Page 85 - Science Class 07

and pressure; erosion and transporta on by and temperature. This is called chemical
wind, water and glaciers; chemical ac on such
as crystal growth, oxida on, hydra on,
carbona on and leaching by water, especially
rainwater, with me.
Natural process that helps in breaking of rocks
is called weathering.
Let’s study about some natural factors of
weathering:
v Flowering water and winds breaks down
the rocks into smaller par cles.
v In some cases, it has been found that the
roots of vegeta on, penetra ng deep
down in of vegeta on, penetra ng deep
down in search of water and nutrients
push the rocks apart. In this way, rocks
break down in smaller par cles.
v Some mes, when it rains, water enters the
cracks or holes of the rocks. When this
water freezes due to very low temperature,
it expands. Because of the expansion, the
rocks are pushed apart and a mechanical
stress is exerted. It leads to disintegra on of
rocks into smaller par cles.
Weather of Rocks
v Natural chemical process is also respon-
sible for the breaking down of rocks. As
you know rocks contain different types of
minerals, these minerals change from one
type to another by flowing water or rain
Science-7
weathering. This process largely depends
on climate of a par cular place. The soil
produced in a warm, moist climate is more
mature and developed than that produced
in cool climate.
v Climate, presence of minerals in parent
rocks, quan ty of water available and
microorganisms found at a par cular place
are some important factors that determine
the type of soil.
v With the passage of me, many other
things mix up with smaller par cles of the
rocks such as humus, water, minerals and
microorganisms. In this way soil is formed.
SOIL PROFILES
There are different types of soil, each with its
own set of characteris cs. Dig down deep into
any soil and you’ll see that it is made of layers,
or horizons (O, A, E, B, C, R). Put the horizons
together and they form a soil profile. Like a
biography, each profile tells a story about the
life of a soil. Most soils have three major
horizons (A, B, C) and some have an organic
horizon (O).
The horizons are:
v O – (humus or organic) Mostly organic
ma er such as decomposing leaves. The O
horizon is thin in some soils, thick in others
and not present at all in others.
v A - (topsoil) Mostly minerals from parent
material with organic ma er incorporated.
A good material for plants and other
organisms to live.
v E – (eluviated) Leached of clay, minerals,
and organic ma er, leaving a concentra on
of sand and silt par cles of quartz or other
resistant materials – missing in some soils,
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