Our Changing Earth

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Topics in the Chapter

Introduction
Endogenic Forces

→ Volcano

→ Earthquakes

Exogenic Forces
Major Landforms

→ Work of a River

→ Work of Sea Waves

→ Work of Ice

→ Work of Wind

Introduction

The earth’s crust consists of several large and some small, rigid, irregularly-shaped plates which carry continents and the ocean floor, called Lithospheric plates.

→ These plates move around very slowly – just a few millimetres each year because of the movement of the molten magma inside the earth.

The earth movements are divided on the basis of the forces which cause them:

→ Endogenic forces: The forces which act in the interior of the earth.

→ Exogenic forces: The forces that work on the surface of the earth.

Evolution of Land Forms

Endogenic Forces

Literal meaning: Endo means inside, genic means origin.
Endogenic forces sometimes produce sudden movements and at other times produce slow movements.
Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

Volcano

A volcano is a vent (opening) in the earth’s crust through which molten material erupts suddenly.

Earthquakes

When the Lithospheric plates move, the surface of the earth vibrates and these vibrations travel all around the earth.

→ The place in the crust where the movement starts is called the focus.

→ The place on the surface above the focus is called the epicentre.

Earthquakes cannot be predicted, but the impact can be minimised with preparedness.
Some common prediction methods used by people include studying animal behaviour.

→ Fish in the ponds get agitated, snakes come to the surface.

Earthquake Preparedness

→ Safe Spot – Under a kitchen counter, table or desk, against an inside corner or wall.

→ Stay Away from – Fireplaces, chimneys, windows, mirrors and picture frames.

→ Be Prepared – Spread awareness among friends and family to face disasters confidently.

Exogenic Forces

Literal Meaning: Exo means outside, genic means origin.

Major Landforms

The landscape is continuously worn away by two processes:

→ Weathering: Breaking up of the rocks on the earth’s surface.

→ Erosion: Wearing away of the landscape by agents like water, wind and ice.

Work of a River

Running water in the river erodes the landscape.
Waterfalls form when rivers tumble at steep angles over hard rocks.
In plains, rivers form bends called meanders.

→ Due to erosion and deposition along meanders, oxbow lakes are created over time.

→ Flooding results in deposition of sediments along banks creating fertile floodplains.

→ Raised river banks are called levees.

→ As rivers approach the sea, they slow down and break into distributaries.

→ Each distributary forms its own mouth.

→ Sediments deposited at all mouths form a delta.

Work of Sea Waves

Sea waves erode and deposit materials forming coastal landforms.

→ Cracks in rocks become wider forming sea caves.

→ Larger cavities leave a roof, forming sea arches.

→ Roof breaks down, leaving walls called stacks.

→ Steep rocky coast above sea level is called a sea cliff.

→ Deposited sediments along shores form beaches.

Work of Ice

Glaciers are large moving masses of ice.

→ Glaciers erode the land and expose solid rock below.

→ Deep hollows carved by glaciers become lakes as ice melts.

→ Deposited material forms glacial moraines.

Work of Wind

Wind is an agent of erosion and deposition in deserts.

→ Mushroom rocks have a narrower base and wider top due to wind erosion.

→ Wind lifts and transports sand; when it stops, sand falls and forms sand dunes.

→ Fine sand carried over long distances gets deposited as loess.

→ Large loess deposits are found in China.

MCQs

1. ___ movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

(a) Slow

(b) External

(d) Erosional

► (c) Sudden

2. An earthquake is measured with a machine called a

(a) Theismograph

(b) Heismograph

(d) Meismograph

► (c) Seismograph

3. The place in the crust where the movement starts is called the

(a) Waves

(b) Focus

(d) Crust

► (b) Focus

4. Which of the following rivers does not form a delta?

(a) Godavari

(b) Mahanadi

(d) Krishna

► (c) Narmada

5. A massive earthquake hit Bhuj Town on

(a) 16th December 2001

(b) 26th January 2004

(d) 16th December 2004

► (c) 26th January 2001

6. Natural cavity of weak rocks formed by action of waves

(a) Sea arches

(b) Stacks

(d) Sea cliff

► (c) Sea caves

7. The place on the surface above the focus is called the

(a) Vent

(b) Earthquake

(d) Epicentre

► (d) Epicentre

8. When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a

(a) Seafall

(b) Waterfall

(d) Ocean fall

► (b) Waterfall

9. Which of the following is NOT exogenic force

(a) Volcano

(b) Wind

(d) Glaciers

► (a) Volcano

10. The earth movements are divided on the basis of the _______ which cause them to move

(a) Forces

(b) Action

(d) Wind

► (a) Forces

11. It is a vent in the earth's crust through which molten material erupts suddenly

(a) Crater

(b) Volcano

(d) Crust

► (b) Volcano

12. When the wind blows, it lifts and transports sand from one place to another. When it stops blowing the sand falls and gets deposited in low hill like structures. These are called

(a) Sand dunes

(b) Stacks

(d) Loess

► (a) Sand dunes

13. The highest waterfall is

(a) Zambia Falls

(b) Angel Falls

(d) Victoria Falls

► (b) Angel Falls

14. ___ is classified as a major earthquake.

(a) 2

(b) 7

(d) 0.5

► (b) 7

15. The forces which act in the interior of the earth are called as

(a) Slow forces

(b) Speed forces

(d) Exogenic forces

► (c) Endogenic forces

16. The steep rocky coast rising almost vertically above sea water is called

(a) Sea arches

(b) Sea cliff

(d) Stacks

► (b) Sea cliff

17. In deserts you can see rocks in the shape of a mushroom, commonly called

(a) Mushroom rocks

(b) Meshrum rocks

(d) Moshrum rocks

► (a) Mushroom rocks

18. What do you mean by erosion?

(a) Moving of plates

(b) Type of exogenic forces

(d) None of these

► (c) Wearing away of landscape