By Jon Zamboni – Updated Aug 30, 2022

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Weathering processes crack, wear away, and weaken rocks, reshaping the landscape over time. Physical weathering—also called mechanical weathering—reduces a rock’s structural integrity without altering its chemical makeup. Chemical weathering, on the other hand, changes the rock’s composition at the molecular level through reactions that exchange ions and cations.

Physical Weathering

Physical weathering describes the mechanical breakdown of a rock’s structure. It is driven by forces such as friction, impact, temperature fluctuations, and the intrusion of expanding substances. While the rock’s chemical composition remains unchanged, repeated mechanical action ultimately fractures and disintegrates it.

Common Types of Physical Weathering

• Wedging – When water freezes in cracks, salts precipitate, or plant roots grow into fissures, the expanding material exerts pressure that splits the rock.
• Exfoliation – Rocks formed under high pressure are released to the surface. As the confining pressure drops, the rocks expand and peel off in parallel sheets.
• Abrasion – Moving rocks collide with one another or with the ground, gradually smoothing and wearing away exposed surfaces.
• Thermal expansion – Diurnal heating causes uneven expansion of mineral grains; the resulting stresses can crack the rock.

Chemical Weathering

Chemical weathering alters the mineral constituents of a rock, making it softer or even dissolving it entirely. Often, chemical degradation weakens the rock, setting the stage for subsequent physical breakdown.

Common Types of Chemical Weathering

• Oxidation – Oxygen reacts with iron-bearing minerals to form rust (iron oxides), which are weaker and more susceptible to mechanical forces.
• Hydrolysis – Water molecules replace ions in mineral structures, producing new, often softer minerals that can be more easily removed.
• Carbonation – Dissolved carbon dioxide forms carbonic acid in rainwater; this acid attacks calcium‑rich rocks such as limestone, producing caves and karst features.
• Acid rain – Sulfur and nitrogen oxides released by combustion mix with atmospheric moisture, forming acids that degrade marble, chalk, and other carbonate stones, damaging monuments and statues.