Comprehensive Study Notes on Rock Weathering and Erosion

Definitions and Comparison of Weathering and Erosion

• Weathering: The break-up and decomposition of rocks in-situ, which means the rocks remain in their original place of origin. It involves the decay of rocks, soils, and minerals through direct contact with the Earth's atmosphere. A key distinction is that weathering does not involve the movement or transport of material.

• Erosion: The action of surface processes, such as wind or water flow, that displaces and moves solid materials—including soil, mud, rock, and other particles—from one location on the Earth's crust to another.

• Key Differences:

  • Movement: In weathering, the material remains in-situ. In erosion, movement of the eroded material occurs.

  • Types: Weathering is categorized into Physical, Chemical, and Biological. Erosion is categorized by its agents: Water, Ice, Wind, Gravity, and Thermal.

  • Process: Weathering causes rocks to break down; erosion (often via water) and transport move those sediments downhill to a different location.

Factors Affecting the Rate of Weathering

• Geology:

  • The type of rock is a primary determinant of weathering rates.

  • Structure: Rocks with existing cracks or joints experience increased rates of physical and chemical weathering because cracks allow water to penetrate the interior.

  • Hardness: Soft rocks are significantly more vulnerable to weathering forces than strong, resistant rocks.

  • Chemical Composition: Certain minerals react more readily. For example, limestones containing high amounts of calcium carbonate ($CaCO_3$) are highly susceptible to carbonation.

• Vegetation:

  • Promotion of Weathering: Areas with heavy vegetation experience rapid biological weathering. Roots can physically break rocks, and mosses can contain chemicals that accelerate chemical breakdown.

  • Inhibition of Weathering: Vegetation can insulate rocks from large temperature fluctuations, reducing the rate of physical weathering. It also intercepts rainwater, which can reduce certain types of chemical weathering.

  • Moisture Retention: Vegetation can hold rainwater in-situ, which increases the time rock is exposed to water, thereby increasing chemical weathering.

• Climate:

  • Temperature: High temperatures increase the rate of chemical reactions, accelerating chemical weathering.

  • Diurnal Range: Areas with high diurnal temperature ranges (the difference between daily maximum and minimum temperatures) see increases in physical weathering processes like exfoliation.

  • Precipitation: Wet areas experience more chemical weathering. The combination of warm and wet conditions promotes both chemical reactions and the growth of vegetation, leading to increased biological weathering.

• Relief:

  • Steep Slopes: These encourage fast surface run-off, which can slow chemical weathering because water does not stay stationary long enough to react. However, steep relief causes weathered debris to fall away quickly, exposing fresh rock surfaces to the elements.

  • Physical Weathering: Steepness can increase certain forms of physical weathering due to gravity-driven movement.

• Aspect:

  • This refers to the compass direction a slope faces. Aspect determines the amount of sunshine a slope receives.

  • Sun-facing slopes: These may have more vegetation, leading to higher biological weathering.

  • Shaded slopes: Slopes not facing the sun may have less vegetation, which can, in some contexts, increase the rates of chemical and certain physical weathering types depending on moisture retention.

• Humans:

  • Human activity influences weathering by adding chemicals to water courses (contributing to chemical weathering).

  • Deforestation or reforestation changes the biological and physical protective layers of the earth.

  • The introduction or removal of animals alters biological weathering patterns.

Categories of Weathering

• Physical (Mechanical) Weathering: The breakdown of rocks caused by physical processes resulting in a change in size or shape, but with no change in the rock's chemical composition.

• Chemical Weathering: The breakdown of rocks caused by a change in their chemical composition through reactions with water, oxygen, or acids.

• Biological Weathering: The breakdown of rock by flora (plants) and fauna (animals). This is sometimes considered a sub-category of physical weathering, but it includes chemical elements (e.g., plant acids).

Physical (Mechanical) Weathering Processes

Physical weathering primarily occurs through temperature changes and crystallization.

• Block Disintegration:

  • Occurs in well-jointed rocks such as granite.

  • Prevalent in areas with a great diurnal temperature range of $10^{\circ}C$ to $15^{\circ}C$ or more.

  • Effective on barren rocks without vegetation, such as in desert regions.

  • The rock splits along joints into large, rectangular-shaped blocks.

• Granular Disintegration:

  • Result of repeated heating and cooling of rocks composed of different minerals.

  • Mineral Response: Dark-colored minerals (e.g., mica in granite) absorb more heat and heat up faster. Light-colored minerals (e.g., quartz, feldspar) reflect light and heat up slower.

  • The alternate expansion and contraction of these different minerals cause the rock to break down into small, grain-like pieces.

• Exfoliation (Onion Weathering):

  • Occurs in regions with high diurnal temperature ranges.

  • During the day, the rock surface heats up and expands; at night, it cools and contracts.

  • These physical fluctuations cause the outer layers of the rock to weaken and peel off in thin sheets, similar to the layers of an onion.

• Freeze-Thaw (Frost Shattering):

  • Occurs in areas with moisture and temperatures that fluctuate above and below $0^{\circ}C$.

  • Process:

    1. Water collects in a rock crack.

    2. As water freezes, it expands by approximately $9\%$, exerting massive pressure on the surrounding rock.

    3. When the ice thaws, more water enters the now-widened crack.

    4. Repeated cycles of expansion and contraction eventually cause the rock to split.

• Salt Crystal Growth:

  • Common in hot, arid areas with high evaporation rates.

  • When water evaporates, it leaves behind salt crystals.

  • As these crystals grow, they expand and exert physical pressure on the rock structure. They can also attack the rock chemically.

Chemical Weathering Processes

• Carbonation (Solution):

  • Rainwater naturally contains dissolved carbon dioxide ($CO_2$), forming a weak carbonic acid ($H_2CO_3$).

  • This acid reacts with rocks like limestone ($CaCO_3$), slowly dissolving them.

  • The dissolved minerals are washed away in solution.

  • This process is most active in areas with standing rainwater and is less effective on steep slopes or in very dry climates.

• Hydrolysis:

  • Hydrogen in water reacts directly with minerals in the rock.

  • Instead of just dissolving the minerals, the water combines with them to form new compounds.

  • For example, feldspar in igneous rocks like granite can be converted into clay minerals.

• Hydration:

  • Certain rocks absorb water into their internal structure, causing the rock to swell and increase in size.

  • This swelling exerts physical pressure and simultaneously changes the chemical structure of the minerals.

• Oxidation:

  • Occurs when iron compounds within the rock react with oxygen.

  • This produce a reddish-brown coating (rust) on the surface of the rock, weakening it.

Biological Weathering Processes

• Plants and Trees:

  • Physical: Roots grow into small cracks and gaps. As they grow, they exert pressure, widening the cracks.

  • Chemical: When dead roots decompose, they release $CO_2$ into the soil, which can convert into carbonic acid. Additionally, some plants like mosses release chemicals that break down minerals.

• Microorganisms and Lichens:

  • Bacteria: Many microorganisms in the soil take nitrogen from the air and minerals (silica, phosphorous, calcium) from the rock for nutrition. Removing these minerals weakens the rock.

  • Lichens: These are symbiotic colonies of fungi and microscopic algae. The fungi produce chemicals to break down rock minerals, which the algae then use for nutrition.

• Animal Activity:

  • Surface Impact: Animals walking on rocks can disturb them, occasionally causing landslides that scrape or smooth surfaces.

  • Burrowing: Animals like badgers and moles break up rock underground or bring buried rock to the surface where it is exposed to other weathering agents.

  • Piddock Shell: A mollusc related to the clam that uses its shell to physically cut holes into rock to create a home.

• Human Activities:

  • Construction and quarrying break up large sections of rock.

  • Foot traffic over long periods creates friction, breaking off tiny particles and causing significant wear and tear on rock surfaces.

Weathering Landforms and Case Studies

• Limestone Pavements:

  • Occur in exposed limestone with no vegetation cover.

  • Rainwater causes carbonation and freeze-thaw weathering.

  • Clints: The high, exposed flat sections of the pavement.

  • Grykes: The deep cracks or fissures that separate the clints.

• Karsts:

  • Formed mainly by carbonation over millions of years.

  • Weaker areas of rock are completely weathered away, leaving behind the stronger sections.

  • Halong Bay, Vietnam: A UNESCO World Heritage site and a prime example of karst topography.

• Tropical vs. Temperate Weathering:

  • Weathering is more rapid in tropical areas due to:

    • Large amounts of rainfall (increasing chemical weathering).

    • Dense vegetation (increasing biological weathering).

    • Proximity to the equator, resulting in high temperatures that accelerate chemical reactions.

    • Specific high-altitude or desert tropical areas may also have high diurnal temperature ranges, promoting physical weathering.