2.1 Mechanisms of physical weathering

Physical weathering breaks down rocks without changing their chemistry. It's like nature's demolition crew, using forces like freezing water, temperature changes, and salt crystals to crack and split rocks into smaller pieces.

This process is a key part of the broader weathering story. It works alongside chemical and biological weathering to shape Earth's surface, creating the landscapes we see and providing the raw materials for soil formation.

Physical Weathering Mechanisms

Mechanical Rock Breakdown

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• Physical weathering breaks down rocks and minerals mechanically without altering their chemical composition
  • Results in smaller fragments and increased surface area
• Frost wedging splits rocks apart when water freezes and expands in cracks
  • Exerts pressure over time as freeze-thaw cycles repeat
• Thermal expansion and contraction fractures rocks through repeated heating and cooling
  • Causes stress as rocks expand when heated and contract when cooled
• Salt weathering fragments rocks as salt crystals grow in pores
  • Exerts pressure within rock structure

Biological and Environmental Factors

• Exfoliation peels off outer rock layers due to pressure or temperature changes
  • Often creates dome-like formations (Half Dome in Yosemite)
• Biological weathering physically breaks down rocks through organism activity
  • Plant roots grow into cracks and widen them
  • Burrowing animals disturb and fragment rock material
• Abrasion erodes rock surfaces through particle collisions
  • Wind, water, or ice carry rock particles that impact other rocks
  • Creates features like ventifacts in deserts

Temperature's Role in Weathering

Thermal Expansion and Contraction Process

• Thermal weathering significantly impacts areas with large temperature fluctuations (deserts)
• Different minerals expand and contract at varying rates within rocks
  • Creates internal stresses leading to fracturing over time
• Coefficient of thermal expansion determines expansion/contraction degree
  • Varies among rock types (quartz expands more than feldspar)
• Repeated heating and cooling cycles cause cumulative rock structure damage
  • Weakens rocks and increases susceptibility to other weathering processes

Thermal Shock and Limitations

• Thermal shock fractures rocks through rapid temperature changes
  • Sudden expansion or contraction can cause immediate breakage
• Thermal weathering effects limited by poor rock heat conductivity
  • Primarily affects surface and near-surface layers (upper few centimeters)
• Effectiveness depends on factors like rock composition and environmental conditions
  • More pronounced in areas with extreme daily temperature swings (hot deserts)

Water's Impact on Physical Weathering

Freeze-Thaw and Hydration Processes

• Water acts as primary agent in multiple physical weathering processes
• Freeze-thaw weathering (frost wedging) breaks rocks when water freezes in cracks
  • Water expands by ~9% upon freezing, exerting pressure on surrounding rock
  • Effectiveness depends on freeze-thaw cycle frequency and water availability
• Hydration weathering expands certain minerals through water absorption
  • Can fracture surrounding rock as hydrated minerals increase in volume
  • Examples include conversion of anhydrite to gypsum or olivine to serpentine

Salt Crystallization and Water's Role

• Salt crystallization weathering occurs as dissolved salts grow within rock pores
  • Exerts pressure as crystals expand, fragmenting rock
  • Particularly effective in coastal areas and arid regions (Death Valley salt pans)
• Water facilitates other weathering processes by:
  • Transporting weathered materials, exposing fresh rock surfaces
  • Increasing chemical weathering rates, working in conjunction with physical processes
• Water-related weathering effectiveness varies with climate and rock properties
  • More impactful in areas with frequent wet-dry or freeze-thaw cycles

Rock Properties and Weathering Susceptibility

Mineral Composition and Structure

• Mineral composition significantly affects rock resistance to physical weathering
  • Some minerals more susceptible than others (mica vs quartz)
• Rocks with minerals having different thermal expansion coefficients prone to thermal weathering
  • Differential stress created by uneven expansion/contraction
• Crystal structure influences mineral cleavage patterns and potential breakage points
  • Minerals with well-defined cleavage (mica) more easily split than those without (quartz)
• Foliated metamorphic rocks often more susceptible to weathering along foliation planes
  • Examples include schist and gneiss

Physical Characteristics and Environmental Factors

• Rock porosity and permeability influence susceptibility to water-related weathering
  • Higher porosity increases vulnerability to freeze-thaw and salt crystallization
• Rock hardness affects resistance to abrasion and mechanical weathering
  • Measured on Mohs scale (talc = 1, diamond = 10)
• Pre-existing fractures, joints, or bedding planes provide pathways for water infiltration
  • Increases vulnerability to physical weathering processes
• Grain size and cementation of sedimentary rocks influence cohesion and weathering resistance
  • Well-cemented sandstone more resistant than poorly cemented varieties
• Environmental factors interact with rock properties to determine weathering rates
  • Climate, topography, and biological activity influence weathering intensity