Weathering is the gradual destruction of rock under surface conditions, dissolving it, wearing it away or breaking it down into progressively smaller pieces. Think of the Grand Canyon or the red rock formations scattered across the American Southwest. It may involve physical processes, called mechanical weathering, or chemical activity, called chemical weathering. Some geologists also include the actions of living things, or organic weathering. These organic weathering forces can be classified as mechanical or chemical or a combination of both.
Mechanical weathering involves five major processes that physically break rocks down into sediment or particles: abrasion, crystallization of ice, thermal fracture, hydration shattering, and exfoliation. Abrasion occurs from grinding against other rock particles. Crystallization of ice can result in force sufficient enough to fracture rock. Thermal fracture may occur due to significant temperature changes. Hydration -- the effect of water -- predominantly affects clay minerals. Exfoliation occurs when rock is unearthed after its formation.
Mechanical weathering does not just affect the earth. It can also affect some brick and stone buildings over time.
Chemical weathering involves the decomposition or decay of rock. This type of weathering doesn't break rocks down but rather alters its chemical composition through carbonation, hydration, oxidation or hydrolysis. Chemical weathering changes the composition of the rock toward surface minerals and mostly affects minerals that were unstable in the first place. For example, water can eventually dissolve limestone. Chemical weathering can occur in sedimentary and metamorphic rocks and it is an element of chemical erosion.
Organic weathering is sometimes called bioweathering or biological weathering. It involves factors such as contact with animals-when they dig in the dirt-and plants when their growing roots contact rock. Plant acids can also contribute to the dissolution of rock.
Organic weathering isn't a process that stands alone. It's a combination of mechanical weathering factors and chemical weathering factors.
The Result of Weathering
Weathering can range from a change in color all the way to a complete breakdown of minerals into clay and other surface minerals. It creates deposits of altered and loosened material called residue that is ready to undergo transportation, moving across the earth's surface when propelled by water, wind, ice or gravity and thus becoming eroded. Erosion means weathering plus transportation at the same time. Weathering is necessary for erosion, but a rock may weather without undergoing erosion.