2.1: Weathering
Physical Weathering
Physical weathering occurs when existing rocks are mechanically broken into smaller pieces with little or no chemical change. The most important types of physical weathering include freeze-thaw (expansion of cracks via freezing of water), biological activity, changes in volume via wetting and drying (especially important with mudrocks), and stress relief weathering. Locally, physical weathering via thermal expansion and contraction and the growth of salt crystals may be important in deserts and coastal areas, respectively.
Chemical Weathering
Chemical weathering is the chemical and/or mineralogical alteration of a rock; it can take place via in situ alteration of minerals or by the removal/relocation of dissolved substances.
Solution (aka dissolution) is a type of chemical weathering that takes place when material is dissolved in water, a process that is enhanced if the water is slightly acidic. Common examples of dissolution include halite in water and the dissolution of marble by acid rain.
Hydrolysis is a type of chemical weathering that takes place when acid and water react with a mineral to form clay minerals, silica, and to release ions into solution (commonly K, Na, and Ca). The most common example is the weathering of feldspar to form clay, a process that results in large amount of fine-grained particles and dissolved material.
Oxidation occurs when compounds lose or share electrons with oxygen, the transformation from pyrite to hematite is a common and important example.
The processes of physical and chemical weathering commonly act together to speed the breakdown of rocks and minerals. Fractures formed by physical weathering can dramatically increase the surface area of a rock. The increased surface area exposes more of the rock to chemical weathering, which may result in further physical destruction of the rock. This interrelationship of physical and chemical weathering can be seen when fractured blocks develop rounded corners ( spheroidal weathering ); the increased surface area to volume ratio near the corners results in preferential weathering. Deeply weathered rocks commonly develop a rounded shape because a sphere has the lowest surface area to volume ratio of any shape.
Some Generalizations
CL imate, O rganisms, R elief, P arent material, and T ime are the variables that influence the weathering of rock and the creation of the resulting soils; the acronym CLORPT is commonly used to remember these processes (Jenny, 1941, Factors of Soil Formation). Climate, particularly temperature and precipitation, probably exerts the most profound influence by controlling the type, speed, and nature of chemical weathering. Despite the complexities inherent in the system, generalizations can be made:
- Because rocks and minerals are most stable under conditions closest to those under which they formed, Bowen’s Reaction Series provides a general framework for predicting igneous rocks' resistance to chemical weathering. Iron-rich ultramafic rocks formed at high temperatures are particularly prone to chemical weathering; silica-rich felsic rocks and minerals formed at relatively low temperatures tend to be the most resistant.
- Quartz, rutile, anatase, magnetite, and zircon are some of the most common stable minerals formed from melts.
- With the notable exception of chert, minerals formed by precipitation (halite, gypsum, etc.) tend to be easily dissolved and prone to weathering.
- Feldspars are prone to hydrolysis and are rapidly destroyed in humid climates.
- Sandstones are common ridge-formers in humid climates; Carbonates are common ridge-formers in arid and semi-arid climates but are valley-formers in humid climates
- Generally speaking, increasing the temperature and the amount water tends to increase chemical weathering rates.