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Unformatted text preview: Rocks (cont.) Rocks Sedimentary Rocks Sedimentary Rocks Rocks formed when sediments are cemented or compacted once these sediments are deposited sediments Constitutes about 5% of the earth’s crust Many of these rocks also contain fossils Many of these rocks are important Many economically (energy resources) economically Classification of sedimentary rocks Classification Can Can be classified as 1) detrital and 2) chemical chemical Detrital sedimentary rocks Detrital Detrital: Detrital: Rocks formed mostly by mechanical weathering mechanical Grouped based on particle sizes: - Coarse (Example: conglomerate, breccia) - Medium (Example: sandstone) - Fine (Example: siltstone) - Very fine (Shale) Chemical Sedimentary Rocks Chemical Sedimentary Sedimentary rock that is formed from chemical sediments, which are precipitated from solution precipitated Formed from chemical weathering Examples: limestone, travertine, coal, Examples: coquina, rock gypsum, rock salt coquina, Coal Coal Made mostly of organic matter Formed from decayed plant structures that Formed had been chemically altered had Successive stages of coal formation formation Formation of peat Formation of lignite (soft, brown coal) Formation of bituminous coal (black, soft Formation coal) coal) Formation of anthracite coal (hard, black Formation coal) coal) Metamorphic rocks Metamorphic Rocks Rocks formed from metamorphism (transformation of one rock form to another) another) Metamorphism occurs when preexisting Metamorphism rock is subjected to temperatures and pressures unlike those in which it is formed formed Metamorphism Metamorphism Metamorphism Metamorphism progresses slowly from slight changes (low-grade metamorphism) to substantial changes (high grade metamorphism) metamorphism) Example: When shale (a sedimentary Example: rock) becomes compact, slate (a metamorphic rock) is formed metamorphic Metamorphism (cont.) Metamorphism Most Most metamorphism occurs in one or two settings: 1) thermal metamorphism and 2) regional metamorphism regional Thermal metamorphism Thermal Rock is intruded by a magma body Change is driven by a rise in temperature Change within the host rock within Regional metamorphism Regional Usually Usually occurs during mountain building mountain Great quantities of rock are subjected to Great direct pressures and high temperatures, resulting in large-scale deformation resulting Metamorphic Metamorphic rocks are a part of many mountain belts and continental interiors mountain Agents of Metamorphism Agents Heat Pressure Chemically active fluids Heat Heat provides the energy that drives chemical reactions that result in the recrystallization of existing minerals recrystallization Pressure as a metamorphic agent Pressure Pressure Pressure increases with increasing depth; the greater the depth, the greater the pressure pressure Pressure causes compact spaces to Pressure close, increasing the density close, Chemically active fluids Chemically Fluids Fluids that surround mineral grains act as catalysts to promote recrystallization by enhancing ion migration enhancing Metamorphic textures Metamorphic Foliated texture Nonfoliated texture Foliated textures Foliated Usually Usually gives the rock a layered appearance appearance Examples: slate, schist, gneiss Nonfoliated texture Nonfoliated Rocks Rocks which lack a foliated texture. lack Usually consist of a single mineral Usually Examples: marble, quartzite, anthracite Examples: coal coal ...
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- Fall '10