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Unformatted text preview: Answers for Chapter 2: Classification and Nomenclature of Igneous Rocks. 1. What are the three principal categories of igneous rocks? What characterizes each? Plutonic (intrusive) rocks crystallize beneath the surface of the Earth. They tend to have coarser grain size as a result. Volcanic (extrusive) flow rocks crystallize at the surface of the Earth. They tend to have very fine grain size or glass as a result. Pyroclastic volcanics result from explosions, collapse, or spray and accumulate much like a sediment. They exhibit fragmental textures. Field expression also distinguishes each type, as will be described in Chapter 4. 2. What is the difference between aphanitic and fine-grained? Aphanitic means that the individual crystals are too fine to see with the naked eye. Fine-grained means that you can see the grains, but they are small (< 1 mm diameter). 3. How does a felsic mineral differ from a mafic mineral? Which minerals on Bowens Series are mafic? Which are felsic? Felsic generally refers to the light colored silicates (feldspars, quartz, feldspathoids, muscovite) whereas mafic refers to the darker ones (biotite, hornblende, pyroxene, olivine). Composition, however, has precedence (e.g. smoky quartz and dark feldspars are ones (biotite, hornblende, pyroxene, olivine)....
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- Fall '09