NCERT CLASS 7 CHAPTER 3 YOUTUBE LINK: https://youtu.be/zB1Q9TL6Klg

1. THE CHANGING EARTH
(VOLCANOES & EARTHQUAKES)
Class 7
NCERT
Chapter- 3

2. Alfred Wegener gave the Continental drift
Theory (1912)- millions of years ago
continents were joined together to a
super continent Pangaea; which broke
and horizontal displacement caused
formation of continents.
 Plate tectonics improved continental drift theory by adding
concept of sea floor spreading
 6 major and 20 minor plates
 Plates are continuously moving; few millimeters per year
 Tectonic activities occur along the plate margins

3. FORCES AFFECTING EARTH’S CRUST

4. VOLCANOES
• Also known as mountain of fire
• Vent- circular opening through which
materials erupt suddenly
• funnel shaped hollow at top of the cone-
crater

5. TYPES OF VOLCANOES

6. DISTRIBUTION OF VOLCANOES
• Most of the active volcanoes form belt around
pacific ocean known as pacific ring of fire
• Second belt is found along Mediterranean sea
known as mid-world mountain belt {has
mostly dormant & extinct volcanoes}
• Fujiyama [Japan] is one of the worlds most
beautiful volcanic mountain
• Deccan Plateau formed due to fissure
eruption

7. EARTHQUAKE
• Shaking of earth’s surface caused by sudden
movement
• Place of origin of earthquake- focus
• Epicentre- point on
earth’s surface vertically
above the focus
• Intensity maximum
near the epicenter.

8. TYPES OF EARTHQUAKE WAVES

9. MEASURING EARTHQUAKES
• Intensity measured on Richter scale
• Earthquake tremors are recorded by
seismograph; the science that deals with it is
called seismology; earthquake scientists are
seismologists
• Intensity is measured in relation to the effect
on human life.

10. DISTRIBUTION OF EARTHQUAKES
Circum-pacific belt- coastal margins of North
America, South America and East Asia;
account for 65% of total earthquakes;
coincides with pacific ring of fire
Mid-continental belt- Alpine mountains,
Mediterranean sea, Northern & eastern Africa
and the Himalayas; 25%of seismic events
Mid- Atlantic ridge belt- Mid Atlantic ridge
and the adjoining islands; moderate
earthquakes due to moving of lithospheric
plates in opposite direction.

11. BHUJ EARTHQUAKE (2001)
• Earthquake at Kachchh region at 8:45a.m. on 26 Jan 2001
• Buildings collapsed, communication and power lines
completely disrupted
• Intensity- 8.1 on Richter scale by Indian Meteorological
Department
• About 50,000 were killed and more than 5,00,000 adversely
affected
• Reasons-
 Sea floor spreading of Indian ocean @5cm/year
 Gradual northward movement of Indian plate
 Reactivated faults below surface of kachchh region