WEATHERING AND EROSION

Weathering and Erosion

• Chapter 8 Pg 234

Earth’s External Processes

• Weathering: Disintegration and decomposition of material at or near the surface.

• Erosion: Incorporation and transport of material by agents (water, wind, ice).

• Mass Wasting: Transfer of rock material downslope under gravity; occurs rapidly.

Types of Weathering

Mechanical Weathering

• Involves breaking rocks into smaller pieces.

• Processes of Mechanical Weathering:

  • Frost Wedging: Freeze-thaw cycles causing rocks to break apart.

  • Expansion and Contraction: Temperature changes leading to rock fracture.

  • Salt Growth: Crystallization of salts which expands and breaks rocks.

  • Unloading: Release of pressure causes rocks to fracture (sheeting/exfoliation).

  • Organisms: Trees and humans can cause mechanical weathering.

  • Jointing: Fractures created from heating/cooling.

Chemical Weathering

• Involves the chemical alteration of minerals in rocks.

• Byproducts: Clay, silica, rust.

• Processes of Chemical Weathering:

  • Hydrolysis: Dissolving minerals through water.

  • Acidification: Dissolving minerals by acid (e.g., Carbonic Acid from acidic rain).

  • Oxidation: Dissolving minerals by oxygen.

Rates of Weathering

• Rates are accelerated by:

  • Mineral Composition and Solubility: High density minerals are more vulnerable.

  • Physical Features: Presence of joints/fractures increases surface area.

  • Climate: High heat and humidity are conducive to weathering.

  • Organisms: Biological activity can enhance weathering.

  • Ice and Snow: Contributes to overall weathering processes.

• Differential Weathering: Non-uniform weathering leading to uneven disintegration.

• Caprock: Resistant layer weathering slower than underlying rock.

• Winnowing & Buckling: Related geological processes.

• Uplift: Movement from tectonic forces.

• Talus: Accumulation of rock debris from frost wedging.

Soil

Characteristics of Soil

• Composition: Mixture of mineral matter, water, air; essential for plant growth.

• Regolith: Parent material made of rock and mineral fragments.

• Soil Texture: Types of soil grains include:

  • Clay

  • Silt

  • Sand

  • Gravel

  • Loam (blend)

• Soil Structure: Defined by peds, types include:

  • Platy

  • Prismatic

  • Blocky

  • Spheroidal

Soil Profile

• Horizons: Zones/layers in soil from surface downward:

  • O Horizon: Organic matter.

  • A Horizon: Organic and mineral matter (zone of leaching).

  • E Horizon: Leached layer with less organic matter.

  • B Horizon: Accumulation of minerals.

  • C Horizon: Unaltered parent material.

• Topsoil: Composed of O and A horizons; true soil includes O through B (solum).

Controls of Soil Formation

• Parent Material (Regolith):

  • Residual Sediment: Derived from weathered bedrock.

  • Transported Sediment: Deposited from elsewhere.

• Time: Longer time allows better soil development.

• Climate: High heat and humidity contribute to soil formation.

• Organisms: Diverse life forms enhance soil development.

• Topography: Mid-slopes (~3° to 30°) promote better soil development.

Paleosols

• Ancient soils that provide records of soil formation conditions; useful in reconstructing past environments but may face issues from burial processes.

Mass Wasting

• Downslope movement of rock, regolith, and soil under gravity.

• Key Factors:

  • Gravity as the main force

  • Friction resisting motion.

• Triggering Factors:

  • Oversteepening of slopes: Exceeding angle of repose increases risk.

  • Removal of Vegetation: Lowered stability of slopes.

  • Ground Vibrations: Earthquakes weaken slope stability.

  • Water Saturation: Reduces cohesion among particles.

Types of Mass Wasting

• Slump: Rapid movement along curved surfaces, often from oversteepening.

• Rockslide: Rapid movement of bedrock, generally caused by steep slopes or vibrations.

• Debris Flow (Mudflow): Rapid movement of debris mixed with water, often confined to channels (e.g., volcanic lahars).

• Earthflow: Movement of saturated soil often following liquefaction triggered by seismic activity.

• Creep: Slow movement of soil and sediment over time.

• Solifluction: Slow movement in permafrost areas.