Maharashtra Board Class 11 Geography Chapter 1 Earth Movements PDF Download

1. Earth Movements

Hills, mountains, plateaus, valleys, etc. are the landforms we see on the earth's surface. They have been developed and shaped by the internal and external forces. The internal processes are classified into slow movements and sudden movements.

The earth's surface changes slowly but continuously. These changes occur due to forces such as tension and compression. The internal processes cannot be observed. However, their effects may be seen on the surface. Formation of mountains and distribution of continents are related to slow movements. There are also sudden processes that operate within the crustal part. Their effects can be noticed on the surface within a few seconds or hours. Earthquake and volcanoes are sudden movements.

Teacher's Note

Earth movements shape our land. When earthquakes happen in India, buildings shake and sometimes collapse, showing the power of these movements.

Exam Trick

Remember: Slow movements = mountains take millions of years to form. Sudden movements = earthquakes happen in seconds. Just like slow cooking vs. quick frying!

Points to Remember

Earth movements are slow or sudden.
Slow movements build mountains and continents.
Sudden movements are earthquakes and volcanoes.
Internal forces cause all earth movements.
We can see the effects of earth movements on the surface.

Evidences Of Earth Movements

The landforms are never permanent in nature. They tend to change. There are evidences to indicate that the earth movements have taken place in the past and have affected the surface of the Earth. Some of them are:

After the Great Tsunami of 2004, the coast around Sumatra island rose by few centimetres.

The formation of three ranges in the Himalayas: the Siwaliks, the Middle Himalayas and the Greater Himalayas.

There is a report that says that due to volcanic eruption near Iceland an island appeared above the sea surface in November 1963. This event was witnessed by some sailors who were passing by the area.

Some islands such as the Megapode Island were reported to have been lost after the tsunami.

To the south east coast of Mumbai, near Mazgaon Dockyard, there are evidences of forest-covered land getting drowned. Even today, trunks of those trees can be found at some depth.

The Kachchh earthquake of June 16, 1819 submerged the coastal areas. This inflicted great damage to ships and country-made boats of the fishermen. The fort of Sindree on the sea coast was completely submerged except a single turret which remained above the water level. The land measuring around 1550 sq.km in area was raised upward because of this earthquake. This raised land is known as "Allah's Bund".

Teacher's Note

The 2004 tsunami in our Indian Ocean changed coastlines. This shows that the earth is always moving and changing shape.

Exam Trick

Remember: Tsunami of 2004 = coast rose. This means earthquakes can lift or push down land. Sumatra coast is a real Indian Ocean example!

Points to Remember

Earth movements happened in the past and keep happening now.
Tsunamis can raise or lower the coast.
The Himalayas have three ranges from folding.
Some islands have disappeared due to earthquakes.
The Kachchh earthquake of 1819 created "Allah's Bund".

Slow Movements

It is important to understand that in reality, these processes are very complex and interrelated. The Earth's movements which are the result of internal forces are known as tectonic movements.

Based on the direction of these movements, they are classified as vertical and horizontal movements.

1) Vertical (Epeirogenic) Movements

Due to the forces in the interior of the earth and the travel of energy, these movements occur. Slow movements keep on taking place either towards the centre of the earth or away from it towards the crust. Due to such movements, an extensive portion of the crust is either raised up or it subsides. When a portion of the crust is raised up above sea-level, it leads to the formation of continents. Hence such movements are also called continent-building movements. Such movement can also cause formation of extensive plateaus. Though these movements are slow they influence huge area. These movements are not related to development of tensions or pressure in the earth crust.

2) Horizontal (Orogenic) Movements

These movements work in horizontal direction. As per the direction, these movements produce compression or tension in the rock strata. These movements lead to either folds or cracks in the surface of the earth. These movement gives rise to mountains. These movements are also slow movements. But their speed is more than the continental-building movements. And their extent is also lesser. These movements produce either folds or faults. Consequently, either fold mountains or block mountains are formed. They are also called mountain-building processes. These forces are further divided into two types:

a) Tensional Forces

When the forces move away from each other and cause stress in the rock strata. This process creates ruptures, cracks, fractures and faults in the rock strata. This leads to crustal fracture and the formation of faults. Rift valleys or block mountains are formed as a result of these forces.

b) Compressional Forces

When the forces operate towards each other, they cause compression and hence are called as converging or compressional forces. These movements cause various types of folding. These forces cause pressure on the layer of rocks. These lead to folding and faulting of the surface.

Teacher's Note

Two types of slow movement push and pull the earth's crust. Vertical movements lift continents up and down. Horizontal movements squeeze and stretch rocks to make mountains.

Exam Trick

Remember: Tension = pull apart (rift valleys). Compression = push together (fold mountains). Think of stretching and squeezing a rubber band!

Points to Remember

Vertical movements build continents by raising crust up or down.
Horizontal movements happen left and right and make mountains.
Tensional forces pull rocks apart and make rift valleys.
Compressional forces push rocks together and make fold mountains.
Both types of forces cause faults and folds in rocks.

Folding

The nature of folding that occurs on the earth's surface depends on many factors. These include the nature of rocks, intensity and duration of force.

Soft and elastic rocks are affected more by these forces. When energy waves move through the layers of rocks on a large scale, folds are formed. Folding results into formation of fold mountains. For example, Himalayas, Alps, Rockies, Andes, etc.

Compressional forces push two portions of crustal rocks together and tend to shorten and thicken the crust. The affected rocks react to compressional forces depending on the strength of the rocks and the speed of the forces. Folding occurs when compressional forces are applied to rocks that are ductile or flexible. Rocks that lie deep within the crust and are therefore under high pressure are generally ductile and particularly susceptible to folding without breaking. As a result rocks deep within the crust typically fold rather than break. Folding is also likely to occur where compressional forces are applied slowly.

Parts Of Fold

Folds develop in earth's crust. Both sides of a fold are called limbs. The axial plane divides a fold into two parts. The axis may be vertical, inclined or horizontal. A fold has two limbs.

As and when limbs slope downward with central portion getting raised up, it is called anticline. As against this, if limbs slope towards each other and the central part located at lower elevation, it is called synclines.

Fold Mountains

Folding leads to development of fold mountains. For example, the Himalayas, the Alps, etc.

Teacher's Note

Soft rocks bend and fold like clay. Hard rocks crack and break. The Himalayas were made by folding, just like folding a piece of paper.

Exam Trick

Remember: Anticline = up (A for up). Syncline = down (S for down). Anticline looks like an arch, syncline looks like a valley!

Points to Remember

Folds are made when rocks bend from pressure.
Anticline folds have a high top and low sides.
Syncline folds have a low top and high sides.
Soft rocks fold more easily than hard rocks.
Fold mountains like the Himalayas take millions of years to form.

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