Weathering Processes

Why This Matters

Weathering is the foundation of the rock cycle's surface processes—it's how solid rock transforms into sediment, releases ions into solution, and creates the raw materials for soil formation. You're being tested on your ability to distinguish between physical and chemical weathering mechanisms, understand the geochemical reactions that drive mineral breakdown, and predict how environmental conditions influence weathering rates. These concepts connect directly to topics like soil development, carbon cycling, karst geomorphology, and mineral stability diagrams.

Don't just memorize a list of weathering types. Instead, focus on what's actually happening at the molecular or mechanical level—is the mineral's crystal structure being destroyed? Are new minerals forming? Is the rock simply breaking into smaller pieces of the same material? When you understand the underlying mechanism, you can predict weathering products, identify dominant processes in different environments, and tackle FRQ scenarios that describe unfamiliar field conditions.

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Physical (Mechanical) Weathering

Physical weathering breaks rocks into smaller fragments without altering their mineralogy. The key principle here is that these processes increase surface area, which accelerates subsequent chemical weathering. Think of it as prepping the rock for geochemical attack.

Freeze-Thaw Weathering

• Water expands ~9% upon freezing—this generates pressures up to 2,100 kg/cm² in confined rock fractures, far exceeding the tensile strength of most rocks
• Repeated freeze-thaw cycles progressively widen cracks through ice wedging, producing angular rock fragments called frost-shattered debris
• Climate dependence makes this process dominant in periglacial environments where temperatures oscillate around 0°C

Thermal Expansion and Contraction

• Differential heating causes minerals to expand at different rates, generating internal stress along grain boundaries
• Diurnal temperature swings in deserts (sometimes exceeding 40°C) create repeated expansion-contraction cycles that fatigue rock surfaces
• Granular disintegration results when individual mineral grains separate, particularly in coarse-grained rocks like granite

Salt Crystallization

• Crystal growth pressure from evaporating saline solutions can exceed 100 MPa, enough to fracture most rock types
• Hydration cycling of salts like $Na_2SO_4$ (thenardite ↔ mirabilite) amplifies damage through repeated volume changes
• Tafoni and honeycomb weathering are diagnostic landforms indicating active salt weathering in coastal and arid settings

Exfoliation

• Pressure release occurs when overlying rock is removed, allowing buried rock to expand and fracture parallel to the surface
• Sheet jointing produces curved, onion-skin layers that peel away from dome structures like Half Dome in Yosemite
• Unloading is the primary mechanism—not thermal stress—distinguishing this from other physical processes

Abrasion

• Particle-on-rock friction mechanically grinds surfaces through impact and scraping by transported sediment
• Glacial striations, river-polished cobbles, and ventifacts all result from abrasion in different transport media
• Sediment load and velocity control abrasion intensity—higher energy environments produce more mechanical wear

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Chemical Weathering Reactions

Chemical weathering transforms primary minerals into secondary minerals and dissolved ions through geochemical reactions. These processes alter the crystal structure itself, creating new phases that are stable under surface conditions. Understanding the specific reactions is essential for predicting weathering products.

Hydrolysis

• $H^+$ ions attack silicate frameworks—water dissociates and hydrogen replaces cations in mineral structures, breaking Si-O bonds
• Feldspar-to-clay transformation is the classic example: $2KAlSi_3O_8 + 2H^+ + 9H_2O → Al_2Si_2O_5(OH)_4 + 4H_4SiO_4 + 2K^+$ (orthoclase → kaolinite)
• Rate increases with acidity—organic acids in soil dramatically accelerate hydrolysis of silicate minerals

Oxidation

• Electron transfer to oxygen converts reduced metals (especially $Fe^{2+}$) to oxidized forms ($Fe^{3+}$)
• Iron-bearing minerals like olivine, pyroxene, and biotite weather to produce red-brown iron oxides and hydroxides (hematite, goethite)
• Redox boundaries in soil profiles mark where oxidation dominates, visible as color changes from gray (reduced) to red/orange (oxidized)

Carbonation

• Carbonic acid formation—$CO_2 + H_2O → H_2CO_3$—provides $H^+$ ions that attack carbonate minerals
• Calcite dissolution—$CaCite + H_2CO_3 → Ca^{2+} + 2HCO_3^-$—is highly effective, producing karst landscapes, caves, and sinkholes
• Atmospheric $CO_2$ levels directly influence carbonation rates, linking this process to climate and carbon cycling

Dissolution

• Direct mineral solubility removes material entirely without producing secondary minerals
• Halite and gypsum are highly soluble—$NaCl$ and $CaSO_4 \cdot 2H_2O$ dissolve readily in undersaturated water
• Karst development depends on dissolution rates, controlled by water chemistry, flow rate, and temperature

Hydration

• Water incorporation into mineral structure causes volume expansion and structural weakening
• Anhydrite-to-gypsum conversion—$CaSO_4 + 2H_2O → CaSO_4 \cdot 2H_2O$—increases volume by ~60%
• Mechanical stress from hydration expansion can crack surrounding rock, bridging chemical and physical weathering

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Biological Weathering

Living organisms contribute to both physical and chemical weathering through mechanical action and biochemical processes. Biological weathering often accelerates other weathering mechanisms rather than operating independently.

Root Wedging and Bioturbation

• Root growth pressure can exceed 1.5 MPa, enough to widen existing fractures and pry apart rock along joints
• Burrowing organisms (earthworms, rodents, insects) physically mix soil and expose fresh rock surfaces to weathering agents
• Organic matter accumulation increases soil water retention, prolonging contact between water and rock

Microbial and Biochemical Weathering

• Organic acid production by bacteria, fungi, and lichens creates localized low-pH microenvironments that accelerate hydrolysis
• Chelation by organic compounds extracts metal cations from mineral surfaces, enhancing dissolution rates
• Mycorrhizal networks actively mine nutrients from rock, demonstrating that biological weathering is often targeted rather than random

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Weathering Patterns and Landforms

These processes describe how weathering varies spatially and produces distinctive landscape features. Understanding differential weathering explains why some rocks persist while others crumble.

Differential Weathering

• Variable rock resistance causes harder lithologies to stand in relief while softer rocks erode faster
• Mesas, buttes, and hoodoos form when resistant cap rocks protect underlying weaker layers
• Structural control means joints, bedding planes, and faults often guide weathering patterns

Spheroidal Weathering

• Corner and edge attack proceeds faster because these surfaces have greater exposure to weathering agents
• Concentric shells develop as chemical weathering penetrates inward along fractures in blocky rock masses
• Core stones remain as rounded boulders surrounded by weathered regolith—common in granite landscapes

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Quick Reference Table

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Self-Check Questions

1. Which two weathering processes both rely on crystal growth pressure in rock fractures, and what environmental conditions favor each?

2. A granite boulder in a humid tropical climate shows rounded edges and clay-rich soil beneath it. Which weathering processes are primarily responsible, and what geochemical reactions are occurring?

3. Compare and contrast hydrolysis and carbonation: what types of minerals does each attack, and what products result?

4. An FRQ describes a limestone plateau with sinkholes, caves, and disappearing streams. Which weathering process dominates, and how would increased atmospheric $CO_2$ affect the rate?

5. Why does physical weathering accelerate chemical weathering, and which physical processes would be most effective at preparing rock for subsequent chemical attack?