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 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 (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.