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Download 7. Classifying Igneous Rocks PPT
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Classifying Igneous Rocks Earth Science Standard 3c: Students know how to explain the properties of rocks based on the physical and chemical conditions in which they formed, including plate tectonic processes. • As magma cools and crystallizes minerals form in predictable patterns • Bowen discovered 2 main patterns: a) Right-branch – continuous, gradual change of mineral compositions in the feldspar group b) Left-branch – minerals rich in iron and magnesium change abruptly as the temperature of the magma decreases 1. Texture (Rate of Cooling) 2. Color (Mineral Composition) • Determined by rate of cooling where and how fast the lava or magma cooled 2 Locations of cooling: a) Intrusive (cooled inside Earth) b) Extrusive (cooled on Earth’s surface) a) Intrusive Igneous Rocks • Not exposed to weather since inside Earth • Very hot within Earth so cooling takes a long time! Slow Cooling • Large mineral grains (can see with human eye) • Hundreds to millions of years to cool • The larger the mineral grains the slower it cooled! • Coarse-grained Texture (phaneritic) Intrusive: Course-grained, interlocking b) Extrusive Igneous Rocks • Exposed to weather since form on top of Earth • Cooler on Earth’s surface so cools more quickly! • Very Fast Cooling = NO mineral grains & Glassy Texture • Hours to Days • Fast Cooling = Fine-grained Texture (mineral grains not seen with human eye (aphanitic) • Days to Weeks • Porphyritic = started cooling slowly and the cooling sped up • Some large crystals (phenocrysts)surrounded by a fine-grained texture Intrusive: Course-grained, interlocking Extrusive: Fine-grained, discrete crystals, often glassy • Determined by mineral composition 4 classifications: a) b) c) d) Felsic Mafic Intermediate Ultramafic a) b) c) d) Felsic = Light colored; white to pink; high silica Mafic = Dark colored; blackish-gray; low silica Intermediate = Gray; between felsic and mafic Ultramafic = black-green; very low silica • Felsic = continental crust • Mafic = oceanic crust • Intermediate = coastlines Classification of Igneous Rocks Felsic Intermediate Extrusive Obsidian Mafic Andsesite Basalt Granite Diorite Gabbro Texture Glassy (noncrystalline) Basaltic Glass Rhyolite Intrusive Pegmatite Ultramafic Finegrained Peridotite Dunite Coarsegrained Very Coarsegrained Formation of Igneous Rocks Pyroclasts Extrusive Intrusive Porphyry: partially crystalline Granite Quartz Orthoclase Granite Biotite Plagioclase Compositional Classification Granite Compositional Granodiorite Classification Granite Quartz Amphibole Plagioclase Granite Compositional Granodiorite Classification Diorite Granite Plagioclase Amphibole Granite Compositional Granodiorite Classification Diorite Granite Gabbro Plagioclase Pyroxene Granite Compositional Granodiorite Classification Diorite Granite Gabbro Pyroxene Olivine Peridotite Intrusive Igneous Rocks Granite Gabbro Extrusive Igneous Rocks Where did these rocks form? How can you tell? Rhyolite Color? Basalt
