Organic debris is the remains of dead organisms such as plants and animals. Dead tree limbs can fall to the ground and physically break rocks apart. More commonly, organic material breaks down rocks chemically as it decomposes. Decomposing organic materials release carbon, which combines with water to create organic acids capable of breaking down rocks.
Living plants, especially large trees, break apart rocks by the actions of their roots. Even plants as small as grass, however, commonly break down rocks. This process can easily be observed in the actions of grass and other plants that grow up through unused sidewalks or pavement. The process occurs when a plant root grows into a crack. As the plant grows, so do its roots. Eventually, the roots will grow too large and widen the crack until the rock breaks apart.
Lichens and fungi commonly grow on rocks, and they weather them as part of their ordinary organic processes. Lichens and fungi secrete acids that break down rocks so that minerals in the rocks may be used to provide lichens and fungi with nutrition. It is also possible for lichens to crack rock at the microscopic level both by penetrating the rock to anchor, and by expanding and contracting in size as it becomes wet and then dries out.
Many animals perform biological weathering. Burrowing animals such as rabbits and groundhogs break apart rocks as they build tunnels. Animals such as earthworms break small particles of soil by eating and digesting them. Some animals, such as Piddock shells, bore into rock to hide. Large animals, including humans, wear down and break down rocks simply by walking on them. Humans sometimes even break apart rocks for fun.