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17/3/2006

Weathering

Weathering comprises the physical and chemical changes of rocks and minerals when they are exposed to, for example, water or biological activity.

Two types of weathering are usually considered, physical and chemical weathering.

Physical weathering can be described as disintegration of minerals and rocks into smaller particles, while the chemical weathering implies a more or less complete dissolution and that the chemical composition of the mineral and the rock, respectively, is changed.


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Weathering comprises the physical and chemical changes (mainly disintegration) of rocks and minerals when they are exposed to, for example, water or biological activity. Two types of weathering are usually considered, physical and chemical weathering. Physical weathering can be described as disintegration of minerals and rocks into smaller particles, while the chemical weathering implies a more or less complete dissolution of the material, and that the chemical composition of the mineral and the rock is changed. Physical weathering appears as the result of different forms of mechanical influence, for example ice formation, rapid changes in temperature, or a pressure caused by roots. Driving force for chemical weathering are liquids, where one of its qualities is its acidity. Sometimes biological weathering is defined as a third kind of weathering. Biological weathering can, though, be classified as either chemical weathering initiated by biological processes, or as physical weathering initiated through for example a pressure by roots.

Physical and chemical weathering are closely linked. By physical weathering the surface areas open to chemical attacks are enlarged, resulting in intensified chemical weathering. The chemical weathering can, in turn, create cavities where ice and roots can exert a pressure.

Chemical changes through chemical weathering imply dissolution of minerals and rocks, and formation of secondary minerals. In a long-term perspective chemical weathering, therefore, is the most important process for mobilisation of nutrient elements into forms easily available for plants. At chemical weathering hydrogen ions are consumed and, therefore, this process also counteracts accumulation of hydrogen ions and the decrease of pH in soils and water. When discussing weathering of mineral soils in connection to preserved production capacity or sensitivity to acidification, it is often chemical weathering that is intended.

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Factors affecting weathering

The rate of chemical weathering mainly depends on mineral composition, grain size distribution, climate, and substances stimulating weathering like acids and organic material. The most common minerals in the soil can be grouped according to increasing weathering capacity:

Mineral

 

 

 

(low)

quartz

 

|

muscovite, potassium feldspar

 

|

plagioclase

 

|

amphibole, pyroxene

 

|

biotite, chlorite, apatite

 

(high)

calcite

Regarding mineral composition the disposition to weathering can be grouped according to:

Rock type

 

 

 

(low)

quartzite, sandstone

 

|

porphyry

 

|

red gneiss & granite

 

|

grey gneiss & granite

 

|

greenstone (amphibolite, basalt, dolerite/diabase, hyperite, gabbro, diorite, porphyrite)

 

(high)

limestone, marble

The rate of weathering also depends on particle size. Specific surface area is, although, a more suitable term than particle size, because weathering, like most soil processes take place at the surfaces of particles. The specific surface area is defined as the total surface area of all particles in a certain amount of soil. For clay particles the specific surface area in general is about 10 - 50 m2/g mineral soil, and for particles in the sand fraction the corresponding figure is less than 1 m2/g mineral soil. In the B horizon in a sandy fine sandy till the specific surface area may be 5 - 10 m2/g. The greater part of the specific surface area in till is constituted of clay particles and humus. A seemingly small increase of clay content in absolute numbers from 2 to 4 per cent by mass almost corresponds to a doubling of the specific surface area. In principle weathering is proportional to the specific surface area. At natural conditions in forest soils there is, although, a risk that the influence of specific surface area is overestimated. A large surface area namely implies small particles and small pores, in turn making it difficult for soil water to move between the soil particles. From a weathering point of view this is a disadvantage because streaming lateral water in forest soils may speed up weathering when it brings acids and removes products from the weathering process.

Climate affects weathering through temperature as well as surplus water (the difference between precipitation and evapotranspiration, transpiration from plants included) trickling (percolating) the soil. The length of the period with frozen soils during winter is another important condition.

The amount of organic matter is decisive to chemical weathering of mineral soils. This is because of the acid qualities of the organic matter, as well as its ability to form complexes with metallic ions and, thus, to promote removal of products form the weathering process, resulting in a lowered ion concentration in the soil solution. At pH values of about 4 - 5, that are normal values for forest soils, the rate of weathering by organic acids are several times higher than the rate of weathering by inorganic, not complex forming acids, like for example sulphuric acid. This implies that a lowering of pH because of acid precipitation at field conditions in forests likely marginally, only, affects the rate of weathering.

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