Rocks break down over time through physical and chemical weathering. Physical weathering splits rocks into smaller pieces, while chemical weathering changes their makeup. These processes work together, with physical weathering exposing more rock to chemical reactions.

Water plays a huge role in both types of weathering. It freezes in cracks to split rocks, carries particles that wear down surfaces, and reacts chemically with minerals. Temperature, acidity, and dissolved gases all affect how water weathers rocks.

Physical Weathering Processes

Physical vs chemical weathering

• Physical weathering mechanically breaks down rocks without altering chemical composition creates smaller fragments with identical mineral makeup
• Chemical weathering alters rock minerals through chemical reactions changes composition often produces new minerals or dissolves existing ones
• Physical weathering increases surface area exposed to chemical processes while chemical weathering weakens rock structures making them more susceptible to physical breakdown

Types of physical weathering

• Frost wedging water freezes and expands in rock cracks repeated freeze-thaw cycles fragment rock (mountain slopes)
• Exfoliation curved rock sheets peel away from main body caused by pressure release as overlying rock erodes (Half Dome, Yosemite)
• Abrasion mechanical wearing of rock surfaces by other particles common in riverbeds, glaciers, and windblown environments (Grand Canyon)
• Thermal expansion and contraction daily temperature fluctuations cause rock to expand and contract leads to fracturing over time (desert landscapes)
• Root wedging plant roots grow into rock cracks widening them eventually splitting rocks apart (cliff faces)

Chemical Weathering Processes

Chemical weathering processes

• Hydrolysis reaction between water and rock-forming minerals breaks down silicates (feldspar) produces clay minerals and dissolved ions
• Oxidation reaction between oxygen and iron-bearing minerals forms iron oxides (rust) common in exposed environments (red rocks)
• Carbonation reaction between carbonic acid and minerals carbonic acid forms when CO2 dissolves in water effectively dissolves limestone (karst topography)
• Dissolution direct dissolving of minerals in water affects highly soluble minerals (halite)
• Hydration addition of water molecules to mineral structures causes expansion and rock weakening (gypsum formation)

Water's role in weathering

• Physical weathering:
  1. Freezes in cracks causing frost wedging
  2. Transports abrasive particles in rivers and glaciers
  3. Facilitates plant growth for root wedging
• Chemical weathering:
  1. Primary reactant in hydrolysis
  2. Solvent for dissolved CO2 in carbonation
  3. Medium for ion exchange in various reactions
• Factors affecting water's potential: Temperature influences reaction rates and freeze-thaw cycles pH affects rate and type of chemical weathering Dissolved gases (CO2) enhance chemical processes
• Water cycle impact: Precipitation delivers fresh water to rock surfaces Groundwater movement facilitates subsurface weathering Evaporation concentrates minerals leading to salt weathering (coastal cliffs)