Page 71 - Fundamentals of Geomorphology

54 INTRODUCING LANDFORMS AND LANDSCAPES

is known as salt weathering (Wellman and Wilson water molecules on their edges and surfaces, or, for simple
1965).Whensaltcrystalsformedwithinporesareheated, salts, in their crystal lattices, without otherwise changing
or saturated with water, they expand and exert pressure the chemical composition of the original material. For
against the confining pore walls; this produces ther- instance, if water is added to anhydrite, which is calcium
mal stress or hydration stress respectively, both of which sulphate (CaSO 4 ), gypsum (CaSO 4 .2H 2 O) is produced.
contribute to salt weathering. The water in the crystal lattice leads to an increase of
volume, which may cause hydration folding in gyp-
Chemical weathering sum sandwiched between other beds. Under humid
mid-latitude climates, brownish to yellowish soil colours
Weathering involves a huge number of chemical reac- are caused by the hydration of the reddish iron oxide
tions acting together upon many different types of hematite to rust-coloured goethite. The taking up of
rock under the full gamut of climatic conditions. Six water by clay particles is also a form of hydration. It leads
main chemical reactions are engaged in rock decom- to the clay’s swelling when wet. Hydration assists other
position: solution, hydration, oxidation and reduction, weathering processes by placing water molecules deep
carbonation, and hydrolysis. inside crystal structures.

Solution Oxidation and reduction
Mineral salts may dissolve in water, which is a very Oxidation occurs when an atom or an ion loses an
effective solvent. The process, which is called solu- electron, increasing its positive charge or decreasing
tion or dissolution, involves the dissociation of the its negative charge. It involves oxygen combining with
molecules into their anions and cations and each a substance. Oxygen dissolved in water is a preva-
ion becomes surrounded by water. It is a mechanical lent oxidizing agent in the environment. Oxidation
rather than a chemical process, but is normally dis- weathering chiefly affects minerals containing iron,
cussed with chemical weathering as it occurs in partner- though such elements as manganese, sulphur, and tita-
ship with other chemical weathering processes. Solution nium may also be oxidized. The reaction for iron,
is readily reversed – when the solution becomes saturated which occurs mainly when oxygen dissolved in water
some of the dissolved material precipitates. The satura- comes into contact with iron-containing minerals,
tion level is defined by the equilibrium solubility, that is, is written:
the amount of a substance that can dissolve in water.
It is expressed as parts per million (ppm) by volume 4Fe 2+ + 3O 2 + 2e → 2Fe 2 O 3 [e = electron]
or milligrams per litre (mg/l). Once a solution is satu-
rated, no more of the substance can dissolve. Minerals Alternatively, the ferrous iron, Fe 2+ , which occurs in
vary in their solubility. The most soluble natural miner- most rock-forming minerals, may be converted to its
als are chlorides of the alkali metals: rock salt or halite ferric form, Fe 3+ , upsetting the neutral charge of the
(NaCl) and potash salt (KCl). These are found only in crystal lattice, sometimes causing it to collapse and
very arid climates. Gypsum (CaSO 4 .2H 2 O) is also fairly making the mineral more prone to chemical attack.
soluble, as is limestone. Quartz has a very low solubility. If soil or rock becomes saturated with stagnant water,
The solubility of many minerals depends upon the num- it becomes oxygen-deficient and, with the aid of anaero-
ber of free hydrogen ions in the water, which may be bic bacteria,reductionoccurs.Reductionistheopposite
measured as the pH value (Box 3.2). of oxidation, and the changes it promotes are called
gleying. In colour, gley soil horizons are commonly a
Hydration shade of grey.
The propensity for oxidation or reduction to occur
Hydration is transitional between chemical and is shown by the redox potential, Eh. This is measured
mechanical weathering. It occurs when minerals absorb in units of millivolts (mV), positive values registering