Weathering (1) for the soil science in india

For the soil of the weathering

1. Chemical & Mechanical
Weathering
How is soil formed?

2. Weathering-the disintegration
and decomposition of rock at or
near the surface of the earth.It
affects the rocks in place and no
transport is involved.This
distinguishes weathering from
erosion.

3. Mechanical/physical weathering-
physical disintegration of a rock
into smaller fragments, each with
the same properties as the
original.Occurs mainly by
temperature and pressure changes.

4. Chemical weathering-process by which the
internal structure of a mineral is altered by the
addition or removal of elements.Change in
phase (mineral type) and composition are due
to the action of chemical agents.Chemical
weathering is dependent on available surface
for reaction temperature and presence of
chemically active fluids.

5. Types of Mechanical Weathering:
Frost Wedging-water expands when it
freezes.This photograph shows the individual
layers within the sedimentary rock breaking
apart through repeated cycles of freeze-thaw.

6. Thermal Expansion and Contraction-
heating causes rock to expand, cooling
results in contraction; different minerals
expand and contract at different rates.

7. Mechanical Exfoliation-rock breaks apart
in layers that are parallel to the earth's
surface; as rock is uncovered, it expands
(due to the lower confining pressure)
resulting in exfoliation.

8. Abrasion-physical grinding of rock
fragments.Here, the photo shows some
pits that have been eroded into the rock by
sandblasting.
Another photograph which shows
the powerful effect of wind
generated abrasion is the Double
Arch from Arches National Park.

9. Types of Chemical Weathering:
Dissolution
Dissolution is very common in areas that have a
great deal of limestone.Acidic waters (from
pollution or natural) dissolve limestone
allowing for additional water to gain entrance.
H
2
O + CO
2
+ CaCO
3
--> Ca
+2
+ 2HCO
3
-
water + carbon dioxide + calcite dissolve into calcium ion
and bicarbonate ion

10. Oxidation(rust)
4Fe
+2
+3O
2
--> 2Fe
2
O
3
ferrous iron + oxygen combine to form ferric
iron oxide (hematite)
Will happen to all iron-bearing silicates to
varying degrees.Common reaction minerals
are hematite, limonite, and goethite.

11. Hydrolysis
2KAlSi
3
O
8
+ 3H
2
0 --> Al
2
Si
2
O
5
(OH)
4
+
4SiO
2
+ 2K(OH)
Silicate minerals (unstable at the earth's
surface) weather to form clay minerals.

12. Factors that influence chemical
weathering:
1.Climate
2.Living Organisms
a.decomposition
3.Time
4.Mineral composition
a. Chemical Weathering Products
b.Clays
c.Metals ores
d.Rounding of boulders

13. Soils and Soil Formation
Dependence of weathering type on the mean
temperature and annual rainfall.
Weathering rates depend on the composition of the
rock, temperature range and rainfall amount.
Weathering producessoils.
Soils may or may not remain in place, and any soil
may be a combination of residual and transported
material.

14. Residual soil: Remains in place; has not been
transported (gruss).
Transported soil: Transported by wind or
water and deposited.

15. A complete soil profilewill have the following components:
O horizon:Organic debris and leaf litter on the surface.
A horizon: Topsoil -leaching, water movement down, Organic
and Mineral material transported downward.
B horizon: Subsoil -accumulation of dissolved material and
fine clays, hardpan.
C horizon: Partially altered parent rock material.
Bedrock:Unweathered parent rock material.

16. Factors in Soil Formation
1.Climate: The greater the rainfall amount, the more
rapid the rate of erosion and leaching.
2.Topography: The steeper the surface slope, the more
likely any eroded material is to be transported out of the
system.
3.Parent Material: Sandstones are more resistant to
weathering than limestone in humid climates, but
limestone are more resistant than sandstones in arid
climates.
4.Plant and Animal activity: Plant and animal activity
produces acids that are powerful erosion agents.
5.Time: Reaction rates are slow, the longer a rock unit
has been exposed, the more likely it is to be weathered.