Formation of Soils from Parent Materials

Soils vary greatly from one location to another around the world. This variation can largely be understood in terms of the way in which certain environmental factors of soil formation affect the four basic processes by which geologic parent materials are changed into soils.

Weathering of Rocks and Minerals

The earth's mantle was originally covered by water and solidified magma, the latter being comprised of igneous rocks such as granite and gabbro.  These rocks in turn were made up of mixtures of specific primary minerals such as quartz, feldspars, micas, hornblende, and biotite.  Weathering broke down the rocks and minerals, destroying some in the process, but also synthesizing new ones (secondary minerals).  Some of the weathering products were transported by water and wind to new locations where they become sediments in lake or ocean bottoms.  These sediments were later recemented into sedimentary rocks such as sandstone and shale.  Some igneous and sedimentary rocks were later subjected to high pressures and temperatures bringing about a metamorphosis (change) in form and structure, and creating metamorphic rocks.  Gneiss, schists, and slate are examples of metamorphic rocks.

Weathering Processes

  1. Physical weathering (disintegration) results in physical breakdown of the rock/minerals into smaller particles without significant change in chemical composition.  Disintegration is enhanced by temperature changes and differential expansion of minerals, frost action and peeling of layers from the parent mass (exfoliation).  Erosion, transport, and deposition by wind, water, and ice are physical processes that grind and break down the rock and mineral particles.
  2. Biogeochemical weathering (decomposition) results in the destruction of the primary minerals and in some cases the simultaneous generation of secondary minerals such as silicate clays.  The latter are formed either by alteration of primary minerals or by recrystallization of decay products into new minerals.  Biogeochemical breakdown is enhanced by the (a) hydrolysis, or the destruction of a mineral by reaction with H+  and OH- ions in water and (b) hydration or the chemical combining of the mineral with intact water molecules, (c) complexation of metal cations by organic ligands.   Oxidation of elements such as iron or manganese in minerals helps break down rocks and releases secondary minerals. Carbonation and other reactions utilizing acids formed by plants and microbes (e.g., H2CO3, HNO3, and H2SO4 and some organic acids) result in the formation of secondary minerals such as silicate clays. All these reactions require or are enhanced by the presence of water, and many of them may lead to the synthesis of secondary minerals.

Five Factors of Soil Formation

Parent material

Climate

Living organisms

Topography

Time

Parent material

Are the starting point for soil development, and their nature profoundly influence soil properties, especially in areas where chemical weathering has not destroyed or greatly modified the original minerals.  Soil texture is often determined by the nature of the parent materials as are the rate and nature of some weathering processes.  For example, limestones resist the acidifying processes while shales resist the rapid downward movement of water.  Also, the type of silicate clays formed in a given area are commonly related to the nature of the parent materials.

Parent materials may have been formed in place from the native rock (residual or sedentary), or they may have been transported from one area to another by water, ice, wind or gravity.  Residual parent materials are common in upland areas that have not been covered by materials transported from elsewhere.  They are the most extensive of all parent materials.

costal.jpg Water-transported materials may have been deposited in former lake bottoms (lacustrine), alongside streams (alluvial), or in ocean bottoms that have since been elevated (marine).  If deposited near the seashore or river bank they are generally coarse textured, but if deposited further from the shore and in still water, they are commonly fine in texture giving rise to soils high in silt and clay.

 

 

 

 

 

 

 

 

 

 

 

Parent materials transported by glaciers (ice) that once covered much of the world's northern latitudes includes a mixture of rocks and minerals. Deposited by the melting glaciers, these heterogenous materials are known collectively as glacial till.  Some till is found in irregular deposits known as moraines.  Coarse textured glacial outwash materials are another locally important parent material.

 

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Parent materials transported by glaciers (ice) that once covered much of the world's northern latitudes includes a mixture of rocks and minerals. Deposited by the melting glaciers, these heterogenous materials are known collectively as glacial till.  Some till is found in irregular deposits known as moraines.  Coarse textured glacial outwash materials are another locally important parent material.

 

glacier_dep_2.jpg (Top) Tongues of a modern-day glacier in Canada. Note the evidence of transport of materials by the ice and the "glowing" appearance of the major ice lobe. (Bottom) This U-shaped valley in the Rocky Mountains illustrates the work of glaciers in carving out land forms. The glacier left the valley floor covered with glacial till. Some of the material gouged out by the glacier was deposited many miles down the valley.

 

 

 

 

 

 

 

 

 

 

 

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Wind blown (eolian) materials have resulted from the action of strong winds that carried fine-textured materials from one area to another. These include, in order of decreasing particle size, dune sands, loess, and aerosolic dusts. Parent materials that move down hill slopes by gravity (colluvium) are not very extensive, but are locally important. Another type of locally important parent material is the accumulated organic debris of partially decomposed plant tissues in which organic soils form, mainly in bogs and other wet areas.

Climate

Largely determines the nature of the weathering that occurs and profoundly influences the living organisms found in an area.  Temperature and precipitation affect the rates of chemical, physical, and biological processes responsible for weathering and for soil profile development.  For every 10oC rise in temperature the rates of biochemical reactions double. Also, weathering reactions involve water. Effective precipitation in soil formation is the water that moves through the regolith. The greater the effective precipitation, the greater is the development of the soil profile.  As a result, soils in desert areas are generally relatively shallow, being influenced mostly by mechanical weathering and by less intensive chemical reactions.  Many soils of the humid tropics are deep, and in them most primary minerals have been destroyed by intensive weathering.

Climate also has a profound influence on the natural vegetation. By its effects on temperature, moisture, natural vegetation, and soil organisms, climate plays a major role in determining weathering patterns and in influencing the kinds of soils that develop.

Living organisms

Organisms, especially natural vegetation, influence the development of soil characteristics in several ways.  First, they are sources of organic matter essential for soil chemical and physical properties.  The accumulation of organic matter in the upper layers of soil is one of the first steps in the development of soil profiles.  Also, living organisms facilitate the cycling of essential plant nutrients. The nutrients are absorbed from the soil by plants, are returned to the soil surface in plant residues, and then back to the soil when soil organisms decompose the plant residues.  Earthworms, termites, and rodents act as mixing agents, moving weathered materials and organic matter up and down the soil profile.  Natural vegetation also helps stabilize the soil and protect it from the ravages of soil erosion.

Soils of grasslands tend to have higher organic matter contents than forested soils. This results in generally more stable soil aggregates in areas with natural grassland vegetation.  Soils developed under pine forests are generally more acid than those formed under deciduous forests since pine tree needles are lower in non-acid cations such as Ca2+  , Mg2+,  and K+.

Topography

Topography influences the rate of runoff and erosion from the soil surface and consequently the infiltration of the water. Consequently, soils on hillsides are commonly not as deep as those on flat terrain, but the removal of excess water is more difficult in the flat lands.  Excess water in depressed areas leads to the formation of peat bogs and in turn organic soils.

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Time

The amount of time that parent materials have been subject to weathering and soil forming processes influences soil properties.  Residual parent materials have generally been subjected to soil forming processes longer than transported parent materials.  Recently deposited alluvium alongside a stream has had too little time for significant soil profile development to take place.  Likewise, soils developed from glacial parent materials and coastal plain areas are generally less weathered than those developed from residual materials.