the rock. Potassium-rich feldspar tends to be some shade of red or pinkish-tan, so a lot of granite is similarly red or pink. But if the rock has a lot of sodium-rich feldspar in it, which is typically white or gray in color, the granite will be gray (Study). I would think that to find a darker color of granite, one would find it deeper than an outcropping. 

Work Cited

Collins, Lorence G. Equal Time For The Origin Of Granite - A Miracle. January 1998. http://www.csun.edu/~vcgeo005/miracle.htm. 14 January 2020. Reply to Thread Filter by: | Clear filters Show: Threaded Hobart M. King, Ph.d., RPG. Geology.com. 2020. https://geology.com/rocks/granite.shtml. Study. Study.com. 2005-2020. https://study.com/academy/lesson/what-isgranite-definition-colors-quiz.html.