Weathering
Weathering is the breakdown of rock at Earth's surface into smaller pieces or new minerals by physical, chemical, or biological processes.
Definition
Weathering is the breaking of rock into smaller pieces by things like ice, water, plant roots, and chemicals in rain, and the rock breaks apart right where it is, without moving. Mechanical (physical) weathering, such as frost wedging, root wedging, and exfoliation, breaks rock into smaller pieces of the same material, while chemical weathering, such as dissolution, oxidation, and hydrolysis, changes the minerals themselves. Climate controls which type dominates: cold, dry climates favor mechanical weathering and warm, wet climates favor chemical weathering. Chemical weathering is driven by reactions with water, dissolved $CO_2$, and oxygen; carbonic acid, $H_2O + CO_2 \rightarrow H_2CO_3$, dissolves calcite and hydrolyzes silicates, converting feldspar to kaolinite clay and releasing dissolved ions. Rates follow Arrhenius behavior, roughly doubling for each $10^\circ C$ rise, and increase with rainfall and surface area. Mechanical weathering feeds chemical weathering by exposing fresh surfaces, and the Goldich stability series predicts that minerals crystallizing at the highest temperatures weather fastest.
Example
Water seeps into a crack in a sidewalk, freezes, expands, and widens the crack; do that for many winters and the concrete crumbles. The red clay of the North Carolina Piedmont is the product of chemical weathering, since feldspar in the bedrock turned to clay and iron turned to rust-colored oxides over millions of years in a humid climate, while frost wedging carved the jagged rock faces along the Blue Ridge Parkway. Granite weathers spheroidally, rounding corners first because they are attacked from multiple sides. Karst landscapes such as Florida's sinkholes and Mammoth Cave in Kentucky form as carbonic acid dissolves limestone along joints.
Key Insight
Weathering breaks rock and erosion moves it, so weathering can happen to a rock that never moves an inch. Breaking a rock into pieces speeds up chemical weathering, because smaller pieces have far more surface area for water and acids to attack. Silicate weathering is Earth's long-term thermostat. Warmer climates weather faster, consuming more atmospheric $CO_2$, which cools the planet, a negative feedback operating over hundreds of thousands of years.