Weathering is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity.
Two important classifications of weathering processes exist — physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals. (link)

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Oxidation takes place when oxygen combines with other elements in rocks to form new types of rock. These new substances are usually much softer, and thus easier for other forces to break apart.

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Hydrolysis occurs when water combines with the substances in rocks to form new types of substances, which are softer than the original rock types. These allows other forces such as mechanical weathering to more easily break them apart.

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Carbonation takes place when carbon dioxide reacts with certain types of rocks forming a solution, that can easily be carried away by water.