When rocks crumble to smaller sediments through time, scientists refer to the process as "weathering." Weathering can alter the chemical composition of the rocks or simply break larger rocks down into smaller pieces. Weathering can occur as a result of human, animal and plant activity and weather conditionsas well as the type of rock being weathered.
How land is used by humans, animals and plants, greatly impacts the rate of weathering. When plants, particularly large trees with strong roots, populate land, the roots can push deep into the ground and cause rocks to fracture. In addition, animals that dig burrows are contributing to the physical weathering process. Human activities also affect weathering rates. For example, a strip mine may remove all ground cover, which exposes rocks that were previously underground to the climate of the region.
Several major contributing weathering factors include rainfall, temperature and wind. When rain hits the ground, small sediments can break off the larger rocks. In addition, the water contributes to chemical weathering by leaching certain ions out of rocks, altering the rocks' chemical composition. If the rain is acidic from pollution, the weathering is more powerful because of acid's ability to break down rocks. The temperature changes also factor in, especially if the temperature swings are larger. For example, when the temperature is warmer, water can seep into cracks in rocks. However, when the temperature falls, the water freezes and the ice expands, enlarging the cracks and potentially breaking off chunks of rock.
Some minerals are more susceptible than others to weathering. Limestone is easily broken down while quartz is very resistant. Igneous and metamorphic rocks are formed deep below the earth's surface, where oxygen and rainfall are not present. When they reach the surface, they react with the oxygen and water and are chemically altered.