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Lab 2: Igneous/Metamorphic Rocks

Lab 2: Igneous/Metamorphic Rocks. GEOL- 103. Igneous Rocks. Form as molten rock cools and solidifies General characteristics of magma Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava Environments

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Lab 2: Igneous/Metamorphic Rocks

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  1. Lab 2: Igneous/Metamorphic Rocks GEOL- 103

  2. Igneous Rocks • Form as molten rock cools and solidifies • General characteristics of magma • Parent material of igneous rocks • Forms from partial melting of rocks • Magma at surface is called lava • Environments • Extrusive: rocks formed from lava (volcanic rocks) • Intrusive: rocks formed from magma (plutonic rocks) • Can also have both stages

  3. Igneous Textures • Used to describe overall appearance of rock based on: • Crystal size • Arrangement of minerals • Factors affecting crystal size • Rate of cooling • Slow rate: fewer, large crystals • Fast rate: abundant, small crystals • Very fast rate: no crystals • % of silica present • Dissolved gases

  4. Igneous Rock Textures • Aphanitic texture • Fine-grained • Rapid cooling rate • Microscopic crystals • Phaneritic texture • Coarse-grained • Slow cooling rate • Large, visible crystals

  5. Igneous Rock Textures • Porphyritic texture • Mineral forms at different depths and temperatures • Large crystals (phenocrysts) are embedded in matrix of smaller crystals (groundmass) • Glassy texture • No crystals • Very rapid cooling

  6. Igneous Rock Textures • Pyroclastic texture • Formed from collisions due to volcanic eruptions • Many different sized particles • Vesicular texture • Formed during volcanic eruptions • Contains holes formed by captured gases

  7. Mineral Compositions • Felsic • Light-colored silicate minerals (quartz, feldspar, muscovite) • High amounts of silica; high viscosity; high gas content • Common in continental crust • Mafic • Dark-colored silicate minerals (olivine, pyroxene, amphibole, biotite, plagioclase) • More dense than felsic rocks • Low amounts of silica; low viscosity • Common in oceanic crust and volcanic islands

  8. Mineral Compositions • Intermediate • Contain 25% or more dark silicate minerals • Associated with explosive volcanic activity • Mix of mafic and felsic compositions • Ultramafic • Rare composition common in mantle • High in magnesium and iron • Composed mostly of olivine (green tint)

  9. Bowen’s Reaction Series

  10. Metamorphic Rocks • Formed through process of metamorphism • Transition of one rock into another by increasing temperature and/or pressure conditions • Produced from: • Igneous rocks • Sedimentary rocks • Other metamorphic rocks • Progresses incrementally from low-grade to high-grade • During metamorphism, rock must remain essentially solid

  11. Metamorphic Settings • Contact metamorphism • Body of magma comes into contact with country rock • Chemical alterations from hot, ion-rich water • Localized event • Regional metamorphism • Occurs during large-scale regional mountain building events • Produces greatest volume of metamorphic rock • Rocks usually display zones of contact and/or hydrothermal metamorphism

  12. Metamorphic Agents • Heat • Most important agent • Recrystallization results in new, stable minerals • Heat sources: contact metamorphism and/or geothermal gradient (25°C/km) • Pressure • Increases with depth • Realigns mineral crystals

  13. Metamorphic Agents • Hydrothermalfluids • Water containing other volatile compounds • Enhances migration of ions to affect recrystallization of existing minerals • Fluid sources: pore spaces, fractures, and hydrated minerals • Parent rocks • Rock with same overall composition of metamorphic rock • Mineral make-up of parent rock influences degree of metamorphism that may occur

  14. Metamorphic Textures • Refers to size, shape, and arrangement of mineral grains • Foliation • Any planar arrangement of mineral grains or structural features within a rock • Formation • Rotation of platy and/or elongated minerals • Recrystallization of minerals in direction of preferred orientation • Changing shape of equidimensional grains into aligned, elongated shapes

  15. Metamorphic Textures • Types of foliation • Slaty: rocks can be easily split into thin, tabular sheets • Schistosity: platy minerals visible and exhibit a layered structure • Gneissic: distinctive banded appearance • Nonfoliation • Develop in low deformation environments • Composed of minerals with equidimensional crystals

  16. Samples Igneous Rocks • Basalt • Diorite • Gabbro • Granite (x2) • Obsidian • Peridotite • PorphyriticAndesite • Pumice • Rhyolite • Scoria • Volcanic Tuff Metamorphic Rocks • Anthracite • Gneiss • Marble • Phyllite • Quartzite • Schist (x2) • Slate

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