Weathering a Erosion

Earth’s Energy Sources

  • Types of Energy Sources:

    • Internal Energy:

    • Radioactive Decay: Breakdown of radioactive isotopes within the Earth creates heat, driving convection currents within the mantle.

    • External Energy:

    • Solar Radiation: Energy from the sun heats the Earth unevenly, influencing climate and weather patterns.

  • Interaction of Energy Sources:

    • The landscape is shaped by endogenic (internal) and exogenic (external) processes:

    • Endogenic Processes:

      • Tectonics

      • Volcanism

      • Metamorphism

    • Exogenic Processes:

      • Weathering

      • Mass wasting

      • Erosion and deposition

Resisting Framework

  • Influences on Landscape Effects:

    • Lithology: Different mineralogy, texture, and strength of rocks respond variably to driving forces affecting landscapes.

    • Structural Features:

    • Spacing of fractures,

    • Location of faults,

    • Juxtaposition of different rock types plays a critical role in rock strength and landscape formation.

Weathering

  • Definition of Weathering:

    • Weathering is the process that breaks down rocks into smaller components.

    • Types of Weathering:

      • Physical (Mechanical) Weathering

      • Chemical Weathering

Physical / Mechanical Weathering

  • Definition: Physical weathering breaks rocks down into smaller pieces, thereby increasing surface area for a given volume of rock.

Mechanisms of Physical Weathering

  • Jointing:

    • Characteristics: Regularly spaced fractures that open without shearing.

    • Formation Depth: Typically occurs at shallow levels (less than a few km).

    • Causes: Caused by tectonic stresses, fluid pressure, or unloading during uplift and erosion.

    • Occurrence: Joints are common features on Earth's surface.

  • Columnar Joints:

    • Formation occurs in cooling lava flows or thick sills due to contraction during solidification, creating hexagonal prisms perpendicular to the surface.

  • Exfoliation Joints:

    • Formed in mountainous regions where concentric slabs “peel off” due to the removal of overlying land masses (e.g., from glacial ice).

  • Spalling:

    • Resulting from differential thermal expansion and contraction, often seen in boulders after intense forest fires.

  • Frost Wedging:

    • Importance: A major mechanism of physical weathering.

    • Density of Water: 1.0 g/cm³;

    • Density of Ice: 0.92 g/cm³;

    • Mechanism: Freezing water expands by approximately 8%, forcing joints apart.

  • Biological Weathering:

    • Enhances physical and chemical weathering through:

    • Root Wedging: Tree roots growing into rock joints.

    • Burrowing: Digging into soil or rock, creating pathways.

    • Bioerosion: Organisms, such as certain bivalves, bore into softer rocks for protection.

    • Acid Secretion: Microorganisms secrete acids that break down minerals for nutrient absorption.

Chemical Weathering

  • Definition: Transformation of rocks and minerals at or near the Earth’s surface, involving the breakdown of minerals into their basic components, including formation of new, more stable minerals.

Mechanisms of Chemical Weathering

  • Rate Dependencies: The rate of chemical weathering is influenced by surface area:

    • Physical weathering increases total surface area, subsequently escalating chemical weathering rates.

    • Chemical weathering can cause pitting, further enhancing surface area for reactions.

  • Carbonic Acid Formation:

    • Reaction: Water reacts with atmospheric CO2 to form carbonic acid:

    • extH2extO+extCO2<br>ightarrowextH2extCO3<br>ightarrowextH++ext(HCO3−)extext{H}_2 ext{O} + ext{CO}_2 <br>ightarrow ext{H}_2 ext{CO}_3 <br>ightarrow ext{H}^+ + ext{(HCO}_3^-) ext{ }

    • This natural process makes rainwater slightly acidic, which is essential for “pre-weathering” processes.

  • Dissolution Weathering:

    • Definition: Chemical weathering where minerals dissolve into ions.

    • Common Susceptible Minerals:

    • Halite: extNaCl<br>ightarrowextNa++extCl−ext{NaCl} <br>ightarrow ext{Na}^+ + ext{Cl}^-

    • Gypsum: extCaSO4∙2extH2extO<br>ightarrowextCa2++extSO42−+2extH2extOext{CaSO}_4 \bullet 2 ext{H}_2 ext{O} <br>ightarrow ext{Ca}^{2+} + ext{SO}_4^{2-} + 2 ext{H}_2 ext{O}

    • Calcite: extCaCO3+extH2extCO3<br>ightarrowextCa2++2extHCO3−ext{CaCO}_3 + ext{H}_2 ext{CO}_3 <br>ightarrow ext{Ca}^{2+} + 2 ext{HCO}_3^ -

  • Hydrolysis:

    • Process where water reacts with a mineral producing a new mineral.

    • Example: Weathering of feldspar leading to clay minerals:

    • 2extNaAlSi3extO8+2extH2extCO3+9extH2extO<br>ightarrow2extNa++2ext(HCO3−)+extH4extSiO4+extAl2extSi2extO5(OH)42 ext{NaAlSi}_3 ext{O}_8 + 2 ext{H}_2 ext{CO}_3 + 9 ext{H}_2 ext{O} <br>ightarrow 2 ext{Na}^+ + 2 ext{(HCO}_3^ - ) + ext{H}_4 ext{SiO}_4 + ext{Al}_2 ext{Si}_2 ext{O}_5(OH)_4

  • Oxidation:

    • Addition of oxygen (and typically water) leading to mineral breakdown.

    • Example: Oxidation of pyrite, resulting in acid mine drainage:

    • 4extFeS2+15extO2+8extH2extO<br>ightarrow2extFe2extO3+8extH2extSO44 ext{FeS}_2 + 15 ext{O}_2 + 8 ext{H}_2 ext{O} <br>ightarrow 2 ext{Fe}_2 ext{O}_3 + 8 ext{H}_2 ext{SO}_4

  • General Chemical Weathering Formula:

    • extPrimarymineral+extacids+extoxygen<br>ightarrowextSecondaryminerals+extdissolvedionsext{Primary mineral} + ext{acids} + ext{oxygen} <br>ightarrow ext{Secondary minerals} + ext{dissolved ions}

Weathering Rates

  • Controlling Factors:

    • Mineralogy: Different minerals weather at different rates.

    • Climate: Temperature and precipitation significantly influence weathering types and rates.

    • Structure: The presence of joints and fractures directly affects surface area available for weathering.

    • Topography: Influences potential for erosion and exposure.

  • Weathering Rate Hierarchy:

    • Slowest to Fastest Weathering Rates:

    • Iron oxides (hematite)

    • Aluminum hydroxides (gibbsite)

    • Quartz

    • Clay minerals

    • Muscovite mica

    • Potassium feldspar (orthoclase)

    • Biotite mica

    • Sodium-rich feldspar (albite)

    • Amphiboles

    • Pyroxene

    • Calcium-rich feldspar (anorthite)

    • Olivine

    • Calcite

    • Halite

Climate Impact on Weathering

  • Key Points:

    • Higher temperatures generally increase rates of chemical weathering.

    • Increased precipitation favors higher rates of chemical weathering.

    • Cooler temperatures favor mechanical weathering methods like frost wedging.

    • Hot and dry environments limit weathering.

  • Climate and Rock Type Influence:

    • The relationship between mean annual temperature and rainfall further dictates the weathering process.

    • Variability in environmental conditions leads to different weathering styles and material interactions.

Soil

  • Definition of Soil: The uppermost layer of land surface, made up of mineral debris, organic matter, rainwater, and gases.

  • Factors Influencing Soil Development:

    • Organisms

    • Climate

    • Time

    • Parent Material

    • Slope and Aspect

Soil Development Processes

  1. Weathering: Physical and chemical processes create loose material and ions in solution.

  2. Water Percolation: Redistribution of ions and fine clay particles.

  3. Organic Interactions: Decomposition alters organic materials.

Importance of Soils

  • Ecosystem Services provided by Soils:

    • Provision of food, fiber, and fuel.

    • Habitat for diverse organisms.

    • Regulation of flood events.

    • Source of pharmaceuticals and genetic resources.

    • Foundation for human infrastructure and construction materials.

    • Cultural heritage significance.

    • Carbon sequestration potential.

    • Purification of water and reduction of soil contaminants.

    • Regulation of climate and cycles of nutrients.

Soil Texture

  • Refers to the proportions of sand, silt, and clay present in the soil.

Soil Horizons

  • Definition: Distinctive layers within soils.

  • Factors Defining Horizons:

    • Texture

    • Chemical Composition

    • Organic Content

    • Structural Characteristics

  • Types of Soil Horizons:

    • O Horizon: Accommodates organic material; generally undecomposed.

    • A Horizon: Topmost, mineral-rich, fertile soil (topsoil).

    • E Horizon: Marks eluviation, light in color due to leaching of clays and organic materials.

    • B Horizon: Accumulates materials leached from A and E horizons; may contain clay and organic material.

    • C Horizon: Comprises parent material, minimally developed soil.

Erosion

  • Definition of Erosion: The process of transporting soil, rock fragments, or dissolved ions away from their origin.

  • Agents of Erosion:

    • Wind

    • Flowing Water

    • Moving Ice

Erosion Characteristics

  • Each transport agent can only carry certain sizes and components based on the energy associated with the agent.

Weathering, Erosion, and Sedimentary Rock Formation

  • Importance of Weathering and Erosion:

    • Fundamental processes for sedimentary rock formation.

    • Weathering creates sediments and dissolved ions essential for sedimentary material.

    • Erosion acts to transport and modify these components, aiding the interpretation of past environments from the sedimentary rock record.