HOW THE SOILS ARE FORMED?
Soil is defined as a natural aggregate
of mineral grains, with or without organic constituents, that can be
separated by gentle mechanical means such as agitation in water. By
contrast rock is considered to be a natural aggregate of mineral
grains connected by strong and permanent cohesive forces.
The process of weathering of the rock
decreases the cohesive forces binding the mineral grains and leads to
the disintegration of bigger masses to smaller and smaller particles.
Soils are formed by the process of weathering of the parent rock.
The weathering of the rocks might be by
mechanical disintegration, and/or chemical decomposition.
Mechanical Weathering
Mechanical weathering of rocks to
smaller particles is due to the action of such agents as the
expansive forces of freezing water in fissures, due to sudden changes
of temperature or due to the abrasion of rock by moving water or
glaciers.
Temperature changes of sufficient
amplitude and frequency bring about changes in the volume of the
rocks in the superficial layers of the earth's crust in terms of
expansion and contraction. Such a volume change sets up tensile and
shear stresses in the rock ultimately leading to the fracture of even
large rocks.
This type of rock weathering takes
place in a very significant manner in arid climates where free,
extreme atmospheric radiation brings about considerable variation in
temperature at sunrise and sunset.
Erosion by wind and rain is a very
important factor and a continuing event. Cracking forces by growing
plants and roots in voids and crevasses of rock can force fragments
apart.
Chemical Weathering
Chemical weathering (decomposition) can
transform hard rock minerals into soft, easily erodable matter.
The principal types of decomposition
are hydmtion, oxidation, carbonation, desilication and leaching.
Oxygen and carbon dioxide which are always present in the air readily
combine with the elements of rock in the presence of water.
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