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Formation of Sedimentary Rocks

Formation of Sedimentary Rocks. Sedimentary rocks account for approximately 5% of the crust. The upper 16 km of Earth. Regardless of this low percentage, approximately 75% of all rocks exposed at the surface is sedimentary rock. Factors leading to the formation of sedimentary rocks include;.

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Formation of Sedimentary Rocks

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  1. Formation of Sedimentary Rocks

  2. Sedimentary rocks account for approximately 5% of the crust. The upper 16 km of Earth. Regardless of this low percentage, approximately 75% of all rocks exposed at the surface is sedimentary rock. Factors leading to the formation of sedimentary rocks include; Formation of Sedimentary Rocks • Erosion and weathering • Transportation • Deposition • Lithification Reference: Tarbuck and Lutgens Pages 41 - 42

  3. In reference to the Rock Cycle, the origin of sedimentary rocks begins with the processes of weathering and erosion. Pre-existing rocks can be broken down both mechanically or chemically to produce sediment. Mechanical Weathering happens when a material is broken into smaller pieces which still retain the characteristics of the original material. Example: freeze – thaw action. Water freezing cracks fragments of rock Erosion and Weathering

  4. Mechanical weathering • Frost wedging

  5. In reference to the Rock Cycle, the origin of sedimentary rocks begins with the processes of weathering and erosion. Pre-existing rocks can be broken down both mechanically or chemically to produce sediment. Chemical Weathering happens when a material is altered by chemical action and a new material is produced which is chemically different than the original material. Examples: Oxidation reactions - Iron rusting Decomposition of limestone (statues and headstones) Erosion and Weathering

  6. Most important processes: • dissolution • oxidation • hydrolysis Chemical weathering Chemical weathering: Processes that break rock components and internal structures of minerals, making new minerals WATER: most important agent

  7. Chemical weathering • Dissolution Dissolving minerals by a liquid agent (such as water) How? Many minerals are water-soluble example: Halite (salt) Adding acid (H+) increases corrosiveness example: carbon dioxide and rain

  8. Example Iron (Fe) + water (H2O) oxygen (O) combines with Fe to form hematite (Fe2O3) Chemical weathering • Oxidation Chemical reaction that causes loss of electrons Oxidation decomposes important minerals: mineral formula where olivine(Mg,Fe)2SiO4 Earth’s mantle pyroxene(Mg,Fe)SiO3 Earth’s mantle hornblendeCa2(Fe,Mg)5Si8O22(OH)2 Continental rocks

  9. Hydrolysis • Liberates potassium from feldspar - important plant nutrient Chemical weathering Decomposition of granite Granite (mainly quartz + potassium feldspar) + carbonic acid (water + carbon dioxide) • makes kaolinite - this left over mineral is clay • - very stable • - main inorganic part of soil • quartz - very stable, glassy appearance • - carried to sea: beach sand, sand dunes • - cements to form sandstone

  10. Rock corners are eroded more rapidly than sides (more surface area at corners) Chemical weathering • Alterations chemical weathering that causes physical changes to rocks

  11. Chemical weathering • Alterations “ spheroidal weathering ” T&L Figure 5.11

  12. Erosional agents (such as, water, wind, and ice,) influenced by gravity, transport the sediment to new locations where it is deposited. Sediment is broken down during the transport phase. This process is called Abrasion. Three main agents tend to transport the sediment, these being; 1) Water(rivers and groundwater) Wind Blown Sand 3) Wind Transportation of Sediment 2) Ice(glaciers)

  13. In dry areas of the world, sediment (sand and silt) can accumulate in areas called deserts. Eventually the sediment is deposited in lakes, river valleys, seas, and more frequently the oceans. Deposition of Sediment

  14. Following deposition, sediment may become lithified, which means turned to solid rock. Sediment is commonly lithified by two processes; 1) Compaction: when the sediment size is small (silt and clay), the weight from the layers of sediment above can compact the lower layers forming solid sedimentary rock. Shale Round Pebbles Conglomerate Lime (Calcite) Limestone Sand Compaction Sandstone Silt and Clay Shale Lithification of Sediment

  15. Following deposition, sediment may become lithified, which means turned to solid rock. Sediment is commonly lithified by two processes; 2) Cementation: Over long period of time mineral matter dissolved in groundwater pass through pore spaces and cement (stick) sediment particles together to form solid sedimentary rock. Cement Sediment Particles Water with dissolved minerals, (calcite or silica) pass through pore spaces Lithification of Sediment

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