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Unformatted text preview: Geol 105 Lab #5 Week of Feb. 13-17, 2012 Igneous Rocks: What Clues Do They Provide Us About Igneous Processes? STATION 1. -- CRYSTAL SIZE & TEXTURE The processes by which igneous rocks form (and the basis for classification of igneous rocks) is reflected by their texture and composition (mineralogy). Texture is the appearance of a rock resulting from the size, shape, and arrangement of the mineral grains or crystals. Textures give us clues about the cooling history of the magma/lava. 1. Arrange the rock samples (#33, 28, 51, 56) from "very coarse" texture to "very fine" texture. Use Fig. 5.6 and 5.7 of lab manual, to determine which rock samples have: (a) fine-grained (aphanitic) texture (b) coarse-grained (phaneritic) texture 2. Thinking about the arrangement of the rocks you made, how does the texture relate to the cooling history? (Hint - think about where and how rocks cool, how long this process takes, and look at the illustration on the graphic.) 3. Compare the textures of the welded tuff and granite rocks, using the magnification. In a few sentences, what difference do you notice? 1 of 9 STATION 2. -- TWO TEXTURES IN ONE ROCK: A HISTORY? 1. Rock samples #48 (2 rocks) do not fit either description of fine-grained (aphanitic) or coarse- grained (phaneritic) texture. Both of these samples are porphyritic texture (see photo page 120). Briefly describe the process of how these rocks may have formed (recall lecture notes). STATION 3. -- TRAYS -- IGNEOUS ROCK COMPOSITION You have a tray with the following 6 rocks: porphyritic andesite, basalt, rhyolite (2 samples), diorite, gabbro, granite (hint: use photos on pages 110-113 and read captions) 1. Using Fig. 5.2 as a reference, and the color graphic, arrange the 3 volcanic rocks in terms of increasing silica (SiO 2 ) composition and list in order here: 2. Pair the volcanic rocks with their plutonic equivalents and list the pairs here: 3. How did you distinguish the volcanic rocks from the plutonic rocks?...
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- Spring '08
- Igneous rocks, Basalt, Mantle