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Weathering—the breakdown of rocks and minerals at Earth's surface—occurs through mechanical (physical fragmentation), chemical (dissolution and oxidation), and biological (root penetration, organic acid production) processes. Climate, lithology, topography, vegetation, and time control weathering rates and landscape development.
Observe weathered outcrops and correlate decay features to rock type and climate. Perform simple dissolution experiments (vinegar on limestone) and oxidation tests (iron-bearing minerals exposed to air). Measure weathering rates using cemetery headstones of known age.
Weathering and erosion are synonymous. Chemical weathering dominates in all climates. Fresh rock surfaces weather uniformly. Weathering is always slow and undetectable during human timescales.
Weathering is the in-place breakdown of rock at Earth's surface — the crucial first step before erosion can transport material away. Think of weathering as the process that loosens and weakens rock, while erosion is the conveyor belt that carries the debris. There are three broad families of weathering, and most real landscapes involve all three working together.
Mechanical weathering physically fragments rock without changing its chemistry. The classic example is frost wedging: water seeps into cracks, freezes and expands by about 9%, and pries the rock apart. Other mechanical processes include thermal expansion from daily heating and cooling cycles, salt crystal growth in pore spaces, and pressure release (or exfoliation) when overlying rock is removed and buried rock expands upward. All of these increase the surface area exposed to attack, which accelerates the next category.
Chemical weathering transforms minerals through reactions with water, acids, and oxygen. If you recall acid-base chemistry, carbonic acid (H₂CO₃) — formed when CO₂ dissolves in rainwater — is the primary agent attacking carbonate rocks like limestone. Feldspars in granite decompose through hydrolysis, reacting with slightly acidic water to form clay minerals and dissolved silica. Iron-bearing minerals undergo oxidation, producing the rust-red stains you see on exposed rock faces. Biological weathering bridges both categories: tree roots mechanically pry apart joints while lichens and soil microbes secrete organic acids that chemically dissolve mineral surfaces.
The rate at which any of these processes operates depends on a handful of controlling factors. Climate is dominant — warm, wet environments drive the fastest chemical weathering because reaction rates increase with temperature and water availability. Cold climates favor mechanical processes like frost action. Lithology matters because minerals differ enormously in susceptibility: the Goldich dissolution series shows that minerals that crystallize at the highest temperatures (olivine, Ca-plagioclase) weather fastest at the surface, while quartz — the last to crystallize from magma — is the most resistant. Topography controls how long water stays in contact with rock: steep slopes shed water quickly, slowing chemical attack but increasing physical erosion. Flat surfaces retain moisture, deepening chemical weathering profiles. Time integrates all other factors — given enough time, even the most resistant rock will decompose.
Understanding these controls lets you read a landscape. A tropical granite outcrop develops thick, clay-rich soil because warm acidic water attacks feldspars for millennia. An arid desert preserves angular boulders because there is too little water for chemical reactions to proceed. A graveyard full of marble headstones of known ages becomes a natural weathering-rate experiment — you can measure letter depth lost per century and see chemical weathering quantified in human time.