Mechanical and Chemical Weathering

There are many examples of mechanical weathering. Abrasion occurs when one rock grinds against another. A rock can also break when plant roots grow into cracks on its surface.

Exfoliation is a repeated cycle during which rocks expand in the daytime heat and contract at night, causing rocks to flake. Frost wedging occurs when cracks fill with water and undergo a repeated cycle of freezing and thawing, causing rocks to crack apart.

Chemical weathering involves the breaking down of rocks by chemical reactions. The three main chemical reactions that decompose rocks are acid reactions, oxidation and hydrolysis.

Acid-producing lichen and tree roots that eat through rock are natural sources of chemical weathering. Many minerals are also relatively unstable and deteriorate in the presence of water and natural chemicals. For example, the feldspar found in granite breaks down into clay.

Chemical weathering can be caused by pollution. Acid rain, created by the burning of fossil fuels, dissolves some types of rocks such as limestone.