| Reference Book | Ratna Sagar Integrated Social Science |
| Class | Class 7 |
| Category | Geography |
| Chapter Number | Chapter 3 |
| Chapter Name | The Changing Earth |
A) Tick (✓) the correct answers.
1) Alfred Wegener was a German
a) Geologist
b) Seismologist
c) Geographer
d) Meteorologist ✓
2) The idea of sea-floor spreading is a part of which theory?
a) Plate Tectonics
b) Pangaea
c) Continental Drift ✓
d) Exogenic forces
3) Which of the following is not a volcanic peak?
a) Kilimanjaro
b) Nanda Devi ✓
c) Etna
d) Vesuvius
4) Which of the following is not an active volcano?
a) Stromboli
b) Etna
c) Rainier ✓
d) Mauna Loa
5) Which of the following terms is not linked with seismic activity?
a) Epicentre
b) Crater ✓
c) Focus
d) Richter Scale
6) The earthquake in Bhuj occurred in
a) 2001 ✓
b) 2002
c) 2003
d) 2004
B) Fill in the blanks
1) Alfred Wegner suggested the Theory of Continental drift.
2) The earth’s crust has six major plates and twenty minor plates
3) Tectonic activities occur mostly along the plate margins.
4) The intensity of an earthquake is maximum near the epicentre.
C) Match the columns
| Column A | Column B |
| 1) Pangaea | Supercontinent |
| 2) Earthquake | Richter scale |
| 3) Ring of Fire | Pacific Ocean |
| 4) Sleeping volcano | Dormant Volcano |
D) Distinguish between
1) Vent and crater
| Vent | Crater |
| It is a circular opening through which hot molten materials erupt suddenly. This opening is called vent. | The volcanic cone or mountain is formed when the erupted materials accumulate around the vent. The vent is connected to the Earth’s interior by a narrow pipe. A funnel shaped hollow at the top of the cone is called crater. |
2) Dormant and extinct volcano
| Dormant Volcano | Extinct Volcano |
| 1) Dormant volcanoes are volcanoes that have not erupted in a long time but are expected to erupt again in the future. | Extinct volcanoes are those which have not erupted in human history. |
| 2) These volcanoes are called sleeping volcanoes. | These volcanoes are called dead volcanoes. |
| 3) They do not show any indication of eruption in the future. | There is no possibility of eruption in future. |
| 4) Examples of dormant volcanoes are Mount Kilimanjaro in Africa and Mount Fuji in Japan. | Example of Extinct volcano is Mt. Kenya in Africa. |
3) Focus and epicentre
| Focus | Epicentre |
| 1) Focus is the location in the Earth where the earthquake starts. | Epicenter is the location on the surface of the Earth directly above where the earthquake starts. |
| 2) The waves that originated from focus are body as well as surface waves. | The waves that originate from the epicenter is body waves |
| 3) Focus is the point where explosion originates | Epicenter is the place on earth where maximum damage is caused |
4) Seismology and Seismologist
| Seismology | Seismologist |
| 1) Seismology is the study of earthquakes and seismic waves that move through and around the Earth. | A seismologist is a scientist who studies earthquakes and seismic waves. |
E) Write short notes on the following.
1) Pangaea – In 1912, Alfred Wegener—a German meteorologist, made an attempt to explain the present-day distribution of continents and oceans. He proposed that millions of years ago, all landmasses were joined together into a supercontinent, called the Pangaea. He suggested that the Pangaea broke and there was a horizontal displacement of the continents.
2) Plate tectonics – The earth’s crust consists of plates, known as lithospheric or tectonic plates. There are six major and 20 minor plates, all of which float independently. The rate of movement of the plates is extremely slow. The energy required to move these plates is produced by the internal heat of the earth. The surface area of the plates is very large in comparison to their thickness. Tectonic activities, such as earthquakes and vulcanicity, occur along the plate margins. The plates are continuously moving.
3) Active volcanoes – They erupt frequently and throw out lava, gases, ashes and rocks. These volcanoes are also called living volcanoes.
4) Primary waves – The primary waves (P waves) or push waves are the first to arrive as they are the fastest. These waves usually travel at a speed of about six km per second. These waves cause relatively small displacements.
5) Surface waves – The surface waves are the last to arrive and travel over the surface of the earth. These waves are very powerful and cause most of the damage. Their effect is not seen at great depths.
6) Richter scale – The intensity of earthquake waves is measured on the Richter scale. It ranges from zero to nine. An increase in one point on the Richter scale indicates that the force of the earthquake is 10 times greater than the strength of the previous number. The earthquakes of intensity six and above are very destructive.
F) Give very short answers for the following questions.
Que 1) Name the force which originate from within the earth.
Ans Endogenic Force
Que 2) What is the name given to those volcanoes which have not erupted for hundreds of years?
Ans Extinct volcanoes which are also known as dead volcanoes.
Que 3) What is the shaking of the earth’s surface due to sudden movements called?
Ans Earthquake
Que 4) Name the instrument used to record earthquake tremors.
Ans Seismograph
G) Give short answers for the following questions.
Que 1) Explain Wegener’s Theory of Continental Drift.
Ans In 1912, Alfred Wegener—a German meteorologist, made an attempt to explain the present-day distribution of continents and oceans. He proposed that millions of years ago, all landmasses were joined together into a supercontinent, called the Pangaea. He suggested that the Pangaea broke and there was a horizontal displacement of the continents. This is called Wegener’s Theory of Continental Drift.
Que 2) Explain how earthquakes occur.
Ans Many factors, such as volcanic eruptions, movement of the lithospheric plates or even bomb explosions can cause the surface of the earth to shake. This shaking of the earth’s surface is called an earthquake. The place of origin of an earthquake is called focus. The point on the earth’s surface, which is vertically above the focus is called epicentre. The vibrations of an earthquake spread out as concentric waves from the focus. The intensity of an earthquake is maximum near the epicentre.
Que 3) Name the major earthquake belts of the earth. Where are they located?
Ans The main earthquake belts on earth are :
1) Circum-Pacific belt – It includes the coastal margins of North America, South America and east Asia. These regions account for about 65 per cent of the total earthquakes of the world. This belt coincides with the Pacific Ring of Fire.
2) Mid-Continental belt – This belt includes the Alpine mountains, Mediterranean Sea, northern and eastern Africa and the Himalayas. About 25 per cent of the seismic events are recorded here.
3) Mid-Atlantic Ridge belt – This includes the Mid-Atlantic ridge and the adjoining islands. It records moderate earthquakes which are caused due to the moving of the lithospheric plates in opposite directions.
Que 4) What are the causes of the Bhuj earthquake?
Ans The reasons for the Bhuj earthquake were:
a) Sea-floor spreading of the Indian Ocean at the rate of five cm per year.
b) Gradual northward movement of the Indian plate.
c) The reactivated faults below the surface in the Kachchh region.
H) Give long answers for the following questions.
Que 1) Explain the concept of Plate Tectonics.
Ans The earth’s crust consists of plates, known as lithospheric or tectonic plates.
a) There are six major and 20 minor plates, all of which float independently.
b)The rate of movement of the plates is extremely slow.
c) The energy required to move these plates is produced by the internal heat of the earth.
d) The surface area of the plates is very large in comparison to their thickness.
e) Tectonic activities, such as earthquakes and vulcanicity, occur along the plate margins.
f) The plates are continuously moving.
Que 2) Explain the different types of forces.
Ans The movement of the lithospheric plates cause various changes on the surface of the earth. The different types of movements of the plates are due to the various forces acting on it. On the basis of their source of origin these forces are divided into two broad categories:
1) The forces which originate from within the earth are called endogenic forces. The endogenic forces can produce sudden or slow movements or both. These forces also cause horizontal and vertical movements.
2) The forces which originate on the surface of the earth are called exogenic forces.
These movements give rise to a variety of relief features. The sudden movements are experienced through eruption of volcanoes and earthquakes.
Que 3) How are volcanoes distributed on the earth?
Ans Most of the world’s active volcanoes form a circular belt around the Pacific Ocean. That is why this belt is called the ‘Pacific Ring of Fire’. This ring coincides with the edge of the lithospheric plates.
The second belt of volcanoes is found along the Mediterranean Sea and is called the Mid-World Mountain belt. This belt has mostly dormant and extinct volcanoes.
Fujiyama in Japan is one of the world’s most beautiful volcanic mountain. The Deccan plateau of India was probably formed due to fissure eruption.
Que 4) Describe the different types of earthquake waves.
Ans According to the mode of travel, earthquake waves are classified into 3 main types :
1) Primary waves – The primary waves (P waves) or push waves are the first to arrive as they are the fastest. These waves usually travel at a speed of about six km per second. These waves cause relatively small displacements.
2) Secondary waves – S waves or shake waves are the next to arrive. Their rate of movement is less than that of the P waves. These waves produce a strong shaking action. They do not pass through liquids.
3) Surface waves – The surface waves are the last to arrive and travel over the surface of the earth. These waves are very powerful and cause most of the damage. Their effect is not seen at great depths.
HOTS
Que 5) Researches show similarity in the fossil remains of vegetation found on the eastern coast of South America and the western coast of Africa. Why?
Ans The fossil remains of vegetation found on the eastern coast of South America and the western coast of Africa are similar because at a point far back in time, the different continents (eg Africa & South America) were actually touching before being separated by plate tectonics. The fossils suggests that South America and Western Africa were once connected as part of a single landmass known as Pangaea and later drifted apart to their current positions.
Que 6) Why does the earthquake belt of the world coincide with the distribution of volcanoes?
Ans The earthquake belt of the world coincide with the distribution of volcanoes because both the natural phenomenon are associated with the tectonic actives and where there is a volcano the energy released will definitely give rise to the earthquake, and where there is a earth quake there can be a volcano sometimes. Most of the volcanic activities and seismic activities on Earth are localised on the boundaries of tectonic plates.
Leave a Reply