Weathering

• Weathering is a set of physical, chemical, and biological processes that alter the physical and chemical state of rocks and soil at or near the earth's surface.
• Effects of Weathering

  • Loss of atoms and molecules from weathered surfaces.

  • Addition of specific atoms to the weathered surface.

  • Breakdown of one mass into two or more masses.
• Chemical Weathering

  -Involves the alteration of the chemical

  composition of weathered material.
• There are different process that result in chemical weathering including:

  – Hydrolysis

  – Oxidation

  – Carbonation
• Hydrolysis

  • Chemical weathering is a process that involves the reaction between mineral ions and the ions of water (OH- and H+), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution through the release of the hydroxide ions.
• Silicate minerals weather by hydrolysis to form clay.
• Oxidation

  Chemical attachment of free oxygen to other elements and compounds.
• Iron-bearing silicate minerals that undergo oxidation include the following:

  – pyroxene

  – amphibole

  – biotite

  – olivine
• Rocks that contain these elements weather by oxidation into an orange color.
• Carbonation

  Dissolving of calcium carbonate (limestone) in acidic groundwater
• Carbonation

  • similar to hydrolysis but all the products are ionic and there is no residue
• Carbonation

  • produces bicarbonate (HCO3), a major part of the dissolved load of most rivers.
  • the carbonation of limestone results in karst topography: caves, sinkholes, etc.
• Carbonic acid in groundwater dissolves limestone.
• limestone dissolved from rocks above the caves is deposited as stalactites and stalagmites.
• Physical Weathering

  -Breakdown of mineral or rock material by entirely

  mechanical methods brought about a variety of reasons.
• Physical weathering examples:

  – Frost Wedging

  – Exfoliation or Unloading

  – Thermal Expansion

  – Plant Wedging
• Frost Wedging

  • Mechanical disintegration, splitting or break up of rock by the pressure of water freezing in cracks, crevices, pores, etc.
• Exfoliation or Unloading

  • Expansion of rock caused by lifting and erosion

  • Rock breaks off into sheets along joints that parallel surface.
• Thermal Expansion

  • Repeated cooling and heating of rock.
• • Heat caused rock to expand and cooling causes rock to contract
• Plant Wedging

  •Plant roots can wedge into cracks in rocks and produce enough pressure to split them
• Biological Weathering

  - Animals and Plants can assist in breaking down rocks into sediment and soil.