Weathering is the process of breaking down rocks, soils, and minerals in place. Erosion, conversely, is the process of transporting the broken-down material—the sediment—from its original location, typically by forces like wind, water, ice, or gravity. Sunlight, or solar radiation, is a primary driver of weathering, which creates the necessary raw material for subsequent erosion to occur.
Physical Breakdown Through Thermal Cycling
Solar energy drives a significant form of physical weathering known as thermal stress weathering, which is particularly effective in environments with large diurnal temperature swings, such as deserts. Rocks are composed of various minerals, and each mineral possesses a different coefficient of thermal expansion, meaning they expand and contract at varying rates when heated or cooled.
During the day, intense solar heating causes the rock’s outer layer and individual mineral grains to expand. As temperatures drop sharply at night, the minerals contract. This repeated, differential movement creates internal stresses and microfractures within the rock structure. Over extended periods, this cyclical thermal fatigue leads to granular disintegration or exfoliation, reducing massive bedrock into smaller, loose fragments available for transport.
Chemical Breakdown Through Photo-degradation
Solar radiation also initiates chemical changes in materials through photo-degradation, primarily driven by high-energy ultraviolet (UV) light. UV radiation possesses sufficient energy to break the covalent bonds that hold molecules together, a process known as photolysis. This bond cleavage creates highly reactive free radicals that readily interact with oxygen, leading to further chemical alteration.
For organic materials, such as paints, plastics, or biological crusts, this photo-oxidation process causes discoloration, brittleness, and a loss of structural integrity. Even in some inorganic minerals, UV exposure can accelerate chemical reactions that weaken the rock matrix. This destabilizes the surface material, making it much more susceptible to physical removal by wind or water.
The Critical Difference Between Weathering and Erosion
The fundamental distinction between the two processes lies in movement: weathering is static decomposition, while erosion is dynamic transport. Sunlight is a powerful agent of weathering because it provides the thermal energy for physical breakdown and the UV energy for chemical alteration. This breakdown happens at the site of the rock, without the material being carried away.
Erosion, however, requires kinetic energy to displace and move the newly created sediment. Solar energy supplies the heat for weathering, but it does not provide the mechanical force needed for large-scale movement. The actual transport is performed by other geological agents, such as wind blowing fine sand particles across a dune or water carrying silt down a river.
Sunlight is best described as an indirect cause of erosion. By thoroughly weathering solid rock into loose material, solar radiation supplies the raw sediment that is then easily picked up and carried away by the true agents of erosion. The processes work sequentially: weathering breaks the rock, and erosion removes the fragments.