1 Weathering and the Breakdown of Rock

Weathering and erosion

Weathering breaks down or alters rock in place at or near Earth’s surface. Unlike weathering, erosion transports weathered rock and sediment. Physical, chemical, and biological processes can all contribute, and they often work together.

Physical weathering

Physical weathering, also called mechanical weathering, breaks rock into smaller pieces without changing the chemical composition of its minerals. The smaller fragments expose more surface area to water and air, which can speed up chemical weathering.

Common processes include:

• Frost wedging: Water enters cracks, freezes and expands, and widens the cracks. Repeated freezing and thawing can eventually split the rock.

• Thermal expansion and contraction: Repeated heating and cooling cause minerals to expand and contract. Differences in how minerals respond can help fracture rock.

• Pressure release: When overlying rock is removed, reduced pressure can allow outer layers to expand and peel away in sheets. This process is called exfoliation.

• Salt-crystal growth: Salty water enters pores or cracks. As the water evaporates, growing salt crystals exert pressure and loosen grains.

For example, repeated freeze–thaw cycles can break exposed rock into angular fragments while its constituent minerals remain chemically unchanged.

Chemical weathering

Chemical weathering changes the minerals in rock through reactions with water, oxygen, acids, or other substances. These reactions may dissolve minerals or transform them into new minerals. Water generally promotes chemical weathering because it enables reactions and carries dissolved substances away.

• Dissolution: Some minerals dissolve in water. Slightly acidic water can dissolve calcite in limestone, gradually enlarging cracks and creating cavities.

• Hydrolysis: Water reacts with minerals. For example, feldspar in granite can be altered into clay minerals.

• Oxidation: Oxygen reacts with minerals that contain iron, producing iron oxides—the rust-like discoloration seen in some rocks.

• Carbonation: Carbon dioxide dissolves in water to form a weak acid that can react with carbonate minerals such as calcite.

Biological weathering

Biological weathering occurs through organisms’ physical and chemical effects on rock. Plant roots can enter existing fractures and push them wider as they grow. Lichens, plant roots, fungi, and microorganisms can also release acids or other compounds that react with minerals. Burrowing animals disturb and mix loose material, exposing it to further weathering. These biological processes connect rock breakdown with soil development.

Factors that control weathering

Weathering rates vary with differences in rocks and environments. The five interacting factors commonly used to describe soil formation also help explain differences in weathering:

• Climate: Temperature and moisture affect both physical and chemical weathering. Frequent freeze–thaw cycles favor frost wedging, while warm, wet conditions generally support faster chemical reactions than cold or dry conditions.

• Parent material: Mineral composition, grain size, fractures, and pores affect how easily rock breaks down. Some minerals are more reactive than others, and fractures give water greater access.

• Organisms: Roots, lichens, fungi, microbes, and soil animals can crack rock, alter minerals, and mix weathered material.

• Topography: Slope affects water movement and how long weathered material remains in place. Steep slopes often lose loose material more readily, while gentler or lower areas can accumulate it.

• Time: Longer exposure gives more opportunity for alteration and soil development, although material can be removed before thick soil forms.

These factors interact. For example, water may enter a crack, freeze and widen it, and later carry dissolved substances deeper into the rock.

From weathered rock to soil

Weathering produces loose mineral fragments and altered material. Regolith is the general name for the layer of loose or weathered material above solid bedrock; it can include rock fragments, sediment, and soil. Soil is biologically active surface material containing mineral particles, organic matter, water, and air. It develops as weathered mineral material interacts with organisms and organic matter accumulates. Over time, layers, or horizons, may form.

A simplified sequence is that bedrock is fractured and chemically altered; fragments and mineral products accumulate as regolith; organisms colonize the surface and add or mix organic matter; and continued soil-forming processes can create a more developed soil profile. This sequence is not always uninterrupted: erosion can remove material, and new sediment can bury an older surface.