Weathering breaks down rocks through physical and chemical processes. Physical weathering fractures rocks, while chemical weathering alters their composition. Climate, rock type, and biological factors influence weathering rates and types, shaping Earth's surface over time.
Weathering produces sediments, soils, and minerals that form new landscapes. It plays a crucial role in the carbon cycle, ecosystem development, and water chemistry. Understanding weathering helps predict how landscapes respond to climate change and human activities.
Physical vs Chemical Weathering
Types of Physical Weathering
- Frost wedging: Water seeps into cracks in rocks, freezes, and expands, causing the cracks to widen and the rock to break apart
- Thermal expansion and contraction: Repeated heating and cooling of rocks leads to stress and eventual breakdown (desert environments)
- Salt crystal growth: Salt water seeps into rock pores, evaporates, and leaves behind salt crystals that grow and exert pressure on the rock (coastal areas)
- Biological activity:
- Root wedging: Plant roots grow into cracks, exerting pressure and widening the cracks
- Animal burrowing: Burrowing animals like moles or rabbits can loosen and break apart rocks and soil
Types of Chemical Weathering
- Dissolution: Minerals dissolve in water, particularly acidic water (limestone, marble)
- Hydrolysis: Minerals react with water, forming new compounds and breaking down the original rock (feldspar)
- Oxidation: Oxygen reacts with minerals, often forming oxides (iron-rich rocks, forming rust)
- Reduction: Minerals lose oxygen atoms, changing their chemical composition (less common than oxidation)
- Carbonation: Carbon dioxide dissolves in water, forming carbonic acid that reacts with rocks (limestone, marble)
Factors Influencing Weathering Type
- Rock type: Some rocks are more susceptible to physical weathering (granite), while others are more susceptible to chemical weathering (limestone)
- Climate: Wet climates favor chemical weathering, while dry and cold climates favor physical weathering
- Environmental factors: Presence of water, acids, and other substances influence the type and rate of weathering
Factors Influencing Weathering
Climate and Water
- Higher temperatures and moisture levels accelerate both physical and chemical weathering
- Water is essential for many chemical weathering reactions (hydrolysis, carbonation)
- Acidity of water influences the rate and type of weathering (acid rain, carbonic acid)
Rock Characteristics
- Mineral composition: Some minerals are more resistant to weathering (quartz) than others (feldspar)
- Rock structure: Rocks with more fractures or pores are more susceptible to weathering
- Surface area: Larger surface areas allow for more rapid weathering (smaller rock fragments weather faster than larger boulders)
Biological and Human Factors
- Lichens, mosses, and other organisms contribute to both physical and chemical weathering (secreting acids, root wedging)
- Human activities like industrial pollution can accelerate chemical weathering (acid rain)
- Deforestation and urbanization can increase erosion and alter weathering patterns
Weathering Products
Sediments and Soils
- Clays, silts, and sands are formed from the breakdown of rocks
- Clay minerals (kaolinite, smectite) are common products of chemical weathering
- Small particle size and high surface area
- Important component of soils
- Weathering provides parent material for soil development
Oxides and Carbonates
- Iron and aluminum oxides/hydroxides (hematite, gibbsite) form from chemical weathering in humid climates
- Carbonate minerals (calcite, dolomite) dissolve during chemical weathering
- Leads to the formation of karst landscapes (caves, sinkholes)
- Characteristics of weathering products (particle size, shape, chemical composition) provide information about weathering processes and environmental conditions
Weathering's Role in Shaping Earth
- Differential weathering leads to distinctive landforms
- Tors: Isolated rock outcrops formed by weathering of surrounding, less resistant rock
- Inselbergs: Isolated hills or mountains rising abruptly from surrounding plains
- Karst towers: Steep-sided limestone hills formed by dissolution of surrounding rock
- Weathering contributes to the formation of sedimentary rocks (weathered materials are transported, deposited, and lithified)
Influence on Ecosystems
- Weathering influences chemical composition of surface water and groundwater
- Implications for water quality and aquatic ecosystems
- Weathering plays a role in the carbon cycle
- Chemical weathering of silicate rocks consumes atmospheric CO2
- Weathering of carbonate rocks can release CO2 back into the atmosphere
- Understanding weathering is essential for predicting landscape changes in response to climate change and human activities