NM

Soil Formation

Soil Formation

2.1 Rock Weathering

  • Definition of Weathering (Merriam-Webster):

    • The action of the weather conditions in altering the color, texture, composition, or form of exposed objects.

    • The physical disintegration and (biogeo)chemical decomposition of earth materials at or near the earth's surface.

Earth's Layers and Scale of Soil

  • Soil Forms:

    • Originates from Earth’s upper crust/lithosphere.

    • Scale:

    • Earth's core: 6378 km

    • Lithosphere: 40-280 km

    • Crust: Less than 100 km

    • Critical Zone: Less than 10 km

    • Soil: Usually less than 0.1 km; makes up less than 0.002% of Earth’s depth and 0.1% of Earth’s crust.

Types of Rocks

  • Igneous Rocks:

    • Formed from cooling of molten magma.

  • Sedimentary Rocks:

    • Formed through reconsolidation of sediments from deposition and cementing materials.

  • Metamorphic Rocks:

    • Formed when igneous or sedimentary rocks are altered under high heat and pressure.

Rock Cycle

  • Elements of the Rock Cycle:

    • Magma: Melting process.

    • Sediment: Unconsolidated particles or rock fragments.

    • Metamorphic Rock Formation: Transformation under heat and pressure.

    • Igneous Rock Formation: Cooling of magma; can be intrusive (coarse-grained, slower cooling) or extrusive (fine-grained, faster cooling).

    • Weathering: Breakdown of rocks into sediments; leads to the formation of sedimentary rocks through compaction and cementation.

Mafic vs. Felsic Minerals in Igneous Rocks

  • Minerals:

    • Mafic: Rich in Fe, Mg, and Ca (e.g., Gabbro, Basalt)

    • Felsic: Rich in Si (e.g., Granite, Rhyolite)

  • Crystallization:

    • Increasing Fe, Mg, and Ca indicates more mafic composition.

    • Increasing Si indicates more felsic composition.

Sedimentary Rocks

  • Coverage: Cover approximately 75% of the Earth's land surface.

  • Types:

    • Glacial, Conglomerate, Sandstone, Shale, Shelly Limestone, Chalk, Anthracite, Banded Iron.

Metamorphic Rocks

  • Examples:

    • Slate, Jadeite, Schist, Gneiss, Garnet, Amphibolite, Marble.

  • Formation: Through recrystallization caused by heat.

  • Types:

    • Marble is derived from limestone.

    • Eclogite formed by high-pressure conditions.

Weathering Processes

  • Types of Weathering:

    • Physical Weathering:

    • Breakdown of rocks into smaller particles through physical forces.

    • Agents:

      • Temperature: Causing expansion and contraction (exfoliation).

      • Water and Ice: Contributing through abrasion or freeze-thaw cycles.

      • Wind: Actions include abrasion and particle movement.

      • Biota: Involvement includes plant roots and animal activity.

    • Biogeochemical Weathering:

    • Alteration and/or breakdown of mineral compounds via chemical reactions.

Agents of Physical Weathering

  • Temperature Changes:

    • Expansion and contraction leading to exfoliation of rocks.

    • Example: Twain Harte Rock in Sierra Nevada exfoliates quickly.

  • Water and Ice:

    • Actions: abrasion, rolling, skipping, and freeze-thaw cycles.

  • Wind:

    • Scraping and scouring by wind-borne particles.

  • Biota:

    • Plant roots exert force, breaking down soil.

Particle Size Categories of Soils and Sediments

  • Soil Separates:

    • Sand: 0.05-2 mm

    • Silt: 0.002-0.05 mm

    • Clay: Less than 0.002 mm

  • Physical Weathering Impact: More physical weathering leads to an increase in silt and clay content.

Mineral Types

  • Primary Minerals: Formed during cooling of magma.

  • Secondary Minerals: Formed by the chemical breakdown of primary minerals.

Impact of Rock Type on Weathering

  • Sedimentary Rocks: Weather more rapidly than igneous rocks.

  • Metamorphic Rocks: Varied weathering rates dependent on composition.

Impact of Mineral Type on Weathering Rate

  • Felsic Minerals: Weather slower.

  • Mafic Minerals: Weather faster.

Chemical Weathering Processes

  • Types of Reactions:

    • Dissolution: Solubilization in water.

    • Calcium sulfate reaction:
      CaSO4 (s) + 2H2O (l)
      CaSO4 ullet 2H2O (s)

    • Hydration: Addition of intact water molecules.

  • Acid-Induced Dissolution:

    • Calcite reaction:
      CO2 (g) + H2O (l)
      ightleftharpoons H2CO3 (aq)

ightarrow CaCO_3 (s) + 2H^+ (aq)
- Results in the formation of bicarbonate.

  • Hydrolysis Process:

    • Breaking down of chemical (covalent) bonds by water via:
      2KAlSi3O8 (s) + CO2 (g) + 11H2O (l)
      ightleftharpoons Si2Al2O5(OH)4 (s) + K^+ (aq) +
      2HCO3^- (aq) + 4Si(OH)4 (aq)$$

  • Electron Transfer in Redox Reactions:

    • Example: iron and copper electron movement during reactions.

Chemical Weathering Reactions Overview

  • Types of Chemical Weathering Reactions:

    • Hydration: Adding water to the chemical structure.

    • Dissolution: Breaking of ionic bonds upon dissociation.

    • Hydrolysis: Breaking of covalent bonds due to water interaction.

    • Complexation: Insoluble ion or molecule surrounded by a soluble molecule.

Impact of Weathering on Minerals

  • Result of chemical weathering results in:

    • Mineral ions + / - precipitation form secondary minerals.

    • Transport of minerals to streams and oceans.

    • Leaching into groundwater.

Stability and Resistance of Minerals to Weathering

  • Primary Minerals (e.g., Quartz): Resistant to weathering.

  • Easily Weathered Minerals (e.g., Calcite): Very low stability.

Summary of Physical and Chemical Weathering Interactions

  • Both processes collaborate throughout the weathering process, actively contributing to soil formation.