Weathering

Weathering Study Guide

Properties of Water

  • Polarity of Water

    • Water is defined as a polar molecule because the oxygen atom pulls electrons more strongly than the hydrogen atoms, creating a distribution of partial charges:

    • Partial negative charge on oxygen (O)

    • Partial positive charge on hydrogen (H)

    • This polarity facilitates hydrogen bonding, which is the attraction of the positive hydrogen atom of one water molecule to the negative oxygen atom of another, resulting in water’s adhesive properties (often described as "sticky").

  • Bond Type

    • Water is composed of covalent bonds that hold the hydrogen and oxygen atoms together.

    • Although the molecule itself is neutral in charge, the uneven distribution of electrons makes it polar, leading to various unique physical behaviors of water.

  • Surface Tension

    • The hydrogen bonding at the water's surface leads to increased cohesion, giving water a characteristic “skin.”

    • This phenomenon allows small insects, for example, to walk on water, and explains why wet sand can form cohesive clumps.

    • Angle of Repose: Refers to the steepest slope that loose sediment can maintain before sliding.

  • Expansion When Frozen

    • Ice forms an open hexagonal lattice structure, which increases water's overall volume and decreases its density.

    • As a result, ice floats on water; specifically, liquid water reaches its maximum density at 4°C, allowing this unique behavior.

  • Specific Heat

    • Water has a high specific heat capacity, which means it can absorb a substantial amount of energy before it undergoes a temperature change.

    • This property plays a crucial role in moderating climate and stabilizing ecosystems, preventing extreme temperature fluctuations.

    • Oceans are significant in their ability to store heat energy and release it gradually, which helps regulate global temperatures.

Physical Weathering

  • Definition

    • Physical weathering involves the mechanical breakdown of rocks without altering their chemical composition, resulting in smaller rock fragments while maintaining their original material identity.

  • Frost Wedging

    • Water infiltrates cracks in rocks, freezes, and expands approximately 9%, which exerts force that eventually splits the rock apart.

    • This process is significantly effective in climates that experience frequent freeze-thaw cycles but is minimal in hot and dry regions.

  • Tree Roots

    • Roots of trees can grow into rock fractures, applying pressure that breaks the rock and can also lead to damage in structures.

  • Abrasion

    • Sediments can grind against one another through processes facilitated by rivers, wind, or waves.

    • The extent of transport time leads to smoother and rounder clast shapes, serving as an indicator of erosion and transport duration and distance.

  • Glaciers vs. Streams

    • Glaciers are capable of grinding the rock beneath them into a fine material referred to as “rock flour.”

    • Streams, on the other hand, abrade clasts by tumbling them, leading to a polishing and rounding effect.

  • Thermal Expansion/Contraction

    • In arid environments, the daily cycle of heating and cooling leads rocks to expand during the heat of the day and contract at night, which produces visible cracks over time.

  • Unloading/Exfoliation

    • Deep rocks that are under significant confining pressure can expand and fracture when the overlying materials are removed, giving rise to sheet-like fractures.

Chemical Weathering

  • Water Dissolving Minerals

    • The polar nature of water allows it to surround ions, disrupting ionic bonds; a good example being the dissolution of halite (table salt).

  • Surface Area Effect

    • Smaller rock particles possess a greater exposed surface area, which correlates with a faster rate of chemical weathering due to increased interaction with chemical agents.

  • Interaction of Physical and Chemical Weathering

    • Physical weathering results in increased surface area, which accelerates subsequent chemical weathering processes.

  • Carbonation

    • The reaction can be represented as:
      ext{CO}2 + H2O
      ightarrow ext{H}2 ext{CO}3

    • This reaction generates carbonic acid, which effectively dissolves calcite in limestone, leading to the formation of caves and sinkholes.

  • Acid Rain

    • Acid precipitation occurs primarily due to the presence of sulfur oxides (SO₂) and nitrogen oxides (NOx) in the atmosphere, leading to the formation of sulfuric and nitric acids.

  • Karst Topography

    • This landscape is characterized by the presence of caves, sinkholes, and disappearing streams, a result of extensive limestone dissolution.

  • Hydrolysis

    • The chemical reaction depicted here involves water reacting with silicate minerals (for instance, feldspar), leading to the formation of clay minerals.

  • Weathering Types

    • Dissolution: Minerals dissolve fully in water.

    • Carbonation: Acidic solutions dissolve carbonate minerals.

    • Hydrolysis: Silicates are converted into clay minerals.

    • Oxidation: Molecular oxygen reacts with iron-bearing minerals, resulting in rust.

  • Granite vs. Limestone

    • In granite, feldspar weathers into clay minerals, whereas quartz remains stable over time.

    • Conversely, limestone (which primarily consists of calcite) weathers quickly and is less stable than granite.

  • Iron Oxidation

    • This process requires oxygen and produces red and yellow rust.

    • Ancient geological formations usually lack this oxidation, implying that early Earth had minimal free oxygen.

  • Biological Weathering

    • Biological weathering involves physical breakage from plant roots as well as the production of organic acids from organisms that chemically weather minerals.

Controls on Weathering & Soils

  • Time

    • Prolonged exposure to weathering elements results in increased weathering rates and the development of thicker soil layers.

  • Climate

    • In colder and drier climates, physical weathering processes are more dominant (freeze-thaw cycles, etc.).

    • In contrast, warmer and wetter climates such as the tropics typically witness a prevalence of chemical weathering processes.

  • Rock Type

    • Quartz-rich rocks, like granite, weather more slowly, while softer minerals such as feldspar and olivine undergo weathering more rapidly.

  • Topography

    • Steep slopes generally lead to thinner soil profiles; conversely, flat or low-lying areas often develop thicker soil profiles.

  • Biological Activity

    • Root systems, burrowing organisms, and organic acid production all enhance both weathering processes and soil formation.

  • Soil Definition

    • Soil is composed of a mixture of:

    • ~45% minerals

    • ~5% organic matter

    • ~25% water

    • ~25% air

  • Soil Horizons

    • O Horizon: Organic layer rich in decomposed plant material.

    • A Horizon: Topsoil composed of organic matter mixed with minerals.

    • E Horizon: Leached zone, often light in color where minerals have been washed out.

    • B Horizon: Subsoil, characterized by the accumulation of minerals.

    • C Horizon: Weathered parent material that remains close to the bedrock.

    • R Horizon: Bedrock, the unweathered layer beneath all soil layers.

    • The thickness of these layers can vary according to climatic conditions.

  • Soil Texture

    • Soil texture is categorized by the proportions of sand, silt, and clay, which directly influence both water flow and fertility of the soil.

  • Soil Types

    • Loess: Comprised of windblown silt, known for its high fertility.

    • Laterites: Found in tropical regions; highly leached and typically exhibit poor fertility.

    • Pedalfers: Humid climate soils that accumulate significant aluminum and iron.

    • Pedocals: Characteristic of arid regions; contain significant calcium carbonate (CaCO₃).

    • Tundra Soils: Contain permafrost and are usually quite thin.

  • Soil Orders

    • There are 12 primary soil orders, categorized based on differences in organic matter content, development stages, volcanic ash presence, and other environmental factors.

Soil Erosion & Mitigation

  • Soil Value

    • Soil is critical for supporting food production and various ecosystems, and due to its slow formation process, it is nonrenewable on human timescales.

  • Main Stabilizer

    • The roots of vegetation play a vital role in holding soil in place, thus preventing erosion.

  • Erosion Causes

    • Key factors leading to soil erosion include:

    • Deforestation

    • Overgrazing by livestock

    • Poor agricultural practices

    • Urbanization and construction activities

    • Climate factors, specifically drought conditions.

  • Dust Bowl (1930s)

    • The convergence of prolonged drought, poor farming methods, and wind erosion resulted in catastrophic soil loss during this period.

  • Erosion Types

    • Sheet Erosion: Involves the removal of thin layers of soil across the surface.

    • Splash Erosion: Caused by the impact of raindrops displacing soil particles.

    • Rill Erosion: Occurs when small channels form in the soil due to concentrated water flow.

    • Gully Erosion: Characterized by larger, more pronounced channels formed by water runoff.

  • Providence Canyon, Georgia

    • Poor agricultural practices led to significant gully erosion in this region, illustrating the impact of human activities on soil erosion rates.

  • Mitigation Strategies

    • Effective soil erosion mitigation techniques include:

    • Introduction of cover crops

    • Implementing contour plowing

    • Constructing terraces on slopes

    • Establishing windbreaks to reduce wind speed

    • Enhancing drainage systems

    • Engaging in reforestation efforts.

  • Salinization

    • Results from the accumulation of salts in soils due to over-irrigation practices, which can be mitigated through improved drainage and flushing of the soil.

  • Contamination

    • Soil can become contaminated through agricultural chemicals, waste products, and heavy metals, which can be addressed through regulatory measures, cleanup efforts, and limiting pollutant introduction.

    • Contamination leads to reduced soil fertility and poses health risks to living organisms.