Weathering breaks down rocks through physical and chemical processes, shaping Earth's surface over time. Physical weathering cracks and splits rocks, while chemical weathering alters their composition. These processes work together, influenced by climate, rock type, and biological activity.

Understanding weathering is crucial for grasping how landscapes form and change. It's the first step in the rock cycle, creating sediments that form new rocks and soils. Weathering also plays a key role in nutrient cycling and soil fertility.

Physical Weathering Processes

Mechanical Weathering

• Mechanical weathering physically breaks rocks into smaller pieces without changing their chemical composition
• Caused by physical forces such as wind, water, ice, and gravity that apply stress to rocks
• Weakens and breaks apart rocks along pre-existing cracks, joints, or planes of weakness (bedding planes)
• Produces sediment ranging in size from large boulders to fine grains of sand and silt
• Effectiveness depends on climate, rock type, and exposure to weathering agents

Frost Wedging and Thermal Expansion

• Frost wedging occurs when water seeps into cracks in rocks and freezes, expanding by ~9% in volume
• Expansion of freezing water exerts pressure on rock, widening cracks and eventually breaking rock apart
• Most effective in climates with frequent freeze-thaw cycles (mountainous or high-latitude regions)
• Thermal expansion and contraction can also break rocks when temperatures fluctuate widely between day and night
• Repeated heating and cooling causes rocks to expand and contract, leading to cracking and flaking (deserts)

Exfoliation

• Exfoliation is a type of mechanical weathering where outer layers of rock peel off in sheets or flakes
• Occurs when rocks are exposed to rapid temperature changes or pressure release (unloading)
• Pressure release happens when overlying rock is removed by erosion or uplift, allowing rock to expand
• Expansion causes curved sheets of rock to detach and peel away from the surface (exfoliation domes)
• Exfoliation is common in granitic rocks and can produce distinctive rounded landforms (Half Dome in Yosemite)

Chemical Weathering Processes

Chemical Reactions

• Chemical weathering alters the chemical composition of rocks through reactions with water, air, and acids
• Weakens rocks by dissolving minerals or transforming them into new minerals (clays)
• Effectiveness depends on climate, rock type, and presence of water and other reactants
• Chemical weathering is most rapid in warm, humid climates with abundant vegetation and acidic conditions
• Primary agents of chemical weathering include water (H2O), oxygen (O2), and carbon dioxide (CO2)

Hydrolysis and Dissolution

• Hydrolysis is a chemical reaction between minerals and water (H2O) that breaks chemical bonds
• Water ionizes into H+ and OH- ions that replace cations in minerals, transforming them into clays
• Hydrolysis is particularly effective at weathering silicate minerals (feldspars) into clays (kaolinite)
• Dissolution is the process by which minerals dissolve in water, completely breaking down the solid
• Highly soluble minerals like halite (NaCl) and gypsum (CaSO4·2H2O) readily dissolve in water
• Dissolution can create karst topography with sinkholes, caves, and underground drainage systems (limestone landscapes)

Oxidation and Carbonation

• Oxidation occurs when minerals react with oxygen (O2), often in the presence of water
• Oxidation commonly affects iron-bearing minerals (pyrite, olivine), producing iron oxides (rust)
• Iron oxides give weathered rocks a reddish or yellowish color (red beds, yellowstone)
• Carbonation is a reaction between minerals and carbonic acid (H2CO3) formed by CO2 dissolving in water
• Carbonic acid reacts with calcium-bearing minerals (calcite) to form soluble bicarbonate ions (Ca(HCO3)2)
• Carbonation is a key weathering process in limestone and other carbonate rocks, leading to karst formation

Biological Weathering

Weathering by Organisms

• Biological weathering is caused by the activities of living organisms, including plants, animals, and microbes
• Plant roots grow into cracks in rocks, widening them and breaking rocks apart (mechanical weathering)
• Roots and decaying organic matter release organic acids that chemically weather minerals
• Burrowing animals mix and aerate soil, exposing fresh rock surfaces to weathering
• Microorganisms like fungi and bacteria produce acids and enzymes that break down rocks and minerals
• Lichen, a symbiotic partnership between fungi and algae, secretes acids that etch and pit rock surfaces
• Biological weathering is most significant in warm, humid climates with abundant vegetation (tropical rainforests)