Download class note geography (weathering)

CLASS NOTE OF
GEOGRAPHY
WEATHERING
WHAT IS WEATHERING
• Weathering is the
general term applied
to the combined
action of all processes
that cause rock to
disintegrate physically
and decompose
chemically because of
exposure near the
earth surface
WEATHERING
• According to Ollier
(1974)
WEATHERING is
the breakdown and
alteration of
materials near the
earth surface to
products that are
more in equilibrium
with newly
imposed physicochemical condition
CONTROLLING FACTORS
• Hardness of rocks and minerals.
• Presence of joints, foliation planes
and Bedding planes
• Nature of ground slopes.
• Changing nature of humidity
• Chemical composition of rocks
• Floral effects
• Time
JOINTS IN BEDROCKS
CLASSIFICATION OF
WEATHERING
1. Mechanical or Physical weathering
2. Chemical weathering
3. Biological weathering
MECHANICAL OR PHYSICAL
WEATHERING
A. Mechanical weathering due to
crystallisation
B. Mechanical weathering involving
super incumbent pressure release
C. Mechanical weathering due to
change of temperature
D. Mechanical weathering involving
chemical change
MECHANICAL WEATHERING
DUE TO CRYSTALLISATION
1. FROST ACTION
2. SALT CRYSTAL GROWTH
FROST ACTION
Most important physical
weathering in cold
climates
When water freezes in
bedrock joints and
bedding planes,it
expands and can split
rock apart
Areas:i)Mountain
summits
ii)Arctic Tundra
iii)Pore spaces
of soil in cold climates
SALT CRYSTAL GROWTH
• Salt crystal
growth can
break rocks
apart grain by
grain,
producing
niches, shallow
caves and rock
arches
Mech weathering involving super
incumbent pressure release
• Mech weathering due to unloading: Rock
disruption due to relief of confining pressure of
underlying rock
• Exfoliation is another term for unloading
• Areas: Rock brought near the surface by erosion
of overlying layers, where rocks expand slightly in
volume
• Expansion causes thick shells of rocks to break
free from the parent mass below and form jointing
and sheeting structure and exfoliation domes
Unloading
Mech weathering due to
change of temperature
• Mech weathering due to expansion and
contraction in rock forming minerals
• Due to intense heating of sun
alternating with nighty cooling, it
exerts powerful disruptive forces on
rocks
Mechanical weathering
involving Chemical change
• Mechanical weathering occurs due to
hydration and oxidation, associated
with changes in volume and resultant
pressure in rocks.
Other causes
• Mechanical weathering also involves wind action:
outer shells of rocks become loose due to
temperature changes and are removed by strong
winds
CHEMICAL WEATHERING
• Chemical weathering involves
decomposition and disintegration of
rocks and involves weathering away
of minerals in rocks. Water vapour
and water are the media of chemical
reactions
Chemical weathering
•
•
•
•
•
•
A. Solution
B. Oxidation
C. Carbonation
D. Hydration
E. Hydrolysis
F. Chelation
Chem weathering-SOLUTION
• First step in chem decomposition. It
refers to the dissolution of soluble
particles and minerals from the rocks
with the help of water. Limestone is
very much susceptible to sol process
in the presence of CO2 content of
water and pH of the solution.
• CaCO3+H2O+CO2—Ca(HCO3)2
Chemical weathering-OXIDATION
• Decomposition by oxidation changes
strong rock into very weak regolith.
• Water mixed with atmospheric oxygen in
contact with iron bearing rocks; iron
oxidises to form ferrous oxides (FeO),
then Ferric oxides(Fe2O3) or Ferric
hydroxides (Fe(OH)2)
• 4FeO+3H2O+O2---2Fe2O3.3H2O…
(Rusting of Rocks)
CHEMICAL WEATHERING
Chemical weathering--Carbonation
• Carbonation is the reaction of carbonate
or bicarbonate ions with minerals. This
process is also known as SOLUTION.
Chemical weathering-Hydration
• Hydration is related to the addition of
water to the minerals. It is an
exothermic change
Chemical weathering-Hydrolysis
• Hydrolysis is a chemical reaction
between mineral and water, i.e.
between Hydrogen (H) ions or
hydroxyl (OH) ions and the ions of
the mineral Rocks and water
molecules decompose
Chemical weathering-Chelation
• Chelation is a complex organic
process– metallic cations are
incorporated into hydrocarbon
molecules.
• Chelate means a co-ordination
compound in which a central metallic
ion is attached to an organic
molecule at two or more positions.
BIOLOGICAL WEATHERING
OR BIOTIC WEATHERING
In all types of weathering in all
climatic regions biotic communities
play some roles in one way or other
• A. Faunal weathering
• B. Floral weathering
• C. Anthropogenic weathering
Faunal weathering
• 1.Tranfer of soils from lower to upper
and upper to lower horizons and
activates weathering
• 2. Burrowing animals include
gophers, prairie dogs, foxes, rabbits,
jackals, termites, rats etc.
FLORAL WEATHERING
• Cracks are widened by root
penetration and consequent root
pressure (physical weathering).
Dense vegetable cover generates
distinct micro climate at the ground
surface—initiate chemical
weathering.
ANTHROPOGENIC
WEATHERING
• The economic and technological man
are most powerful weathering agent
through mining ,road and dam
construction, deforestation etc.