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Sedimentary Rocks

Sedimentary Rocks. How do sedimentary rocks form?. They form when sediments, produced by weathering and erosion, become connected together. . Two types of weathering exist:. Physical weathering: pieces of rocks break off of bigger rocks (minerals are chemically unchanged)

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Sedimentary Rocks

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  1. Sedimentary Rocks

  2. How do sedimentary rocks form? • They form when sediments, produced by weathering and erosion, become connected together.

  3. Two types of weathering exist: • Physical weathering: pieces of rocks break off of bigger rocks (minerals are chemically unchanged) 2. Chemical weathering: minerals in a rock are dissolved or otherwise chemically changed (when “glue” dissolves, other minerals break away in chunks)

  4. There are 4 main agents of erosion: • Wind • Moving water • Gravity • Glaciers

  5. In the case of glaciers, materials are not well sorted. Large boulders, sand, mud, are all carried along and dumped.

  6. Sedimentary layers • Sediments generally settle into layers. The _____________ particles settle at the bottom of a depression (on land or sea) and the _____________ grains settle at the top.

  7. Some processes dump materials instead of layering them: • Glaciers • Landslides

  8. Lithification begins with Compaction

  9. After compaction comes cementation: • A new mineral (like calcite, CaCO3) grows between sediment grains as dissolved minerals precipitate out of groundwater • Or, existing mineral grains grow larger as more of the same minerals precipitate from groundwater and fill in around them.

  10. Cementation with a different mineral (iron oxide forms between quartz)

  11. Sedimentary rocks often form bedding • Graded bedding • Particle sizes are heavier and coarser at the bottom • Cross-bedding • Graded bedding turned sideways: sediment is carried across a surface and dumped onto the front face (Fig. 6.6 in textbook)

  12. Clastic sedimentary rocks:Formed from deposits of loose sediments • Coarse-grained • Conglomerate (rounded stones from beaches and rivers) • Breccia (angular stones that didn’t travel far from their source) • Medium-grained • Sandstone (made of sand ; presence indicates old stream and river channels; very porous) • Fine-grained • Shale (made of silt and clay particles)

  13. Examples of clastic rocks:

  14. More clastic sedimentary rocks:

  15. Chemical sedimentary rocks • Formed when minerals are dissolved during chemical weathering and carried into lakes and oceans. Water evaporates; dissolved minerals are left behind. • Evaporites: layers of chemical sedimentary rocks, found esp. in arid areas, oceans • Examples: Gypsum, halite (NaCl); limestone (CaCO3)

  16. Evaporites

  17. Organic rocks • Formed from once-living things • Examples = limestone; coal

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