Maharashtra Board Class 9 Geography Chapter 3 Exogenetic Processes Part I PDF Download

Exogenetic Processes Part- I

See the given pictures. Observe the physical appearance of the rocks in each picture. You can see that rocks are broken, fractured and have holes in them. In a picture you can also see that the statue has been deformed. Why are the rocks in such a condition? Think about them and briefly tell the reasons you can think of. Discuss the reasons. Check with the teachers if your reasons are relevant.

Do You Know?

Landforms formed on the earth's surface as a result of internal movements are called primary and secondary landforms. For example continents, mountains, plateaus, plains, etc. Because of the external processes like weathering, erosion, transportation and deposition, the primary and secondary landforms give way to the tertiary landforms. For example, valleys, sand dunes, delta, U-shaped valleys, etc.

Many landforms are formed due to the internal movements. Many processes occurring on the earth's surface also lead to the formation or degradation of landforms continuously. In this lesson, we will be studying the exogenetic (external) processes and the landforms formed by them.

External processes occur because of the forces working on the earth's surface. They are mainly solar energy, gravitational force and kinetic energy associated with the moving objects on the earth's surface.

Teacher's Note

Rocks break down slowly over time into smaller pieces. This happens because of sun, water, wind and plants. Like how an old building slowly falls apart without care.

Exam Trick

Remember: Exogenetic = external = outside the earth. Endogenetic = internal = inside the earth. Think EXO = EXternal.

Points to Remember

Weathering is the breaking of rocks into smaller pieces.
External processes happen on the earth's surface because of sun, gravity and moving objects.
Primary landforms are made by inside forces. Tertiary landforms are made by outside forces.
Three types of weathering exist: mechanical, chemical, and biological.

Can You Tell?

See the given pictures. Observe the physical appearance of the rocks in each picture. You can see that rocks are broken, fractured and have holes in them. In a picture you can also see that the statue has been deformed. Why are the rocks in such a condition? Think about them and briefly tell the reasons you can think of. Discuss the reasons. Check with the teachers if your reasons are relevant.

Geographical Explanation

Breaking or weakening of rocks is a natural phenomenon. It is called weathering. Weathering can be of three types: mechanical (physical), chemical and biological. In arid climates, mechanical weathering is dominant while in humid climates, chemical weathering is more effective. Biological weathering occurs because of living organisms.

Mechanical Weathering:

You will notice that just as we can remove each and every outer layer of the onion, similarly, in nature rocks undergo such a process. The exposed part of the rock heats more while the inner part is comparatively cooler. As a result, the outer layers of the rocks fall apart from the main rock. This is called exfoliation of the rock.

Release of pressure: It is not that tension is created in the rocks only because of temperature, freezing of water or crystallization. The outer layers of the rocks exert pressure on the inner or lower layers. When this pressure ceases to exist, the lower or inner layers get freed from the pressure. This also leads to weathering.

Water: Some areas experience more rainfall than others. In such areas, soaking of rock water also causes weathering of some rocks like sandstone, conglomerate. These rocks are formed because of pressure on the agglomeration of sand particles. Mud also makes sand particles come together. When water penetrates such rocks, the particles get loose and separate from the main rock. This is called granular weathering.

Sometimes both temperature and water are responsible for weathering. Difference in temperature cause contraction and expansion widening the joints or the cracks in the rocks. Water accumulates in such wide joints and big blocks of rocks separate from each other. This is called block disintegration.

Try This

Take an onion.

Cut it in the middle.

Observe the cut parts.

Try to remove each skin layer of these parts.

Teacher's Note

Exfoliation happens when rock layers peel off like onion skin. You can see this on hills and mountains in India where rocks crack naturally.

Exam Trick

Remember: Exfoliation = skin peeling. Like sunburned skin peeling off your body, rock layers also peel off one by one.

Points to Remember

Exfoliation is when outer rock layers fall off from the main rock.
Heat and cold make rocks expand and contract.
Release of pressure also causes rocks to break.
Water soaking in rocks can cause granular weathering.
Block disintegration happens when both heat and water work together.

Mechanical weathering mainly occurs because of the following reasons:

Temperature, Frost, Crystal growth, Release of pressure, Water

Temperature:

The minerals in the rocks expand because of heat and contract when temperature decreases. Due to such continuous contracting and expanding, tension develops in the rock particles. Each mineral reacts differently to the temperature. Some minerals expand more while others do not expand as much. Consequently, the tension formed in the rocks also increases and decreases. As a result, cracks develop in the rocks and they break. In areas, where the diurnal range of temperature is higher, weathering of this type is common in the hot deserts.

Frost:

You know that the volume of the water increases when it freezes. In areas where the temperatures drop below 0° C for quite some time, the water accumulated in the cracks and crevices in the rocks freezes. Its volume increases. This leads to tension in the rocks and they shatter.

Crystal Growth:

In rocky coasts, waves hit the sea cliff. The water is alkaline. Some water droplets hit the cracks in the rocks. In this alkaline water, the soluble materials in the rock get dissolved. This leads to formation of small holes in the rocks. This is the effect of solution. Alkaline water gets stored in these holes. Because of heat, this water turns into water vapor and only crystals of alkaline materials remain in the rocks. Crystals occupy more space. This causes tension in the rock. Holes are formed in the rocks. It looks like a honeycomb.

Teacher's Note

Frost action breaks rocks in cold mountains like the Himalayas. When water freezes in rock cracks, it pushes the rock apart like a stone being pushed by ice.

Exam Trick

Remember: Frost = ice = expands. Water freezing in rocks is like inflating a balloon inside the rock. The rock bursts!

Points to Remember

Different minerals expand and contract at different rates due to heat.
Frost action happens when water freezes and expands inside rocks.
Crystal growth happens when salt water evaporates in rock holes.
Pressure from upper rock layers can cause rocks to break when released.
Mechanical weathering is strong in hot deserts with big temperature changes.

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