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Geology Review 2 - Geology Review 2 Chapter 4 Igneous Rocks...

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Geology Review 2 Chapter 4- Igneous Rocks Key Terms Magma -a body of molten rock found at DEPTH, including any dissolved gases and crystals Lava -magma that reaches earth’s surface Intrusive -igneous rock formed below Earth’s surface Extrusive -igneous activity that occurs at earth’s surface Plutonic -igneous rocks that form at depth Volcanic -pertaining to the activities, structures, or rock types of a volcano Granite -an igneous rock composed almost entirely of light-colored silicates, Phaneritic and felsic Diorite -dark grey intermediate igeous rock, phaneritic Gabbro -dark, coarse grained rock, phaneritic and mafic Rhyolite - igneous, extrusive volcanic rock, aphanitic and felsic Andesite -intermediate, aphanitic Basalt - aphanitic, mafic Components of magma - 1. Liquid portion-melt, mostly silicon and oxygen 2. Solid component-silicate minerals that crystallized in the melt 3. Gaseous phase-volatiles, H2O and CO2, SO2 Igneous textures Aphanitic -igneous rocks that form at the surface, rapid cooling=fine-grained texture Phaneritic -igneous rocks that form below the surface, slow cooling=coarse-grained texture Porphyritic -large crystals imbedded in a matrix of smaller crystals Vesicular -voids left by gas bubbles that escape a lava as it solidifies Scoriaceous -vesicular texture, produced by basaltic magma Relationship between rate of cooling, silica, gas content, and igneous texture Amount of Silica o More Silica=slower cooling, (higher cooling temperature, lower melting point) o Silica component of melt becomes enriched as magma evolves Gas content o Water and gas can lower melting point, causes magma to rise o When gas evaporates can leave a vesicular texture Mafic - dark colored, contains substantial dark-silicate minerals and calcium-rich plagioclase feldspar Intermediate- 25% dark silicate minerals, other dominant mineral plagioclase feldspar Felsic - rock composed almost entirely of light-colored silicates Key Questions 1. Igneous rocks form by crystallization of magma. As temperature of the liquid drops, the ions pack closer and closer together as they slow their rate of movement. Silicon and oxygen molecules link together first. SO2 tetrehedra join with other ions to create an igneous rock. 2. Igneous rocks that form below earth’s surface exhibit a coarse-grained texture because more time is allowed for crystallization. Igneous rocks that form above earth’s surface exhibit a fine- grained structure because they crystallize very quickly. 3. Texture is the size, shape, and distribution of the particles that collectively constitute a rock. 4. Factors that contribute to the texture of an igneous rock include, rate of cooling, amount of silica, amount of dissolved gasses. 5. Common igneous rock textures include aphanitic, phaneritic, porphyritic, glassy (molten rock quenched very quickly), pyroclastic, and pegmatitic (exceptionally large crystals, form in fluid- rich environment.
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6. Igneous rocks are named by mineral content and cooling history. Rocks named for mineral content can be classified as felsic, intermediate, and mafic.
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