What is weathering and its types

Weathering of rocks

Under the influence of the atmosphere, rocks are subject to physical (mechanical) and chemical processes of destruction, which are referred to as rock weathering. The type of weathering depends on the climate (temperatures and temperature fluctuations, air humidity) and the agents acting on the rock (oxygen, trace gases, acids, water).

 

I. Physical (mechanical) weathering

1. Frost blast: The freezing of water is associated with an increase in volume of about 9%. At –22 ° C, H2O exerts a pressure of 2,100 kg / cm² (~ 200 N / mm²). In order for the frost splitting to take effect, the pore and capillary space of a rock must be at least 91% filled with water.


2. Temperature weathering: Due to the different heating and the different expansion of the minerals during insolation (solar radiation), tensions are generated in the rock, which lead to the destruction of the structure. Daily temperature fluctuations also have the same effect.


3. Salt weathering: The crystallization of salts in hairline cracks in the rock creates a crystallization pressure (1000 kg / cm² / ~ 100 N / mm²) which, like frost blasting, destroys the structure of the rock. Hydrated crystals of some salts take up to 300% larger volume than their aqueous solution (e.g. CaSO4, Na2SO4, MgSO4, Na2CO3).


4. Physico-biological weathering: The turgor pressure of plant cells (›10 kg / cm²), e.g. root network, destroys the structure of the rock.

 

II. Chemical weathering

1. Solution weathering: Minerals, especially salts, are dissolved in water, e.g. the solubility of gypsum is 2.5 g / l.


2. Carbonic acid weathering: Carbon dioxide forms carbonic acid when dissolved in water. Atmospheric carbon dioxide dissolves in the rain, so that normal rainwater has a pH value around 5.6. Carbonic acid reacts chemically with calcium carbonate (calcite) and water-soluble calcium hydrogen carbonate is formed. 1l of water dissolves 14mg CaCO3 at 20 ° C, the amount increases fivefold when carbon dioxide is dissolved in the water.

CaCO3 + H2CO3 -> Ca (HCO3) 2


3. Flue gas weathering: The anthropogenic flue gases such as sulfur dioxide (SO2) and nitrogen oxides (NOx) produced by burning fossil fuels (crude oil, lignite and hard coal) form strong acids in combination with water, which are able to dissolve minerals and thus destroy the structure of the rock.

SO2 + 2 H2O -> H3O + + HSO3-


4. Oxidative weathering: is based on the oxidizing effect of atmospheric oxygen with the contribution of water; this breaks down minerals.

4 FeCO3 + 6 H2O + O2 -> 4 FeOOH + 4 HCO3- + 4 H +
Bivalent iron (from the siderite) is converted into trivalent iron

4 FeS2 + 14 O2 + 4 H2O -> 4 FeSO4 + 4 H2SO4
Bivalent-negative sulfur (from pyrite) is converted to tetravalent positive


5. Hydrolytic Weathering: By absorbing water into the crystal lattice of a mineral (hydration), ions can be released from the crystal lattice. This form of chemical weathering occurs with silicates and is therefore also called silicate weathering.


6. Chemical-biological weathering: Acid secretions of bacteria, fungi, algae, lichens etc. dissolve minerals and thus destroy the structure of the rock.