Chapter 3 – Earth Movements and Earthquakes
1. The Lithosphere and Lithospheric Plates
The lithosphere is the solid outermost shell of the Earth, which includes the crust and the uppermost part of the mantle. Interestingly, the lithosphere is not a single unbroken sheet; instead, it is broken into a number of large and small pieces called lithospheric plates.
-
These plates are like giant puzzle pieces covering the surface of the Earth.
-
They are in slow motion—moving only a few millimetres per year.
Reason for movement:
Beneath the lithosphere lies molten magma in the mantle. This magma moves in a circular pattern (convection currents). The heat from Earth’s interior causes this molten rock to rise, move laterally, and then sink again, dragging the lithospheric plates along with it.
2. Forces Causing Earth Movements
Movements on Earth’s surface are the result of two main types of forces:
A. Endogenic Forces (Internal forces)
These originate inside the Earth and cause changes on the surface.
They are of two types:
-
Sudden Forces – Act very quickly, causing immediate changes:
-
Earthquakes
-
Volcanoes
-
Landslides
-
-
Diastrophic Forces – Act very slowly over thousands or millions of years:
-
Mountain Building (e.g., formation of the Himalayas)
-
Plateau formation
-
B. Exogenic Forces (External forces)
These work on the surface of the Earth due to agents like:
-
Rivers
-
Wind
-
Sea waves
-
Glaciers
They cause erosion (wearing away of land) and deposition (laying down of material).
3. Sudden Movements – Volcanoes and Earthquakes
A. Volcano
A volcano is a vent or opening in the Earth’s crust through which molten material (lava), ash, and gases escape from beneath the surface.
-
Eruptions may be explosive or quiet.
-
They can alter landscapes dramatically and cause widespread destruction.
B. Earthquake
An earthquake is the shaking or trembling of the Earth’s surface caused by the sudden movement of lithospheric plates.
Key Terms:
-
Focus: The location inside the Earth where the earthquake originates.
-
Epicentre: The point on Earth’s surface directly above the focus.
-
Seismic waves: Vibrations that travel outward from the focus and epicentre.
Damage pattern:
The strongest shaking and most severe damage occur nearest to the epicentre, and the intensity decreases as you move away.
4. Types of Earthquake Waves
-
P Waves (Primary or Longitudinal Waves)
-
Travel fastest.
-
Can move through solids, liquids, and gases.
-
Cause particles to move back and forth in the direction of the wave.
-
-
S Waves (Secondary or Transverse Waves)
-
Slower than P waves.
-
Can travel only through solids.
-
Move particles up and down or side to side, perpendicular to the wave’s direction.
-
-
L Waves (Surface Waves)
-
Travel along Earth’s surface.
-
Cause the most damage due to rolling and swaying motion.
-
5. Measuring Earthquakes
-
Instrument: Seismograph – records the intensity and duration of vibrations.
-
Scale: Richter Scale – measures magnitude.
Richter Scale guide:
-
2.0 or less: Barely felt.
-
Over 5.0: Damage possible (falling objects, cracks).
-
6.0 and above: Very strong, structural damage likely.
-
7.0 or more: Major earthquake, widespread destruction.
6. Earthquake Prediction
Although exact prediction is not possible, certain traditional indicators have been observed:
-
Sudden change in animal behaviour – fish become restless, snakes emerge from their holes.
-
Groundwater level fluctuations.
-
Unusual sky glows or sounds.
7. Earthquake Preparedness
Preparedness can save lives and reduce damage.
Safe Spots during an earthquake:
-
Under a sturdy table, kitchen counter, or desk.
-
Against an inside wall or corner.
Places to avoid:
-
Near fireplaces, chimneys, windows, mirrors, or picture frames.
Other precautions:
-
Spread awareness among friends and family.
-
Conduct drills to know how to react.
-
Keep an emergency kit ready (water, torch, first-aid).
8. Summary Table
| Force Type | Nature | Examples |
|---|---|---|
| Endogenic | Internal | Earthquakes, Volcanoes, Mountain building |
| Exogenic | External | River erosion, Wind deposition, Glacial action |
Conclusion
The Earth’s surface is dynamic and constantly changing due to internal (endogenic) and external (exogenic) forces. Understanding how lithospheric plates move and how earthquakes occur helps us reduce damage through preparedness and awareness. While we cannot stop these natural processes, we can certainly learn to live more safely with them.
