The processes that shape the Earth's surface from forces above and at the surface are called exogenetic processes. These include processes of degradation that destroy material and processes of aggradation that build up material. Processes of degradation are weathering, mass movement, erosion, and transportation. Weathering breaks down rocks through physical and chemical processes. Physical weathering is caused by factors like temperature changes, frost action, and plants and animals. Chemical weathering results from reactions with substances like water, carbon dioxide, and oxygen. These exogenetic processes gradually wear down and alter the Earth's surface over time.

2. The exogenetic processes are the processes that shape the land by forces
coming on or above the Earth's surface (exogenetic forces).
Exogenetic Processes
These are processes that take place on the earth's surface.
They are subdivided in two categories:
Processes of degradation
Processes of aggradation
Processes of degradation are those that destroy what has been built up by
endogenetic/aggradation processes. These are
Weathering
Mass movement
Erosion
Transportation
Processes of aggradation are those that build up material. Deposition
of weathered and eroded sediments
The destruction of the landscape is known as denudation.
Denudation is the long-term sum of processes that cause the wearing away
of the earth’s surface leading to a reduction in elevation and relief of
landforms and landscapes (weathering, erosion, and mass wasting).

3. • Weathering
• Weathering is the breaking and decomposition of rocks at/near the
earth's surface by physical & chemical processes.
• Weathering may be defined as the mehanical fracturing or chemical
decomposition of rocks by natural agents at the surface of the earth
(B.W.Sparks).
• Firstly, it involves two types of changes in the rocks
• (1) physical or mechanical changes, wherein rocks are disintegrated
through temperature changes (heat factor), frost action (frost factor),
biological activities (biotic factors), and wind actions. (2) chemical
changes wherein rocks are decomposed through static water, oxygen,
carbon dioxide, and biological activities.
• Secondly, the breakdown of rocks occurs at the place of rocks (in situ).
• Thirdly, there is no large-scale transport of weathered materials except
mass movement or mass translocation of weathered materials (rock-
waste) down the slope under the force of gravity.

4. • Factors Influencing the Character and Rate of
Weathering
• These factors include:
• A) Composition and structure of rocks: Mineral composition,
joint pattern, layering system, faulting, folding etc. largely affect
the nature and intensity of weathering. For example, carbonate
rocks having more soluble minerals are easily affected by
chemical weathering. Rocks having vertical strata are easily
loosened and broken down due to temperature changes, frost
action, water and wind action. The rocks having horizontal
strata are more compact and are less affected by the
mechanisms of disintegration and decomposition.
B) The gradient of the slope: The rocks in the region of steep
hillslope are easily disintegrated due to mechanical
weathering and the weathering materials are moved down in
the form of rock fall, debris fall etc and expose for further
weathering. The regions of gentle and moderate ground
slope are less affected by mechanical disintegration.

8. C) Climate:
The greater the amount
of rainfall the higher the
rate of chemical
weathering.
Temperature changes
accelerate the rate of
mechanical weathering.
Seasonal variations in
climate generate
different conditions for
weathering.
In monsoon climate
rocks are subjected to
mechanical
disintegration due to hot
and dry summer months
whereas chemical and
biological weathering is
more dominant during
wet monsoon months.

9. D) Vegetation cover : Vegetation bind the rocks through roots and thus
protect them from weathering and erosion but the same time the
penetration of roots weakens the rocks by breaking them into several
blocks. Dense vegetations protect the ground surface from the direct
sun rays. The micro-organisms associated with the roots encourage
decomposition and disintegration of rocks through physico-
biochemical weathering.
– Time
– Man's activities
• There are three types of weathering agents:
• 1. Physical or mechanical weathering agents
• (i) moisture and water
• (ii) Frost
• (II) Insolation (temperature)
• (iv) Wind
• 2. Chemical weathering agents
• (I) Oxygen
• (II) Carbon dioxide

11. • (III) Hydrogen
• 3. Biological weathering agents
• (I) vegetations
• (II) animals mainly microorganisms.
A. Physical or mechanical weathering: The physical or mechanical
weathering leads to fragmentation and breakdown of rock masses
into big blocks and boulders, cobbles and pebbles, sands and silts.
1. Block disintegration due to temperature:
Thermal stress weathering (sometimes called insolation
weathering)results from expansion or contraction of rock, caused
by temperature changes.
Some rocks are less affected by temperature changes such as
clastic sedimentary rocks (shales, sandstones) because the
particles are separated by thin cementing laminae of silica.
On the other hand, crystalline rocks, like granites are affected by
temperature changes as particles are closely associated with each
other and these particles expand and contract with increase and
decrease of temperature respectively.

12. • c
Clastic rock
Crystalline rock

13. • If temp. of granite rocks is increased by 65.5 degree C, the rock
expands by 2.54cm per 30.48 m distance. The repetition of
expansion and contraction of outer rock layers due to diurnal
range of temperature in the hot desert areas causes tension and
stresses which introduce parallel joints in the rocks. The rocks,
then, are disintegrated along these joints and broken big blocks
of rocks are dislodged from the main rock mass and fall down
the slope under the impact of gravity.

14. 2. Granular disintegration due to temperature:
The coarse-grained rocks are more affected by shattering process in
those hot deserts which are characterised by high range of daily
temperature. If the rocks are coarse-grained and are of different
colors they absorb insolation differently. And thus different parts of
the same rocks are affected by differential expansion and contraction
which causes stresses within the rocks due to which they are
disintegrated into smaller particles. Marble is an example.

15. 3. Block disintegration due to frost:
This process is more active in those areas which are very
often characterised by alternate process of freezing and
thawing of water mainly during night and day respectively.
The more compact and highly consolidated rocks (granites)
are less affected by freeze-thaw actions while less compact
loosely consolidated rocks (sedimentary) are more affected
by frost actions. Water present in between particles of
porous rocks freezes during night due to fall of temperature
below freezing points and thus expands due to increase in
temp. during day time. 10% thaws during day time and
contracts by 10% due to freezing. This diurnal freeze and
thaw cycle causes alternate expansion and contraction
which introduce tension and stresses due to which rocks
are disintegrated into smaller particles.

16. An example of rocks susceptible to frost action is chalk, which
has many pore spaces for the growth of ice crystals. When water
that has entered the joints freezes, the ice formed strains the
walls of the joints and causes the joints to deepen and widen.
When the ice thaws, water can flow further into the rock.
Repeated freeze-thaw cycles weaken the rocks which, over time,
break up along the joints into angular pieces.

17. 4. Exfoliation due to temperature and wind:
It occurs in places like hot deserts where the day temperature
often soars to 40 degrees Celsius and night temperature falls
to below 10 degrees Celsius. The repeated heating by day and
cooling by night cause the outer layers of the rock to expand
and contract alternately. Stress is created in the rock,
producing cracks and gradually, the outer layers of rock will
peel off.
It is also known as onion weathering, refers to peeling off
concentric shells of rocks due to combined actions of heat and
wind in hot arid and semi-arid regions and monsoon lands. It is
more common over crystalline rocks. The outer shells of rocks
become loose due to high temperature during day time and
comparatively low temp. during night respectively and these
loosened shells are removed (peeled off) by strong winds.

18. In pressure release, also known as unloading, overlying materials
(not necessarily rocks) are removed (by erosion, or other
processes), which causes underlying rocks to expand and fracture
parallel to the surface. Often the overlying material is heavy, and
the underlying rocks experience high pressure under them, for
example, a moving glacier. Pressure release may also cause
exfoliation to occur.
Pressure Release of
granite.

20. 5. Salt wedging: As water enters the holes and cracks in the
surface of rocks, it often carries salt with it. As the water later
evaporates, the salt is left behind. Over time, these salt
deposits build up, creating pressure that can cause rocks to
split and weaken.

21. • Chemical weathering is the process by which rocks are
decomposed, dissolved or loosened by chemical processes to form
residual materials. Some of the processes involved in chemical
weathering are given below :
• CARBONATION process by which dissolved carbon dioxide in
rainwater or moisture in surrounding air forms carbonic acid and
reacts with the minerals in the rock. This process weakens the rock
thus breaking it down in the process.
• e.g.: Calcium Carbonate + Water + Carbon Dioxide ---> Calcium
Carbonate (soluble-easily dissolved)
• HYDROLYSIS
• chemical reaction between the minerals in the rock and hydrogen in
rain water. For example, during hydrolysis, the feldspar in granite
changes to clay mineral which crumbles easily, weakening the rock
and causing it to break down.
• OXIDATION The process by which oxygen combine with water and
minerals in the rock such as calcium and magnesium. When iron
reacts with oxygen, reddish -brown iron oxide is formed. The iron-
oxide crust crumbles easily and weakens the rock.
• e.g. : Iron + Oxygen --> Iron Oxide (crumbles)
•

22. • Comparison of unweathered
(left) and weathered (right)
limestone
•Oxidize pyrite cubes

23. SOLUTION process by which minerals in the rocks dissolve
directly in water.
• HYDRATION process where minerals in the rock absorb water
and expand, creating stress which causes the disintegration of
rocks.
• EG: Unhydrated Calcium sulphate + Water --> Hydrated Calcium
Sulphate (expands)

24. • Biological weathering is a form of weathering caused by the
activities of living organisms – for example, the growth of
roots or the burrowing of animals. Tree roots are probably
the most significant agents of biological weathering as they
are capable of prising apart rocks by growing into cracks and
joints. Plants also give off organic acids that help to break
down rocks chemically
Biological weathering of lava by
lichen, La Palma

25. •