Earth - Portrait of a Planet - Sediments and Soils

Sediments and Soils

What is Sediment

  • Sediments are loose fragments of rocks, minerals, shells, shell fragments, or mineral crystals that precipitate out of water.

How Sediments are Produced

  • Sediments are produced from bedrock, soil, and unconsolidated sediment.

Physical Weathering

  • Clast Diameter:
    • Boulders: More than 256 millimeters.
    • Cobbles: Between 65 millimeters and 256 millimeters.
    • Pebbles: Between 3 millimeters and 64 millimeters.
    • Sand: Between 1/16 of a millimeter and 2 millimeters. Represented as (116)(\frac{1}{16}) millimeter to 2 millimeters.
    • Silt: Between 1/256 of a millimeter and 1/16 of a millimeter. Represented as (1256)(\frac{1}{256}) millimeter to (116)(\frac{1}{16}) millimeter.
    • Mud: Less than 1/256 of a millimeter. Represented as (1256)(\frac{1}{256}) millimeter.
Jointing
  • Vertical bedding joints in sandstone.
  • Exfoliation joints in granite.
Frost Wedging
  • A crack grows due to water freezing and expanding.
  • A block is lifted and pushed out.
Root Wedging
  • Tree growing in a joint.
  • Eventually, the blocks tumble to the base of the cliff.
Salt Wedging
  • Salt wedging in rock on a beach cliff in Scotland yields "honeycomb weathering".

Chemical Weathering

Dissolution
  • Water molecules (H2O)(H_2O) are polar.
  • Bonds hold ions together in grains on the crystal surface.
  • Dissolution causes grain surfaces to become pitted.
  • Water molecules hold ions in solution.
  • Water seeping into joints in limestone produced troughs.
Oxidation
  • Weathered pyrite crystals.

Rate of Chemical Weathering

  • Relative Stability of Minerals at the Earth's Surface (Fastest Weathering to Slowest Weathering):
    • Halite (Least Stable)
    • Calcite
    • Olivine
    • Ca-plagioclase
    • Pyroxene
    • Amphibole
    • Na-plagioclase
    • Biotite
    • Orthoclase (K-feldspar)
    • Muscovite
    • Clay (various types)
    • Quartz
    • Gibbsite (aluminum hydroxide)
    • Hematite (iron oxide) (Most Stable)

Physical and Chemical Weathering

  • Intact rock containing quartz, feldspar, and biotite.
  • Chemical weathering leads to minerals turning into clay.
  • More cracks result in more surface area for weathering.
  • Surface area examples:
    • Fewer cracks, less surface area - 6m26 m^2
    • Increased cracks, more surface area- 12m212 m^2
    • Most cracks, more surface area - 60m260 m^2
  • Transport of weathered materials.

Differential Weathering

  • Weathering attacks an edge on two sides.
  • Weathering attacks a corner on three sides.
  • Weathering attacks a face on one side.
  • Weak shale vs. strong sandstone.