Weathering – Physical, Chemical, Biological


Weathering - Physical and Chemical

Weathering is a geological process that naturally breaks down rocks and minerals at or near the Earth’s surface. It occurs over time scales ranging from years to millennia. Weathering plays a pivotal role in shaping the Earth’s landscapes and influencing the cycling of nutrients and elements. This process differs from erosion, which involves the physical removal and transport of material by agents such as water, wind, or ice.

Types of Weathering

There are two broad types of weathering: physical (or mechanical) and chemical weathering. Sometimes biological weathering is considered a separate type, but more often it gets included in the two main categories.

Examples of Physical and Chemical Weathering

Arches National Park is a result of salt and frost weathering (physical weathering). The rounded hills of the Appalachians result largely from chemical weathering processes.

Physical Weathering

Also known as mechanical weathering, physical weathering involves the breakdown of rocks and minerals into smaller pieces without changing their chemical composition. Various environmental factors drive this process, including temperature fluctuations, pressure changes, and biological activity.

Frost Weathering

Frost weathering or freeze-thaw weathering occurs in regions with temperature fluctuations around the freezing point. Water enters cracks in rocks, freezes, and expands, exerting pressure that enlarges the cracks and eventually fragments the rock.

Thermal Stress Weathering

Thermal stress weathering results from extreme temperature changes. This is common in desert environments. Rocks expand when heated and contract when cooled. Repeated cycles cause stress and eventual fracturing.

Pressure Release or Unloading

Pressure release weathering happens when overlying rock layers erode away, reducing the pressure on underlying rocks. Lowering the pressure makes the rock expand and fracture parallel to the surface. Granite domes are an example of a feature that results from this process.

Salt Weathering

Salt weathering is common in arid and coastal regions. Salty water evaporates the salt crystallizes in rock pores and cracks. As the salt crystals grow, they exert pressure on the rock and break it. Note that while sodium chloride is a common salt, other chemical compositions are important, too.

Biological Contributions

Plants and animals also contribute to physical weathering. Roots growing into rock crevices exert pressure and cause mechanical fracturing. Burrowing animals move rocks and aid in their breakdown.

Chemical Weathering

Chemical weathering involves the chemical alteration of minerals within rocks, forming new minerals and soluble salts. This type of weathering contributes to soil formation.

Dissolution

Dissolution is the process where minerals dissolve in water. Carbonate rocks like limestone are particularly susceptible, leading to karst landscapes with features like sinkholes and caves.

Hydrolysis

Hydrolysis is the reaction of minerals with water, forming new minerals and releasing ions. This process is essential in breaking feldspar minerals into clay.

Carbonation

Carbonation occurs when carbon dioxide dissolves in water and forms weak carbonic acid. The acid reacts with minerals like calcium carbonate in rocks and breaks them up.

Oxidation

Oxidation involves the reaction between minerals and oxygen. It commonly occurs as rusting in iron-rich rocks. This process weakens rocks and changes their composition.

Hydration

Hydration is the absorption of water into the crystal structure of minerals. This makes them expand and weaken. This is distinct from simple wetting.

Biological Contributions

Plants, fungi, and bacteria contribute to chemical weathering through the production of organic acids which enhance mineral breakdown. Bird droppings and bat guano and the chemicals released by lichens also cause chemical weathering.

Factors Influencing Weathering Rates

Several factors influence the rate of weathering:

  • Climate: Temperature and precipitation patterns heavily influence both physical and chemical weathering processes.
  • Rock Type: Different rock types have varying susceptibilities to weathering.
  • Topography: Slope and aspect affect moisture retention and exposure to environmental forces, influencing weathering rates.
  • Vegetation: Plant roots and organic acid production accelerate both physical and chemical weathering.
  • Time: The duration of exposure to weathering processes affects the extent of rock degradation.

Where Do the Types of Weathering Occur?

Physical weathering is prevalent in arid and cold climates. For example, thermal stress and frost weathering are dominant processes in deserts and high mountain regions. Chemical weathering is more significant in warm, humid climates. For example, abundant rainfall and high temperatures accelerate chemical reactions in tropical rainforests.

Weathering of Manmade Structures

Similar to natural geological processes, the weathering of buildings, statues, and other manmade structures involves both physical and chemical mechanisms. Human activities and environmental pollution often accelerate these processes.

Physical Weathering of Human-Made Structures

Physical weathering in human-made structures occurs due to various factors:

  • Thermal Expansion and Contraction: Materials expand when heated and contract when cooled. This causes cracking and spalling in materials like concrete and brick.
  • Frost Weathering: Water entering cracks and pores in materials freezes and expand, similar to frost weathering in natural rocks. This deteriorates masonry and stonework in cold climates.
  • Salt Crystallization: Salt accumulates in the pores of materials in coastal areas or where de-icing salts are used. Crystallization of these salts exerts pressure and causes disintegration of the material.
  • Mechanical Erosion: Wind-driven sand and other particles erode the surfaces of buildings and statues, gradually wearing them down or smoothing their features.

Chemical Weathering of Human-Made Structures

Chemical weathering also plays a significant role in the degradation of human-made structures:

  • Acid Rain: Pollutants like sulfur dioxide and nitrogen oxides in the atmosphere combine with water vapor to form weak acids. When acid rain falls on buildings and statues, it causes significant chemical weathering through dissolution and surface erosion. Limestone and marble are particularly susceptible to acid rain.
  • Oxidation: Metal components of structures, such as iron reinforcements or bronze statues, are susceptible to oxidation (rusting). Oxidation is especially common in humid environments. It weakens structural integrity and causes aesthetic damage.
  • Carbonation: Carbonation of concrete is where calcium hydroxide reacts with carbon dioxide to form calcium carbonate. This leads to a reduction in pH and corrodes steel reinforcement, compromising structural integrity.
  • Pollution: Urban environments often have higher concentrations of corrosive pollutants, which accelerate the chemical degradation of building materials.

Biological Weathering

Biological factors also contribute to the weathering of human-made structures:

  • Microbial and Algal Growth: Microorganisms and algae grow on surfaces, especially in damp conditions. This causes discoloration and potential physical damage. Chemical damage also occurs when organisms produces acidic compounds.
  • Root Damage: Roots from nearby vegetation grow into cracks and crevices. This causes physical disruption and sometimes introduces moisture and organic acids.

Prevention and Conservation

Various strategies mitigate weathering of human-made structures:

  • Material Selection and Design: Choosing weather-resistant materials and designing structures to minimize water retention and thermal stress reduces weathering.
  • Protective Coatings: Applying water repellents, anti-graffiti coatings, or corrosion inhibitors protect against weathering processes.
  • Regular Maintenance: Regular cleaning, repairing cracks, repointing masonry, and removing vegetation help prevent or slow down weathering.
  • Environmental Controls: Reducing pollution or acid rain reduces the impacts on structures.

References

  • Blatt, Harvey; Tracy, Robert J. (1996). Petrology : Igneous, Sedimentary, and Metamorphic (2nd ed.). New York: W.H. Freeman. ISBN 0716724383.
  • Fry, E. Jennie (1927). “The Mechanical Action of Crustaceous Lichens on Substrata of Shale, Schist, Gneiss, Limestone, and Obsidian”. Annals of Botany. os-41 (3): 437–460. doi:10.1093/oxfordjournals.aob.a090084
  • Hall, Kevin (1999). “The role of thermal stress fatigue in the breakdown of rock in cold regions”. Geomorphology. 31 (1–4): 47–63. doi:10.1016/S0169-555X(99)00072-0
  • Murton, J. B.; Peterson, R.; Ozouf, J.-C. (2006). “Bedrock Fracture by Ice Segregation in Cold Regions”. Science. 314 (5802): 1127–1129. doi:10.1126/science.1132127
  • Zambell, C.B.; Adams, J.M.; Gorring, M.L.; Schwartzman, D.W. (2012). “Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux”. Chemical Geology. 291: 166–174. doi:10.1016/j.chemgeo.2011.10.009