Soils & Sediments

 

Sediments: broadly defined, consists of:

  • loose fragments of rocks or minerals broken off bedrock
  • mineral crystals the precipitate directly out of water
  • shells or shell fragments

 

They form a surface veneer, or cover, over bedrock. Also called regolith

 

 

 

This cover's thickness varies from zero where bedrock is exposed to several kilometers

 

Some sediments transform into soil.

 

 

 

 

 

Sediments are produced by weathering of preexisting rock. Weathering is the combination of processes that corrode and/pr break up solid rock into loose debris.

  • physical weathering
  • chemical weathering
    • dissolution
    • hydrolysis
      • water chemically reacts with minerals and breaks them down to form other minerals. For example, hydrolysis reactions transform feldspar and many other silicate minerals into clay.
    • oxidation
    • hydration
      • absorption of water into the crystal structure of minerals. Causes some minerals, such as certain types of clay, to swell.

 

 

 

 

Physical & Chemical Weathering

 

Physical and chemical weathering work together to:

Increase surface area (physical)
Dissolve minerals and cements (chemical)
Alter hard minerals into soft minerals (chemical)

 

 

Differential Weathering

 

 

Weathering occurs faster at edges and corners of a block than on a flat face.
 

 

 

 

 

 

 

Different rocks in an outcrop weather at different rates.
 

 

 

 

 

Chemical Weathering

Inscriptions on a granite headstone (left) last for centuries, but those on a marble headstone (right) may weather away in decades. These gravestones are in the same cemetery and are about the same age.
 

 

 

 

 

 

Soil Formation

 

 

Soils forms through:

  • debris production
  • interaction with water
  • interaction with organics
     

All of these produce unique layers, called horizons, that are seen in a soil profile.
 

 

 

 

 

 

 

 

 

 

 

 

 

Soil-Forming Factors

 

 

Weather & Climate:

 

 

 

 

 

 

 

Substrate composition:

Mineral composition and resistance to weathering are important.

 

 

 

 

 

Slope steepness and wetness:

  • Regolith easily washes from steeper slopes.
  • Wetter soils contain more organic material.
  • Flat soils tend to hold more moisture and develop thicker soils.
     

 

 

 

 

 

Time:

  • Soils require time to develop.
  • Younger soils tend to be thinner.
     

 

 

 

 

Example Soils

 

 

 

 

 

 

 

Soil destruction

            Dust Bowl                                        

 

 

 

Water erosion forming Badlands