3,3

• Sedimentology


  The study of sedimentary rocks and the processes by which they are formed, includes and is related to a large number of phenomena

  
• Paleochannels (now filled with sediment), like that of the Nile, are incised deeper than the current sea level
• General rule: river can't be deeper than the body into which it empties
• These types of prehistoric canyons, that cut deeper than the current sea level, and that is now filled up with sediment are called PALEOCHANNELS
• Evaporite minerals form on evaporation, and – as such – implies periods of drying up or substantial sea level lowering (of, in this instance, the Mediterranean
• Mud and fine sediment deposit when the sea level rises, flooding the seafloor with river sediment
• When the strait of Gibraltar opened, the Mediterranean could flow freely into the Atlantic Ocean
• Sediment is broadly defined as

  • Loose fragments of rocks or minerals broken off of bedrock (e.g sand and mud)
  • Mineral crystals precipitated directly out of water (e.g halite and gypsum)
  • Shells or shell fragments

  
• Sediments form through weathering, which is the physical and chemical breakdown of pre-existing rock at or near the Earth's surface
• Sediments form a surface veneer or cover from 0 km where bedrock outcrops, to 20 km where deep sediment-filled depressions called sedimentary basins have developed
• Interactions with water and organisms may, over time, modify sediment into soil which is essential for life
• Regolith


  Any loose material (sediment or soil) covering bedrock at Earth's surface
• Fresh rock contains the minerals and textures that it had when it formed

  
• At Earth's surface and near-surface, this is affected by the atmosphere and hydrosphere (both are supplying chemicals to interact with the rock), and living organisms that can alter Earth's materials
• Weathering


  The combination of processes that break up and corrode solid rock, and that may eventually transform it into loose debris or detritus

  
• Erosion


  The movement of weathered material
• Physical or mechanical weathering is the process whereby intact rock is broken into unconnected clasts (grains or chunks), but not employing chemical reactions

  
• Clasts classified by grain diameter

  • Boulders: > 256 mm
  • Cobbles: 65mm < grain diameter < 256mm
  • Pebbles: 3 mm < grain diameter < 64mm
  • Sand: 0.0625mm < grain diameter < 2mm
  • Silt: 0.0039mm < grain diam < 0,0625 mm
  • Mud: < 0.0039mm

  
• Phenomena contributing to physical weathering

  • Jointing
  • Frost wedging
  • Salt wedging
  • Root wedging
  • Thermal expansion
  • Animal attacks
• Jointing


  Natural cracks forming in rock due to decreasing overburden pressure and cooling
• Joints can be smooth and planar, to fairly irregular and jagged or broad curves
• Intervals of sedimentary rocks typically have sets of planar vertical joints which, because they intersect bedding, break rocks into rectangular blocks, that can collect at the base of a slope in a rock rubble deposit called talus

  
• Granite plutons undergo exfoliation whereby they split along joints oriented nearly parallel to the rock surface to create curving sheets - like the layers of an onion
• Wedging


  When plant roots, crystallised salt, or freezing water pushes the opposite sides of joints apart
• Thermal expansion results when intense heat bakes a rock, causing its outer layer to expand, and on cooling it contracts, resulting in sufficient forces to break the rock off in sheet-like scales

  
• Animal attack also results in physical weathering through burrowing creatures (e.g. earthworms, moles, gophers) that move rock fragments
• Chemical weathering or decomposition

  The chemical reactions that alter or destroy minerals when they come into contact with water solutions or air
• Types of chemical weathering

  • Dissolution – reaction with acid
  • Hydrolysis – reaction with water
  • Oxidation – reaction with oxygen
  • Hydration – interaction to water
• Minerals in water separate into ions that become surrounded by water molecules through the process of dissolution
• During hydrolysis, water reacts chemically with minerals, breaking them down to form other minerals
• Oxidation reaction results when an element loses electrons, commonly occurs when elements combine with oxygen
• Rusting is an example of the oxidation of iron
• Hydrolysis


  Reactions that slowly break down silicate minerals like amphibole, pyroxene, mica and olivine to create various types of clay
• Quartz


  Can undergo hydrolysis but typically at a very slow rate leaving the quartz grains to survive in most climates
• Orthoclase feldspar


  Weathered by hydrolysis to form kaolinite (clay)
• Oxidation


  Reaction that results when an element loses electrons, commonly occurs when elements combine with oxygen
• Oxidation


  • Rusting is an example of the oxidation of iron
• Oxidation of rock


  Iron-bearing minerals (e.g. biotite; pyrite) becoming a weak, rusty-brown mixture of various iron-oxide and iron-hydroxide minerals
• Pyrite (FeS2)


  A sulfide mineral that reacts with air to transform into iron oxide
• Hydration


  The absorption of water into the crystal structure of minerals, such as some clay types, causes the rock to expand and weaken the rock