Weathering is the process by which rocks break down into smaller pieces. There are three main types of weathering: mechanical, chemical, and biological. Mechanical weathering occurs when temperature changes cause rocks to expand and contract, breaking them into smaller pieces. Chemical weathering happens when acid rain forms from carbon dioxide and nitrogen in the atmosphere, and the acid dissolves the minerals in rocks. Biological weathering is caused by plant roots and small creatures penetrating cracks in rocks and breaking them apart.