Chemical weathering occurs when rocks are broken down by chemical processes. One example is oxidation, which occurs when iron interacts with oxygen and water. This is commonly called rusting. Another type of chemical weathering is dissolution, which occurs when rocks are dissolved in water. A third type of chemical weathering is known as carbonation. This occurs when acid in water dissolves rocks. Human activity sometimes produces rain containing nitric and sulfuric acid (acid rain), which exacerbates the process of chemical weathering.
Physical weathering occurs when rocks are mechanically broken apart. Anything that moves -- or erodes -- rock can be an agent of physical weathering, including wind, water and gravity. The physical weathering of rocks by movement is known as abrasion. Frost wedging occurs when water enters a crack or pore in a rock and then freezes and expands. This widens cracks in rocks and sometimes breaks them apart. Insolation weathering occurs when rocks crack from heating during the day and cooling at night. Cycles of wetting and drying can also cause rocks to break apart, a type of physical weathering known as slaking.
When an organism helps to break down rock, the process is a form of biological weathering. Root wedging is one of the most common types of biological weathering. This occurs when a root enters a crack in a rock. Over time, the root grows larger along with the plant, until it breaks the rock apart. Lichens chemically break down rocks in order to obtain nutrients. Burrowing animals also commonly break down rocks in the course of building tunnels.
Water is one of the primary agents of erosion. Water in streams moves sediments and small particles of soil downstream; fast-moving water is capable of moving larger rocks. Over time, the action of ocean waves can erode shores and even high cliffs by slowly removing rocks and sediment from the base. Even rainwater that is not absorbed by the ground -- known as runoff -- is capable of moving small pieces of dirt and soil. Erosion by runoff is of particular concern to farmers, who go to great lengths to minimize erosion and maintain the fertility of their soil.
Wind is capable of picking up small pieces of dust, dirt and soil. Much like water, as wind moves faster, it is able to carry larger and larger pieces of sediment. Wind erosion is sometimes referred to as deflation. Erosion by wind is the force that created the great dust storms of the 1930s in the American Midwest.
Gravity is a force of nature that we often take for granted, but it is the third primary agent of erosion. The downhill movement of rocks, soil and sediment due to the action of gravity is sometimes referred to as "mass wasting." Rockfall occurs on steep slopes and cliffs, whereas rockslides occur on more gentle slopes. A third type of mass wasting -- mudflows -- are generally precipitated by heavy rains, which increase the weight of the soil.