Formation Processes

Most rocks exposed at the earth's surface are not chemically stable and are constantly undergoing the process of breakdown known as weathering, a process that involves both mechanical disintegration (physical weathering) and chemical decomposition (chemical weathering).  Weathering is the first stage in soil formation.

Chemical Weathering:

Involves the breakdown of the primary minerals in the rock to new secondary minerals that are more stable in the surface environment or to material that may be carried away in solution.  The extent to which the breakdown has occurred may be given as the index of weathering, usually expressed as the ratio of a common element like aluminum or iron present in the secondary mineral compared to the total present in the soil.  Water is the essential agent in chemical weathering, either reacting with the minerals directly or carrying dissolved species which themselves react with the minerals.  The reactions involved are many and complex, but they can be grouped into major categories as follows:

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Hydration is addition of the entire water molecule to the mineral structure and commonly occurs in clay minerals, sometimes causing them to swell.

Carbonation is the reaction with carbonic acid, which forms when carbon dioxide from the atmosphere dissolves in rain water:

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Oxidation is the bonding of oxygen, abundantly available dissolved in surface water, to the metallic elements (potassium, calcium, magnesium, and iron) of the primary minerals.  A common example is the formation of the rusty brown and orange oxides of iron on the surface of iron-containing rocks.  Thus in the case of the olivine mineral, fayalite (Fe2SiO4), iron released by hydrolysis (a), undergoes oxidation to ferric oxide (b):

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Ion-exchange involves the transfer of charged atoms (ions) of calcium, magnesium, sodium, and potassium between waters rich in one of the ions and a mineral rich in another.  It is particularly important  in the alteration of one clay mineral to another (e.g., illite, the K-rich clay mineral may lose potassium into solution and take up Mg²⁺ to form Montmorillonite).

Chelation is the taking of metal atoms or ions into hydrocarbon molecules and relates to the biological processes that take place in soil formation.

Common secondary minerals produced on weathering of major rock-forming silicate minerals are shown in Fig. 11.1.   This figure also emphasizes the fact that the different primary minerals weather at different rates, i.e., their weatherability as shown in the figure, is highly variable.  Although variable, it is systematic and the resistance to weathering of the primary silicates can be rationalized in terms of their crystal structures.  For example, olivine, a mineral that contains SiO₄ tetrahedra linked by Fe or Mg ions, is much less resistant than quartz which is made up of SiO₄ tetrahedra linked by their corners to form a complete framework of these stable units.   Thus, quartz has a very low solubility.  Because it is also hard, resists abrasion, and is a common primary mineral in many rocks, it is a common constitute of soils.

Figure 11.1

As figure 11.1, clearly shows, the secondary minerals that result from weathering processes depend directly on the nature of the primary minerals and hence the type of underlying bedrock.  Therefore, rock-type (parent material) exercises a major control over the kind of soil that forms.  The typical weathering products of a granite are given in Table 11.1 and show that in chemical weathering, many reactions are taking place simultaneously.  It is also important to note that many of the rocks exposed at the surface of the earth are sediments that are already made largely of the secondary minerals in Fig. 11.1, redeposited after one (or more) weathering cycles.

Table 11.1

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Climate is second only in importance to parent material as a controlling factor in soil formation.  The climate controls weathering and soil formation directly, through the amount of precipitation and the temperature, and indirectly through the kinds of vegetation that can cover the land.  The importance of climate can be illustrated by considering four contrasting examples:

Humid tropical climates lead to intense chemical weathering that produces soils largely made of insoluble residues - iron oxides (laterites) and aluminum oxides (bauxites).  The process of removal of metal atoms in forming bauxite from an original igneous rock may follow a sequence of the type shown below for the breakdown of the potash feldspar resent:

4KAlSi3O8     +    4H+         +   18H2O   ------->   Al4Si4O10(OH8)  +   8H4SiO4    +     4K+

Feldspar + H ions in solution + water   ------>  kaolinite + silicic acid in solution + K ions in solution

Humid midlatitude climates with seasonal freezing allow much greater accumulation of vegetational debris--a humus layer--and dissolved species may not be removed but recombine to form stable clay minerals. 

Hot arid climates allow for the growth of little vegetation and provide too little water to permit much chemical weathering.   Consequently, such regions often do not develop true soils.  Instead, salts may be left at or near the surface from the evaporation of the little available water, or a variety of rock like crusts may form such as the calcium carbonate-rich calcrete (or caliche).  Weathering involves rapid mechanical and chemical breakdown of the less resistant silicates.  Clays may be blown away to leave only sands made up largely of quartz.

Cold climates may also be very dry because all the water has turned into the solid form (snow, ice, frost) and is useless for chemical weathering.   The biological activity of plants and microorganisms is also much reduced, although the slow rate of decay of organic material can lead to its accumulation forming peat bogs and the thick peat accumulations found in Canada and known as muskeg.  Mechanical breakdown (by frost wedging) is the major weathering process.

The degree to which biological processes, chiefly involving vegetation and microorganisms, contribute to soil formation is dependent on   the temperature and available moisture.  Living plants take up certain elements (as essential nutrients), and these are returned to the surface soil when the plant sheds its leaves or dies.  Plants also control the moisture content of the soil by transpiring water and serve to protect soils from erosion.  Animals living in (or burrowing into) the soil may also play an important role, for example, earthworms rework the soil by burrowing and passing the soil through their intestinal tracts.  These biological processes are, in turn, influenced by climate and by parent material from which the soil forms, since these control the development of vegetation which, in turn, may permit animals to flourish.  A soil is, therefore, a complex and constantly changing (or dynamic system) in which many interacting physical, chemical, and biological processes are going on at the same time.

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In physical weathering, rocks are broken down to smaller pieces by various natural agencies, the importance the importance of which will depend on the type of rock being weathered and the climate under which weathering occurs.  Wind, rain, frost action, and the differential expansion and contraction during rapid heating and cooling all contribute to the breaking down of rocks.   Most rocks already contain planes of weakness or planes along which fracturing has occurred.  The release of the confining pressure of overlying rocks, when material originally formed at depth is exposed at the surface, causes expansion, fracturing, and the formation of joints.  Joint planes are also formed when an igneous rock cools.   Bedding planes (original sedimentary layers) in sedimentary rocks, or fracture planes introduced into rocks when major earth movements (tectonism) or minor movements (such as landslips) occur, are other examples of planes of weakness.  In the   broad temperature belts, frost wedging is probably the most important physical weathering agent.  When water, trapped in fractures or pore space in rocks at the surface, freezes, its volume increases by about 9 percent.  The maximum pressure force that can be generated when confined water freezes is about 2100 tons per square foot (about 40 times greater than the force needed to break and average granite).   Although such maximum forces do not occur, because ice itself is not strong enough to seal water into a rock crack, frost wedging does produce stresses capable of disintegrating the hardest rocks.  Plants and animals can also contribute significantly to physical weathering through the wedging action of plant and tree roots and the activities of burrowing animals.

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