weathering, collective term for the processes by which rock at or near the earth's surface is disintegrated and decomposed by the action of atmospheric agents, water, and living things. Some of these processes are mechanical, e.g., the expansion and contraction caused by sudden, large changes in temperature, the expansive force of water freezing in cracks, the splitting caused by plant roots, and the impact of running water; others are chemical, e.g., oxidation, hydration, carbonization, and loss of chemical elements by solution in water. Weathering is important because it aids in the formation of soil and prepares materials for erosion. Construction materials for buildings and roads are subject to weathering by water, carbon dioxide, aerosol gases, freeze-thaw cycles, and salt (see also formation of potholes). New techniques in road construction, allowing a minimum of weathering, more weather-resistant aggregates, and better building materials have lowered costs for maintenance and repair.