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)$</p></li><li><p><strong>Hydration</strong>: Addition of intact water molecules.</p></li></ul></li><li><p><strong>Acid-Induced Dissolution</strong>:</p><ul><li><p>Calcite reaction: <br>$CO2 (g) + H2O (l)
    ightleftharpoons H2CO3 (aq)

ightarrow CaCO_3 (s) + 2H^+ (aq)$<br>- Results in the formation of bicarbonate.</p><ul><li><p><strong>Hydrolysis Process</strong>:</p><ul><li><p>Breaking down of chemical (covalent) bonds by water via:<br>$2KAlSi3O8 (s) + CO2 (g) + 11H2O (l)
ightleftharpoons Si2Al2O5(OH)4 (s) + K^+ (aq) +
2HCO3^- (aq) + 4Si(OH)4 (aq)$$

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.