Class 7 Geography
Chapter 3: Our Changing Earth
Table of Contents
1. Define Continental Drift?
2. Define Plate Tectonics?
3. What forces act that cause movement of plates?
4. Write the difference between Exogenic and Endogenic forces?
5. Define Volcano?
6. What is a vent?
7. What is a Crater?
8. What are the constituents of molten lava?
9.How are volcanoes classified?
10.Write the difference between Active, Dormant and Extinct Volcanoes?
11. What are Calderas ?
12. Where are volcanoes distributed mostly?
EARTHQUAKES
1. Define Earthquake?
2. How Earthquake occur?
3. What are Seismic waves?
4. What are the other modes that causes the Earth surface to shake?
5. What are various kinds of Earthquake waves ?
6. What is the difference between Primary, Secondary and Surface waves?
7.Define:
8. Define a Richter scale?
9. Write about the distribution of Earthquakes?
1. Define Continental Drift?
- It is proposed by Alfred Wegener in 1912.
- He proposed that super continent called Pangea broke into different continents around 200 million years ago
- The continents drifted (moved) to the present location.
- This is continental drift theory.
2. Define Plate Tectonics?
It was proposed by 1968.
The outer layer of Earth is made up Of rock plates known as Lithospheric tectonic plates.
These plates move extremely slow each year they move about a few millimetres.
3. What forces act that cause movement of plates?
- There is movement of plates that occur due to various forces acting on it.
- They are exogenic and endogenic forces.
4. Write the difference between Exogenic and Endogenic forces?
| Exogenic forces | Endogenic forces |
| Exogenic forces are the ones that act on the Surface of Earth. | Endogenic forces act in the interior of earth. |
| Exogenic forces are erosional and depositional works by river, wind, glaciers, sea waves. | Endogenic forces act in the interior of earth. |
| It causes slow destruction over period of time. | Endogenic forces produce slow as well as sudden movements like Earthquake, Volcano it causes massive destruction. |
5. Define Volcano?
Volcanoes are opening in Earth’s crust through which molten material comes out from the interior of Earth.
6. What is a vent?
Opening of a volcano is called a vent
7. What is a Crater?
- Conical mountain formed around a vent.
- Crater is a large hole at the top of a volcano.
- It is also defined as a funnel shaped basin surrounding the vent.
8. What are the constituents of molten lava?
Molten lava consists of lava, ash, hot molten rocks, steam, gas, solid rock particles.
9.How are volcanoes classified?
- Volcanoes are classified based on the nature and frequency of eruptions.
- They are classified into Active volcanoes, Dormant volcanoes, Extinct volcanoes.
10.Write the difference between Active, Dormant and Extinct Volcanoes?
| Active volcanoes | Dormant volcanoes | Extinct volcanoes |
| It is also called as Living volcanoes | It is also called as sleeping volcanoes. | It is also called as dead volcanoes. |
| Active volcanoes erupt frequently and gives out ashes, gases, lava rocks. | Dormant volcanoes have not erupted! for a long period of time. They become destructive once they comes to life | Extinct volcanoes are inactive volcanoes. They will not at all erupt. |
| Ex: Mt. Etna in Italy, Barren island in Andaman Sea | Ex: Mt. Vesuvius. in Italy | Ex: Kilimanjaro in Africa, Rainier in USA |
11. What are Calderas ?
Calderas are lakes that are formed from extinct volcanoes.
12. Where are volcanoes distributed mostly?
Most of the world active and dormant are located in two distinct belts. volcano
a) Circum-pacific Belt: It encircles the Pacific ocean.
It is also Known as the Pacific Ring of fire.
Most volcanoes are located in Andes of South America and Rockies of North America
b) Mid world Mountain belt:
It consists of extinct and dormant volcanoes that lie along the Mediterranean sea.
EARTHQUAKES
1. Define Earthquake?
The Sudden Shaking of the Earth caused by internal forces of Earth is called as Earthquake.
2. How Earthquake occur?
- Earthquake originates from the interior of Earth and the starting point is called Focus.
- Point on the surface of Earth exactly above the Focus is called Epicentre
- Earthquakes send shock waves around and destroy life and property.
3. What are Seismic waves?
- The vibrations of an earthquake spread out as concentric waves from the focus. These vibrations are called Seismic waves.
- The intensity of the earthquake is maximum at its Epicentre.
4. What are the other modes that causes the Earth surface to shake?
The surface of the Earth can shake due to movements of lithospheric plates, volcanic eruptions or by bomb explosions.
5. What are various kinds of Earthquake waves ?
There are three types of waves, Primary waves, Secondary waves, Surface waves.
6. What is the difference between Primary, Secondary and Surface waves?
| Primary waves | Secondary Waves | Surface waves |
| Primary waves fastest and foremost waves. | These waves arrive after P-waves. | These waves are the last waves to arrive. |
| It is also called p waves or push waves. | It is also called s waves or Shock waves. | |
| These waves travel at a speed of 6km/sec. | The rate of P wave is higher than S wave | Surface waves travel over the Surface of Earth. These waves are so powerful. |
| Small displacements. are caused by these waves. | Strong shaking actions are produced by S waves | Most of the damage is caused by these waves. |
7.Define:
- Seismology = Science that deals study of earthquakes are called as seismology.
- Seismologists = The experts / scientist who study the pattern of earthquake are called as seismologist.
- Seismograph = It is an instrument that records the intensity of earthquake waves.
8. Define a Richter scale?
- It was designed by Charles Richter in 1935.
- Richter scale has range from 0-9.
- Richter scale states that force of the earthquake is ten times greater than that previous one, if there is an increase at one point.
- Earthquakes that measure above 6 are termed as destructive.
9. Write about the distribution of Earthquakes?
Earthquakes occur more frequently in some specific areas. they are Circum pacific belt, Mid continental belt, Mid atlantic ridge belt.
Circum Pacific Belt
- It consists of East Asia, South America, Coastal Margin of North America.
- 65% of total Earthquake in this belt.
Mid Continental Belt
- It includes Northern, Eastern Africa, Alphine mountains, Himalayas Mediterranean sea.
- 25% of total seismic event occur here.
Mid Atlantic Ridge Belt
- It includes Mid Atlantic Atlantic Ridge, adjoining Islands.
- Moderate earthquake occur here due to moving of lithosphere plate in opposite direction.
- In India, Earthquake belt is confined to Ganga Brahmaputra valley, Himalayan region.
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