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Rocks Notes Igneous Rocks -All start as igneous rocks -Intrusive igneous rocks – rocks created when magma cools underground (plutons) -Extrusive igneous rocks – rocks created when magma cools on the surface (volcanoes, etc.) Igneous rock identification Composition: Color -Igneous rocks are classified based upon chemical composition, however most of the time color is a good indication of rock type and can be used for identification. -Color used to be used more widely for ID in what was called the Color Index Felsic –Light in color (White, light gray, tan or pink) -Rick in Silica (SiO2) Intermdiate – Intermdeiate color -Equal amount of light and dark minerals Mafic – Dark in color (Black to brown) -Poor in silica (SiO2) but rich in Fe and Mg Exceptions -Obsidian is felsic in composition but is dark black or brown in color -Dunite is felsic in composition but light green or greenish yellow in color Texture -Texture in igneous rocks does not refer to the roughness or smoothness, but the crystal size -Pegmatic – large crystals >2-3 cm -Phaneritic – crystals are large enough to see but smaller than 2 cm, entire rock is composed of crystals -Aphanitic – fine grained, crystals are too small to see -Porphorytic – made of crystals of 2 different sizes -Large crystals are called phenocrysts and smaller crystals are not seen and are called groundmass -Glassy – made entirely of glass, few if any crystals can be seen -Vesicular – lava solidifies before gas can escape leaving holes and bubbles in the rock -Pyroclastic – made of volcanic fragments (ash, lapilli or bombs) Sedimentary Rocks -Rocks that are composed of sediment -Sediment is deposited by wind or water -Classification is based on texture and make-up -Texture represents the origin or make-up of sedimentary rocks -Texture types 1) Clastic sedimentary rocks – contains clasts (small pieces of rocks or minerals) -Gravel-sized particles (>2mm) – conglomerate -Sand-sized particles (1/16 mm – 2 mm) – sandstone -Clay and silt-sized particles (<1/16 mm) – siltstone, shale 2) Chemical sedimentary rocks -Sedimentary rocks made of a particular mineral -4 important types 1) Quartz – white to clear, hardness = 7 (Hard) 2) Gypsum – clear, hardness = 2 (soft, scratched by fingernail) 3) Halite – clear, common table salt, hardness = 2.5 4) Calcite – hardness of 3 (reacts with HCl) 3) Biological sedimentary rocks -Formed of biological sediment or affected by biological activity -Ex. -Coal – made of decomposed plant material -Coquina – made of shell fragments (calcite) -Fossil limestone – limestone containing fossils Metamorphic Rocks -Rocks that have been changed by heat and or pressure through a process called metamorphism -Parent Rock – original rock before metamorphism -Types of Metamorphism 1) Regional metamorphism – when high temperature and pressure affect large regions of Earth’s crust. 2) Contact metamorphism – when molten rocks come into contact with a solid rock 3) Hydrothermal metamorphism – when very hot water reacts with rock to changes its chemistry and mineral structure -Common near igneous intrusions and volcanoes -Texture -Foliated – rocks with wavy layers or bands of minerals -Pressure during metamorphism causes minerals with flat or needle like crystals to arrange with their long axis perpendicular to the pressure pg. 136 -Nonfoliated – rocks without layers or bands of minerals