What Factors Determine the Rate of Weathering?

Weathering, or the breakdown of rocks, plays a key role in supporting life on earth. Weathering produces the soil that allows our planet to have a wide range of terrestrial plant life. Newly formed soils consist primarily of weathered rock and mineral particles. As plants grow, die and decompose, the soil becomes enriched with organic matter, also known as humus. The rate at which rocks decompose is influenced by a number of factors.

One type of weathering, known as chemical weathering, works at different rates depending on the chemical composition of affected rocks. Two of the main chemical weathering processes are oxidation and carbonation. Oxidation, which is better known as rusting, weakens rock exposed to air. The process produces red or brown discoloration, as in weathered basalt. Rocks high in iron are most susceptible to oxidation. Carbonation occurs when carbon dioxide from the atmosphere mixes with water to form weak carbonic acid. Carbonation chiefly effects rocks high in calcite, such as limestone and marble.

Silicate minerals consist of crystal lattices based on chemical combinations of silicon and oxygen that form a repeating grid. If the silicon-oxygen groups bond directly to one another, weathering proceeds more slowly. However, if some of the oxygen atoms bond to an intermediary element, the lattice is less durable. For example, the crystal lattice for quartz, a slow-weathering rock, uses only silicon-oxygen bonds. In contrast, olivine weathers very quickly. In the olivine lattice, many of the oxygen atoms link up to magnesium or iron rather than silicon.

Climate affects the rate of weathering in two different ways. Chemical weathering proceeds more quickly in warm environments since increased temperature speeds many chemical reactions that break down rocks. In contrast, rates of physical weathering are higher in cooler regions, particularly those that hover close to freezing. In such areas, frost wedging is a key weathering process, in which liquid water seeps into pores or fractures in rock and then freezes.

Both chemical weathering and physical weathering are maximized in wet environments. Frost wedging depends on availability of water, and the chemical process of carbonation requires both water and carbon dioxide. Water also can directly weather rock through hydraulic action or through production of acid rain. Areas with high salt content also experience increased weathering due to the phenomenon of salt wedging. When salt water seeps into rock, small fissures can be pried apart by growth of salt crystals when the water evaporates.