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• Earth's surface is constantly changing
• Rock is disintegrated and decomposed, moved to lower elevations by gravity, and carried away by water, wind, or ice
• External processes


  Weathering, mass wasting, and erosion that occur at or near Earth's surface and are powered by energy from the Sun
• Internal processes


  Processes that derive their energy from Earth's interior, such as mountain building and volcanic activity
• Weathering, mass wasting, and erosion are responsible for transforming solid rock into sediment
• Weathering is a basic part of the rock cycle and a key process in the Earth system
• Weathering contributes to the formation of some of Earth's most spectacular scenery and causes the deterioration of many human-made structures
• Mechanical weathering


  Physical forces that break rock into smaller and smaller pieces without changing the rock's mineral composition
• Chemical weathering


  Chemical transformation of rock into one or more new compounds
• Weathering is the response of Earth materials to a changing environment
• Mechanical weathering processes

  1. Frost wedging

  2. Salt crystal growth

  3. Expansion resulting from unloading

  4. Biological activity
• Erosional agents such as wind, glacial ice, rivers, and waves disintegrate rock debris as they move it
• Mechanical weathering


  Increases the surface area available for chemical weathering
• Frost wedging occurs when water freezes in cracks and expands, exerting a force great enough to break rock
• Salt crystal growth in cracks and pore spaces can also split rocks as the crystals grow larger
• Sheeting


  The process where concentric slabs of igneous rock, particularly granite, break loose due to the reduction in pressure when overlying rock is eroded away
• Joints are fractures in rock that allow water to penetrate deeply and start the process of weathering
• Biological activity, such as plant roots growing into fractures and burrowing animals, contributes to both mechanical and chemical weathering
• Deeply buried igneous pluton

  • Uplift and erosion of overlying rock
  • Exfoliation dome
• Biological Activity


  1. Plant roots grow into fractures, wedging rock apart

  2. Burrowing animals break down rock by moving fresh material to the surface

  3. Plant roots, fungi, and lichens produce acids that promote decomposition

  4. Bacteria extract compounds from minerals and use the energy to supply their life needs
• Mechanical weathering increases the surface area of a rock, which influences chemical weathering
• Water can cause mechanical weathering by freezing in cracks and fractures, expanding and wedging the rock apart
• Exfoliation dome formation

  1. Uplift

  2. Expansion and sheeting
• Joints promote weathering by allowing water to penetrate and start the weathering process long before the rock is exposed
• Biological activity contributes to both mechanical and chemical weathering
• Water


  The most important agent of chemical weathering
• Carbonic acid


  Formed when carbon dioxide dissolves in water, a weak acid that decomposes many rocks
• Calcite dissolution in carbonic acid

  CaCO3 + (H + HCO3) → Ca2+ + 2HCO3-
• Potassium feldspar weathering in carbonic acid

  2 KAlSi3O8 + 2(H + HCO3-) + H2O → Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2
• Quartz is the only common mineral very resistant to both mechanical and chemical weathering
• Weathered products of common silicate minerals

  • Sodium, calcium, potassium, and magnesium ions forming soluble products
  • Iron combining with oxygen to produce insoluble iron oxides
  • Aluminum, silicon, and oxygen joining with water to produce residual clay minerals
• Spheroidal weathering


  • Corners and edges of angular rock masses are attacked more readily, causing them to become more rounded
  • Successive shells may loosen and break off as weathering penetrates deeper into the rock
• Carbonic acid is formed when carbon dioxide dissolves in water, both from the atmosphere and from decaying organic matter in the soil
• When carbonic acid reacts with limestone (calcium carbonate), the insoluble calcium carbonate is transformed into soluble products
• When carbonic acid reacts with potassium feldspar, the products are a clay mineral, potassium ions, bicarbonate ions, and silica
• The rounded boulders in Figure 5.10A formed through spheroidal weathering, where the corners and edges are attacked more readily, causing the rock to take on a more spherical shape
• Factors influencing weathering rates

  • Rock characteristics (mineral composition, solubility, physical features like joints)
  • Climate (temperature, moisture, vegetation)
• Granite headstones are more resistant to chemical weathering than marble headstones, which readily dissolve
• Silicate minerals weather in the same order as their order of crystallization in Bowen's reaction series, with olivine being the least resistant and quartz the most resistant
• Optimal environment for chemical weathering is warm temperatures and abundant moisture, whereas in polar regions it is ineffective due to lack of moisture, and in arid regions there is insufficient moisture