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GEOLOGY STUDY GUIDE 2: Chapters 4, 5, 6, 7, 8 Chapter 4 – Igneous Rocks Some key terms or ideas: Magma- Body of molten rock at depth, including dissolved gases and crystals Lava- Magma that reaches Earth’s surface Intrusive- Igneous rock that formed beneath Earth’s surface Extrusive- Igneous activity that occurs at Earth’s surface Plutonic- Igneous rocks that form at depth Volcanic- Pertaining to activities, structures, or rock types of a volcano Granite- Intrusive, felsic, phaneritic igneous rock made of light-colored silicates Rhyolite- Extrusive equivalent of granite, felsic Andesite- Extrusive, intermediate, medium-gray, fine-grained Diorite- Intrusive, intermediate, plutonic equivalent of andesite, porphyritic Basalt- Extrusive, mafic Gabbro- Intrusive equivalent of baslat, mafic Components of magma- Liquid portion = melt, solid portion = silicate minerals already crystallized from melt, volatiles = gases Igneous textures- Factors that contribute: cooling rate, silica content, and amount of dissolved gas Grain size, crystalline vs. glassy, vesicles Aphanitic- Crystals are too small for individual minerals to be distinguished w/out a microscope (rock cooled quickly) Phaneritic- Crystals are roughly equal in size and large enough for minerals to be identified w/out a microscope (rock cooled slowly) Porphyritic- Rock has two crystal sizes, small (groundmass) and large (phenocrysts) Vesicular- Holes due to trapped CO2 gas Scoriaceous- VERY vesicular (pumice) Relationship between rate of cooling, silica, gas content, and igneous texture Lower temp = higher viscosity, more silica = higher viscosity, low volatiles = higher viscosity Mafic- Magnesium and iron, considered silica deficient, 45% silica, dark in color Intermediate- Between felsic and mafic, Felsic- 65% or more silica, tend to be light in color Bowen’s reaction series- In magma, which minerals crystallize first (most mafic crystallize first) Fractional crystallization- Some minerals crystallize, are dense and sink to bottom of melt, first formed crystals are segregated from rest of cooling magma Assimilation- Incorporate surrounding rock Magma mixing- When one magma chamber intrudes on another Partial melting- How most igneous rocks melt; not all minerals melt at the same temperature; if their temperature is not reached, they will not, but other minerals surrounding them may = melt rich in felsic minerals Some key questions or concepts: Describe how igneous rocks form. Compare igneous rocks that form below Earth’s surface with those that form above Earth’s surface. Explain how cooling rate affects the resulting crystal size of minerals within igneous rocks.
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