Weathering and Soil Formation

Mechanical vs. Chemical Weathering

  • Weathering is the breakdown of rocks into smaller particles.
  • Mechanical Weathering: Physical breakdown of rocks into smaller pieces without changing their chemical composition.
  • Chemical Weathering: Chemical alteration of rocks.

Key Vocabulary

  • Abrasion
  • Bedrock
  • Chemical Weathering
  • Erosion
  • Fertility
  • Humus
  • Mechanical Weathering
  • Oxidation
  • Sediment
  • Soil
  • Soil Horizon
  • Soil Profile
  • Uniformitarianism
  • Weathering

Agents of Weathering

  • Flowing water, rain, ice, glaciers, acid rain, wind, cold, heat, carbon dioxide, and oxygen break down rocks.
  • Small particles created by weathering are called sediment.
  • Erosion transports sediment to new locations.

Uniformitarianism

  • The principle that present-day natural processes can explain past geological changes.
  • "The present is the key to the past."

Factors affecting the rate of weathering

  • Rock Type: Rocks with minerals that dissolve easily or react readily with acids/oxygen weather faster.
  • Climate: Temperature and precipitation influence weathering rates. Warm, humid climates favor chemical weathering; cold climates favor mechanical weathering.

Mechanical Weathering Processes

  • Freezing and Thawing (Ice Wedging): Water expands when it freezes, cracking rocks.
  • Pressure Release: Rocks crack when pressure is reduced (e.g., glacier melting).
  • Plant Growth (Root Wedging): Roots grow into cracks, widening them.
  • Animal Activity: Burrowing animals bring rock particles to the surface.
  • Abrasion: Rocks rubbing against each other.

Chemical Weathering Processes

  • Dissolution: Minerals dissolve in carbonic acid.
  • Oxidation: Reaction of a substance with oxygen (e.g., rust).
  • Acid Rain: Accelerates weathering due to human-released gases.

Soil Formation

  • Soil is an accumulation of loose weathered material.
  • Soil is a mixture of rock particles, minerals, decayed organic matter (humus), water, and air.
  • Bedrock: Solid rock layer beneath the soil.
  • Humus: Decayed organic matter, rich in nutrients, enhances soil fertility.
  • Fertility: Ability of soil to support plant growth.

Soil Texture

  • Clay: Fine-grained, poor drainage.
  • Sand: Coarse, drains quickly, doesn't hold nutrients well.
  • Silt: Mid-range grain size, holds water moderately.
  • Loam: Ideal mixture of clay, silt, sand, and humus; good drainage and nutrient retention.

Soil Profile

  • Cross-section of soil layers (horizons).
  • Horizon O: Organic matter.
  • Horizon A: Topsoil, mineral-rich.
  • Horizon B: Subsoil, weathered bedrock.
  • Horizon C: Substratum, weathered rocks.

Rate of Soil Formation

  • Depends on climate, parent material (rocks and minerals), mineral composition, texture, and layering

Soil Classification

  • Soils classified into 12 orders based on climate, plant production, and soil composition.
  • Examples: Gelisols (frozen), Spodosols (sandy), Oxisols (weathered), Aridisols (dry), Mollisols (fertile).
  • Soil classification determines land use.

Organisms and Soil

  • Humus formation from plant and animal decay.
  • Decomposers (microbes, bacteria, fungi, earthworms) break down plant litter.
  • Earthworms and burrowing animals mix the soil.