Types of Weathering

Why This Matters

Weathering is the foundation of nearly every landscape process you'll encounter in Earth Surface Processes. Before erosion can transport material, before soils can form, before sediments can become sedimentary rocks—weathering must first break down the parent material. You're being tested on your ability to distinguish between mechanical disintegration and chemical decomposition, and to recognize how climate, rock type, and biological activity control which weathering processes dominate in different environments.

Don't just memorize a list of weathering types. Instead, focus on the mechanisms behind each process and the environmental conditions that favor one type over another. When you see an exam question about limestone caves, you should immediately connect it to carbonation and dissolution. When asked about talus slopes, think freeze-thaw. The key is understanding why certain weathering processes operate where they do—and how they work together to shape Earth's surface.

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Mechanical Breakdown: Physical Weathering Processes

Physical weathering shatters rocks into smaller pieces without altering their mineral composition. The surface area increases dramatically, but the chemistry stays the same—setting the stage for accelerated chemical attack.

Physical Weathering (Overview)

• Mechanical fragmentation breaks rocks through applied stress—no chemical reactions involved
• Increased surface area from fragmentation accelerates subsequent chemical weathering rates
• Climate controls determine which physical processes dominate: freeze-thaw in periglacial zones, thermal stress in deserts

Frost Weathering

• Freeze-thaw cycles drive this process—water expands ~9% when freezing, generating pressures up to 2,100 kg/cm²
• Periglacial and alpine environments with temperatures oscillating around 0°C experience the most intense frost action
• Talus slopes and blockfields are classic landforms produced by sustained frost shattering

Thermal Weathering

• Diurnal temperature swings cause repeated expansion and contraction, fatiguing rock surfaces over time
• Desert environments are hotspots—surface temperatures can shift 50°C+ between day and night
• Exfoliation and granular disintegration result as outer rock layers peel away from the cooler interior

Salt Weathering

• Crystal growth pressure occurs when dissolved salts precipitate in pore spaces as water evaporates
• Coastal and arid regions with high evaporation rates see the most aggressive salt attack
• Honeycomb weathering (tafoni) is a distinctive landform created by salt crystallization in rock cavities

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Chemical Transformation: Reactions That Alter Minerals

Chemical weathering changes the molecular structure of minerals, producing new compounds and dissolved ions. Water is almost always involved, acting as a solvent, reactant, or transport medium.

Chemical Weathering (Overview)

• Mineral alteration transforms primary rock-forming minerals into secondary minerals like clays
• Hydrolysis, oxidation, and carbonation are the three dominant reaction types you need to know
• Warm, humid climates accelerate chemical weathering because reaction rates increase with temperature and moisture

Oxidation

• Electron transfer to oxygen transforms minerals—iron-bearing rocks develop characteristic red-orange rust ($Fe_2O_3$)
• Structural weakening occurs as oxidized minerals expand and become more friable
• Near-surface environments with abundant atmospheric oxygen see the most oxidation

Carbonation

• Carbonic acid formation occurs when $CO_2$ dissolves in water: $CO_2 + H_2O \rightarrow H_2CO_3$
• Limestone and marble are highly susceptible—calcium carbonate dissolves readily in weak acid
• Karst landscapes including sinkholes, caves, and disappearing streams result from sustained carbonation

Dissolution

• Direct mineral solubility removes material entirely—no solid residue remains
• Calcite, halite, and gypsum are among the most soluble common minerals
• Cave systems and sinkholes form where dissolution removes enough material to create voids

Hydration

• Water molecule incorporation into mineral crystal structures causes expansion and weakening
• Anhydrite to gypsum transformation ($CaSO_4 \rightarrow CaSO_4 \cdot 2H_2O$) is the classic example—volume increases ~60%
• Clay mineral behavior depends heavily on hydration, affecting soil stability and landslide potential

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Biological Agents: Life as a Weathering Force

Biological weathering harnesses living organisms to break down rock through both physical and chemical means. Organisms often accelerate weathering rates by orders of magnitude compared to abiotic processes alone.

Biological Weathering

• Root wedging occurs when growing roots exert physical pressure in rock fractures—forces can exceed 150 kPa
• Organic acid production by lichens, fungi, and bacteria chemically attacks mineral surfaces
• Burrowing organisms mix soil and expose fresh rock surfaces to weathering agents

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Quick Reference Table

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Self-Check Questions

1. Which two weathering processes both rely on crystal growth to fracture rocks, and what environmental conditions favor each?

2. A limestone cliff in a humid tropical climate shows extensive pitting and cave development. Which weathering processes are primarily responsible, and how do they work together?

3. Compare and contrast hydration and hydrolysis—how does each process use water differently to weather minerals?

4. Why does physical weathering often accelerate chemical weathering, even though the two processes operate through different mechanisms?

5. An FRQ presents a coastal desert environment and asks you to explain the dominant weathering processes. Which types would you discuss, and what landforms might result?