Lecture: Weathering

The physical and chemical breakdown of rocks at Earth’s surface

Exposure to new environmental conditions Expansion and contraction Water interactions Biological activity Atmospheric influences Mechanical and chemical weathering often work togehter

Physical/mechanical weathering Chemical weathering

Physical forces break rock into smaller pieces No change in mineral composition Increases the surface area of rocks making them more susceptible to chemical weathering

Chemical breakdown of rock into new minerals and dissolved ions and compounds Converts original minerals into new, stable minerals or dissolves compounds/ions Works alongside mechanical weathering Plays a major role in soil formation and nutrient cycling

Essential for soil formation Releases life-sustaining nutrients to sustain ecosystems Plays a key role in the geologic carbon cycle Responsible for shaping landscapes

Frost Wedge Salt crystal growth Jointing Sheeting biological activity

Water enters cracks in rock, freezes, expands, and causes the rock to break apart Repeated freeze-thaw cycles amplify the breakdown of rock over time Often leads to rockfalls and landslides in high altitude environments

Saltwater seeps into rock crevices and evaporates leaving salt crystals behind As crystals grow, they exert pressure on rock, causing it to weaken and break Contributed to the formation of honeycomb weathering patterns in some rock formations

Refers to natural fractures in rock that develop due to cooling, pressure release, or tectonic forces Joints provide pathways for water infiltration accelerating weathering processes

When overlying rock layers are removed by erosion, pressure on the underlying rock decreases This reduction in pressure causes the rock to expand and fracture into onion-like layers, forming exfoliation domes Can create large, curved slabs which gradually peel away from the main rock body

Root wedging: plant roots grow into cracks in rocks, expanding and breaking them apart Burrowing animal activities

Water is the primary agent in chemical weathering Acts as a solvent Transports reactive substance Facilitates dissolution, oxidation and hydrolysis Enhances chemical reactions by mixing with gases such as carbon dioxide and oxygen

Certain minerals dissolve in water, separating into individual ions Halite (salt) dissolves readily in pure water Carbon dioxide from the atmosphere dissolves in rainwater to form carbonic acid, enhancing dissolution Calcite dissolves in acidic water, forming caves and sinkholes

Rusting Oxygen reacts with iron, forming iron oxides (rust), weakening rocks This process is responsible for the reddish and yellowish hues seen in weathered rocks Ferromagnesian minerals like olivine are especially susceptible to oxidation Pyrite reacts with oxygen and water to produce sulfuric acid, contributing to acid mine draignage an environmental hazard -> weakens rock making them more suspectible to further breakdown

Water breaks down minerals by replacing cations, forming more stable compounds Silicate minerals decompose into clay minerals Releases solub;e ions which are transported to oceans and contribute to salinity Plays a key role in soil formation

Clay minerals Soluble cations Iron oxides Silica Bicarbonate ions

Weathering attacks edges and corners of rocks more rapidly than flat surfaces Causes rocks to develop rounded shapes over time Water infiltrates cracks, causing chemical reactions along fractures

Surface area: more exposure increases the rate of chemical weathering Rock characteristics: mineral composition and solubility affect how quickly rocks break down Climate: temperature and precipitation control both mechanical and chemical weathering rates