Weathering, as we saw in Chapter 5, is the general process by which rocks are broken down at Earth’s surface. Weathering produces all the clays and soils of the world, as well as the dissolved substances that streams carry to the ocean. Chemical weathering occurs when the minerals in a rock are chemically altered or dissolved. Physical weathering takes place when solid rock is fragmented by mechanical processes that do not change its chemical composition. Chemical and physical weathering reinforce each other. Chemical weathering weakens rocks and makes them more susceptible to physical weathering. The smaller the pieces produced by physical weathering, the greater the surface area available for chemical weathering.
Once weathering reduces rocks to particles, they may accumulate as soil, or they may be removed by erosion, transported, and deposited elsewhere as sediments. Erosion is the process by which particles produced by weathering are dislodged and removed from their source, usually by means of currents of water or air. Erosion moves particles from hillslopes to the starting points of stream channels. Mass wasting includes all the processes by which weathered and unweathered Earth materials move downslope in larger amounts and in large single events, usually by means of gravity. The products of mass wasting—particles released by weathering as well as large masses of unweathered rock—are also transported to the starting points of stream channels. Once these materials reach stream channels, streams and rivers can efficiently transport them farther downslope, perhaps across continents and all the way to the ocean. The transport of sediments by streams from their source areas in mountains to their sinks in the world ocean will be covered in more detail in Chapter 18.
Weathering is one of the major processes of the rock cycle. It shapes Earth’s surface topography and alters rock materials, converting all kinds of rocks into sediments and soils. The early sections of this chapter emphasize chemical weathering because it is in some ways the fundamental driving force of the weathering process. For example, the effects of physical weathering, which we will examine next, depend largely on the chemical decay of minerals. Before we look at either type of weathering in detail, however, let’s examine the factors that control weathering.