Weathering and Weathering Series
Weathering and weathering series
Weathering is the in situ (in position) breakdown of rocks by natural forces into sediments or chemical constituents. Weathering may be physical or chemical. Physical weathering is the mechanical disintegration of rocks into finer particles. Chemical weathering is the decomposition of rocks according to the weathering series, a list of minerals arranged in order of their relative chemical stability at the earth's surface.
Chemical weathering occurs via a variety of processes such as dissolution, oxidation, hydration, or carbonation. These processes alter minerals at the molecular level either producing, as weathering products, different minerals or non-mineral chemical constituents. Based on their chemical stability, some minerals are more susceptible to the agents and processes of chemical weathering than others. A mineral's stability is determined to a large extent by the conditions under which it formed. Many igneous and metamorphic minerals that equilibrated deep within the earth will be less stable in the very different conditions found at the surface of the earth. These minerals will then be more susceptible to the agents of chemical weathering. This susceptibility follows a general progression called the weathering series. Below is the weathering series with the least stable minerals, the ones that will weather first, at the top. It progresses downward toward the more stable and long-lived minerals.
- Olivine—Calcic plagioclase
- Hornblende—Alkalic plagioclase
- Potassium feldspar
The minerals in the weathering series are essentially the same as Bowen's Reaction Series , the order in which minerals crystallize from magma . It differs, however, in that the weathering series is not a successive progression of weathering products. It does not mean, for example, that olivine will break down to form augite, merely that olivine will tend to decompose before augite, if both are present.
Physical weathering is the mechanical fragmentation of rock in place. It is differentiated from erosion, or mass wastage, which involve the transport of material. Physical weathering is accomplished dominantly by the processes of expansion and cracking due to the unloading of pressure and expansion from crystal growth. Unloading is the release of lithostatic pressure experienced by a body of rock after it has been uplifted to the surface of the earth where the pressure is much less than where it formed deeper within the earth. This
pressure change will cause a rock to expand in all directions, dislodging grain boundaries. Often, this type of expansion results in concentric fractures that cause curved portions of the rock to slough off, or exfoliate. Crystal growth includes ice formation. Water that has permeated a rock, if frozen, will expand and create fractures. A rock can also experience secondary mineral growth, often of evaporite minerals that were transported in solution and infiltrate the rock by capillary action. Other minor agents of physical weathering include vegetation, which can create fractures by root growth, and thermal expansion of rock caused by climate changes.
The rate of rock weathering, whether chemical or physical, is influenced by the type of rock, climate, topography , and vegetation. The type of rock includes the mineralogy , which determines where in the weathering series the rock lies. It also includes lithologic structures such as number and size of fractures or bedding planes, both of which can be sites for focused weathering activity. Climate influences the rate of weathering as well as which type of weathering processes will predominate. For example, frost heaving will not be factor in a very warm climate, and a moist climate will experience more chemical weathering than a dry one. Topography determines how much rock will be exposed to the elements. It also influences the amount of vegetation that may take hold. Vegetation root systems, in addition to physically weathering rock, also produces carbon dioxide and humic acid, two chemical weathering agents.
See also Soil and soil horizons