how soil is formed what is weathering types of weathering mechincla weathering chemical weathering how weathering cause soil formation

1. soil

2. Topic # 1
soil formation
• Soil :
“ the upper layer/weathered layer of earth crust
is called soil”
• Edaphology :
“ the study of soil is called edaphology “

3. How soil formed :
Over million of years, the forces of weathering and
erosion, slowly but constantly have produced a layer of
broken pieces of rock all over the earth’s surface. The
size of these rock pieces varies from boulders to the
finest bits. The layer composed of pieces of rock is
called ROCK MANTLE.
As the forces of wind and water go on breaking and
decomposing the rock mantle, a layer of fine loose
materials is formed. This is referred to as SOIL. It is
made up of tiny grains of rocks and materials.

4. • The natural processes of weathering and
erosion bring about important changes on the
surface rock. These processes consistently
break, carry off and deposit loose surface
materials from where they are formed to
other places. The continued action of water
and wind changes the rock pieces into finer
particles, thus forming boulders, gravel, sand
and clay.

5. • To these surface materials are added the
decayed remains of plants and animals called
HUMUS. Organic substances are returned into
the soul after they decompose. Because of
their finer texture, these enable the soil to
hold much water.

6. How soil is formed ?
Soil is formed by weathering
“the breakdown of soil is called weathering “
There are two types of weather
1. Mechanical weather
2. Chemical weather

7. Mechanical weathering
• Mechanical weathering includes processes that break rocks
to pieces without changing their chemical composition.
“the breakdown of rock by cracking through various agencies
is called mechanical weathering”
 It is brought about by severe environmental conditions
such as
I. Temperature
II. Rain water
III. Snow
IV. Wing
V. Plants and animals

8. Mechanical weathering
1. Temperature
Temperature influence the breaking of rock by expanidation an contraction of
rocks .During the day the rock are heated and then expand. At night, they are
cooled and then contract. Since rocks are composed of minerals, these
minerals expand and contract too. This alternate expansion and contraction
causes the rocks to crack, especially if the minerals expand and contract in
unequal amounts.
.

9. Mechanical weathering
.
water freezing also increase
Weathring .
Water may enter the cracks or crevices in rocks. When it freezes
it expands by about 9% of its volume, causing the rock to split.
Sometimes this alternate expansion and contraction may cause the rocks to peel off
in thin sheets. This peeling off process is called exfoliation

10. Mechanical weathering
• 2. Rain water
The rain water flowing on the rock also cause
the break down of this rock

11. Mechanical weathering
• 3. snow :
• Snow moves down the rocks from glaciers and
grinds the underlying particles and change to
soil

12. Mechanical weathering
• 4. wind :
• Wind carries small fragments of rocks from
one place to other and it help in soil formation

13. Mechanical weathering
• 5. plants and animals :
• Plant root grow inside rock and break it to
form soil
• Animal burrows which disintegrate the rocks

14. Chemical weathering
In chemical weathering, the composition
of the rock is altered. A new product which
easily crumbles to pieces is formed.
Chemical weathering is brought about by a
chemical reaction between the minerals in
the rocks and carbon dioxide, oxygen or
water.
Chemical weathering may cause due to
I. Hydrolysis
II. Hydration
III. Carbonation
IV. Oxidation
V. Solvent action

15. Chemical weathering
• Hydrolysis :
Break down of rock mineral by the action of water
• Hydration
Water dissociate into ions and H and OH reacts with different
compounds , and reaction cause the break down of rock
• Carbonation :
Water and CO2 react to form carbonic acid which alter the
composition of rock
• Oxidation
Rock which contain iron undergo oxidation which make it porous and
break able
• Solvent action
Water as a universal solvent dissolve some minerals and cause the
breaking of soil