Weathering causes the breakdown of rock at the Earth's surface. The main causes of weathering are plant and animal life, the atmosphere, and water. Weathering breaks down and loosens the surface minerals of rock so they can be transported by erosion. There are two types of weathering: chemical and mechanical. Chemical weathering changes the composition of materials, while mechanical weathering breaks rocks apart without changing their chemical makeup. Biological weathering also contributes to rock breakdown through processes like plant and microbial growth.