Lithosphere – I Endogenetic Processes — Class 9 Social Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Social Science, Geography — Chapter 1. Earth's interior, plate tectonics, volcanoes, and earthquakes.

1. About this lesson

Geography Chapter 1 introduces the Earth's internal structure and the endogenetic (internal) forces that shape the planet — plate tectonics, volcanic activity, and earthquakes. It explains the composition of Earth's layers, how plates move, and the constructive and destructive effects of these forces.

2. The Four Spheres of the Earth

3. Structure of the Earth's Interior

The Earth's interior is divided into three layers:

Key facts about the core:

• Large quantities of iron create Earth's gravitational force.
• As Earth rotates, the liquid outer core spins over the solid inner core → generates Earth's magnetic field.
• This is responsible for the functioning of the magnetic compass.
• Due to high pressure, inner core materials remain solid.
• Magma is found in the mantle (NOT the crust or core).
• SIAL floats over SIMA because SIAL (silica + aluminium) is lighter than SIMA (silica + magnesium).
• The outer core is made up of liquid iron. The inner core is solid.

4. Geomorphic Processes

Geomorphic processes — the formation and deformation of landforms on Earth's surface.

Endogenetic (Internal) Processes

• Forces acting from Earth's interior → surface.
• Build the landscape; create topographic relief.
• Generate heat; eject materials from deep below the crust.
• Internal radioactivity is the principal source of power.
• Connected to: volcanism, earthquakes, plate tectonics, folding and faulting.

Exogenetic (External) Processes

• Forces acting on the surface: running water, glaciers, wind, waves, etc.
• Tear down the landscape into low elevated plains. (Covered in Ch 2.)

5. Plate Tectonics

• The lithosphere is divided into huge slabs of rock called Tectonic Plates (major and micro-plates).
• These plates float independently over the mantle.
• Collisions produce mountain ranges and irregular surface features — Plate Tectonics.
• The movement of tectonic plates is induced by thermal energy from the mantle.

Types of Plate Boundaries

Supercontinents

• Many million years ago, India was part of the supercontinent Gondwana (NOT Pangaea directly — Pangaea later split into Laurasia and Gondwana).
• In the ancient period, Gondwanaland moved towards the north.

6. Diastrophism — Folding and Faulting

• Diastrophism is connected to fold/fault (NOT volcanism or earthquakes directly — it's the process of deformation).
• Fold: When tectonic plates collide, compression causes rocks to bend into folds → fold mountains (e.g., Himalayas).
• Fault: When stress and tension stretch rocks causing them to crack, resulting in faults → block mountains, rift valleys.
• The movement of plates that creates stress and tension causing rocks to stretch and crack results in fault.

7. Volcanoes

A volcano is a vent or opening on the Earth's crust through which hot solid, liquid, and gaseous materials (magma) erupt out from the interior to the surface.

Major Components

Classification by Periodicity of Eruptions

Common mistake: Mt. Kilimanjaro is extinct, NOT dormant. Mt. Fuji is dormant.

Classification by Structure

Effects of Volcanoes

Constructive: Enrich soil fertility → agriculture; geothermal energy; tourist spots; building materials.

Destructive: Earthquakes, floods, mudslides, rockfall; lava destroys everything in its path; dust/ash affects breathing; alters weather; disrupts transport.

8. Earthquakes

• Earthquakes are sudden vibrations in the Earth's crust spreading outward in all directions as waves.

Key Terms

Seismic Waves

9. Tsunami

• Tsunami is a Japanese term meaning "harbour waves."
• Large seismically generated sea waves caused by earthquakes, submarine explosions, and landslides.

10. The Pacific Ring of Fire

• A zone around the Pacific Ocean with high volcanic and seismic activity.
• Composite volcanoes are commonly located around the Pacific Ocean.

11. Igneous Rocks — Primary/Mother Rocks

• Igneous rocks are called Primary Rocks or Mother Rocks because all other rocks are directly or indirectly formed from them.

12. Worked examples

Example 1. What are the three layers of the Earth? Crust (SIAL), Mantle (SIMA), Core (NIFE).

Example 2. Where is magma found? In the mantle.

Example 3. What is the point of origin of an earthquake called? Focus (Hypocentre). NOT epicentre — epicentre is the point on the surface.

Example 4. Name an extinct volcano. Mt. Kilimanjaro, Tanzania.

Example 5. Why does SIAL float over SIMA? SIAL (silica + aluminium) is lighter than SIMA (silica + magnesium).

13. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

1. ______ is the rigid outer layer of the Earth — (a) Core / (b) Mantle / (c) Crust / (d) Inner core. Ans: (c) Crust.

2. ______ layer is made up of liquid iron — (a) Inner core / (b) Outer core / (c) Mantle / (d) Crust. Ans: (b) Outer core.

3. Magma is found in the ______ — (a) Crust / (b) Mantle / (c) Core / (d) None. Ans: (b) Mantle.

4. Diastrophism is connected to ______ — (a) Volcanism / (b) Earthquakes / (c) Tectonics / (d) Fold/Fault. Ans: (d) Fold/Fault.

5. The movement of tectonic plates is induced by ______ — (a) Hydel / (b) Thermal / (c) Wave / (d) Tidal. Ans: (b) Thermal.

6. In the ancient period, Gondwanaland moved towards ______ — (a) North / (b) South / (c) East / (d) West. Ans: (a) North.

7. Many million years ago, India was part of the supercontinent ______ — (a) Gondwana / (b) Laurasia / (c) Panthalasa / (d) Pangea. Ans: (a) Gondwana.

8. Movement of plates creating stress, stretching rocks and cracks results in ______ — (a) Fold / (b) Fault / (c) Mountain / (d) Earthquake. Ans: (b) Fault.

9. ______ refers to a bowl-shaped depression at the top of a volcano — (a) Crater / (b) Vent / (c) Chamber / (d) Volcanic cone. Ans: (a) Crater.

10. The point of origin of an Earthquake is called the ______ — (a) Epicentre / (b) Focus / (c) Seismic wave / (d) Magnitude. Ans: (b) Focus.

II. Match the following

III. Consider the given statements

11. (i) Mt. Fuji is a dormant volcano. (ii) Mt. Kilimanjaro is a dormant volcano. (iii) Mt. Tanzania is a dormant volcano. — (a) (i) is true / (b) (ii) is true / (c) (iii) is true / (d) all true. Ans: (a). (ii-wrong: Kilimanjaro is extinct; iii-wrong: Mt. Tanzania is not a volcano — Kilimanjaro is in Tanzania.)

12. Statement: Magma gushes out when it finds vents. Reason: Interior of Earth contains compressed hot magma. — (a) Both true / (b) Statement true, reason false / (c) Statement false, reason true / (d) Both false. Ans: (a).

13. Statement I: Mountain ranges formed by collision of tectonic plates. Statement II: Movement of tectonic plates due to thermal energy from the mantle. — (a) I false, II true / (b) Both false / (c) I true, II false / (d) Both true. Ans: (d).

IV. Answer in brief

14. Various spheres of the Earth — Lithosphere (solid outer part), Atmosphere (gases), Hydrosphere (water), Biosphere (life).

15. Layers of the interior — Crust/SIAL (outer, 5–30 km), Mantle/SIMA (~2,900 km, magma), Core/NIFE (innermost, hottest; solid inner + liquid outer). Core large iron → gravity + magnetic field.

16. Plate tectonics — Lithosphere divided into tectonic plates (major + micro). Float over mantle. Collisions → mountains, ocean features. Movement driven by thermal energy.

17. Volcanic components — Magma Chamber (reservoir), Vent (passage), Crater (bowl-shaped top depression), Volcanic Cone (mountain).

18. Earthquake and its occurrence — Sudden vibrations in Earth's crust; spread outward as waves from the focus. Measured by seismograph.

19. Seismic waves — P-Waves (fastest, first to reach, 5.3–10.6 km/s), S-Waves (only through solids, 1–8 km/s), L-Waves (surface, most destructive).

20. Tsunami — Japanese term "harbour waves." Large sea waves from earthquakes, submarine explosions, landslides.

21. Pacific Ring of Fire — Zone around Pacific Ocean with high volcanic and seismic activity. Composite volcanoes common here.

V. Give reasons

22. SIAL floats over SIMA — SIAL (silica + aluminium) is lighter than SIMA (silica + magnesium). Therefore SIAL floats over SIMA.

23. Igneous rocks are Primary/Mother Rocks — All other rocks are directly or indirectly formed from igneous rocks.

VI. Distinguish between

24. Core vs Crust — Core: innermost, hottest, NIFE (Ni + Fe), solid inner + liquid outer, below mantle. Crust: outermost, SIAL (Si + Al), rigid, above mantle.

25. Epicentre vs Hypocentre — Epicentre: point on surface directly above focus; maximum impact. Hypocentre (Focus): point of origin inside Earth; generates elastic waves.

26. Divergent vs Convergent boundaries — Divergent: plates pull apart; mid-oceanic ridges; volcanic islands. Convergent: plates move together; subduction zone; one plate sinks under another.

27. Primary vs Secondary waves — P-Waves: fastest, first to reach epicentre, 5.3–10.6 km/s, travel through solids and liquids. S-Waves: only through solids, shake perpendicular, 1–8 km/s.

28. Shield volcano vs Volcanic Dome — Shield: intense viscous lava, gently sloping, lava flows in all directions (e.g., Mauna Loa). Dome: slow ejection of viscous silica-rich lava, circular mound, doesn't flow far (e.g., Paricutin).

VII. Answer in a paragraph

29. Describe the structure of the Earth.

    • Crust: outer layer (5–30 km), SIAL (Silica + Aluminium), rigid, continental + oceanic.
    • Mantle: beneath crust (~2,900 km), SIMA (Silica + Magnesium), upper solid, lower molten (magma).
    • Core: innermost, NIFE (Nickel + Iron), solid inner + liquid outer. Iron → gravity + magnetic field. Magnetic compass works due to this.

30. Internal and external processes of Earth.

    • Geomorphic processes: formation/deformation of landforms.
    • Endogenetic (internal): forces from interior → surface; build landscape; radioactivity-powered; volcanism, earthquakes, tectonics.
    • Exogenetic (external): forces on surface (water, glacier, wind, waves); tear landscape down.

31. Classification of volcanoes by periodicity.

    • Active: constantly ejecting (Mt. St. Helens).
    • Dormant: inactive for long; may suddenly explode (Mt. Fuji).
    • Extinct: permanently stopped (Mt. Kilimanjaro).

32. Effects of volcanoes.

    • Constructive: fertile soil, geothermal energy, tourism, building materials.
    • Destructive: earthquakes, floods, mudslides, lava destroys everything, ash affects breathing, alters weather, disrupts transport.

VIII. Map Skill

On world outline map, mark: (a) Pacific Ring of Fire, (b) Earthquake-prone zones (any two), (c) Two active volcanoes, (d) Himalayas and Alps, (e) Rift valley of East Africa.

IX. HOTS

33. Sources of information about Earth's Interior — DIRECT vs INDIRECT:
    • Direct: Volcanic eruptions (bring deep materials to surface), deep mining, drilling projects.
    • Indirect: Seismic activity (wave behaviour reveals layer properties), Earth's magnetism, meteorites (composition similar to Earth's interior).

14. Common mistakes

• Mistake: Magma is found in the core. Fix: Magma is found in the mantle (lower part).
• Mistake: The point of origin of an earthquake is the epicentre. Fix: The focus (hypocentre) is the origin point. Epicentre is on the surface above it.
• Mistake: Mt. Kilimanjaro is a dormant volcano. Fix: Mt. Kilimanjaro is extinct (dead). Mt. Fuji is dormant.
• Mistake: The rigid outer layer of Earth is the mantle. Fix: The crust is the rigid outer layer.
• Mistake: Outer core is solid. Fix: The outer core is liquid iron. The inner core is solid.
• Mistake: Diastrophism is connected to volcanism. Fix: Diastrophism is connected to fold/fault (deformation of the crust).
• Mistake: Gondwanaland moved south. Fix: Gondwanaland moved towards the north.

15. Quick revision

• Geography Ch 1 · Lithosphere – I Endogenetic Processes.
• Earth's 4 spheres: Lithosphere, Atmosphere, Hydrosphere, Biosphere.
• Interior layers: Crust/SIAL (5–30 km) → Mantle/SIMA (~2,900 km, magma) → Core/NIFE (solid inner + liquid outer; iron → gravity + magnetic field).
• Plate tectonics: plates float on mantle; divergent (pull apart), convergent (subduction), transform.
• India was part of Gondwana (moved north).
• Volcanoes: Active (St. Helens), Dormant (Fuji), Extinct (Kilimanjaro). Components: Chamber, Vent, Crater, Cone.
• Earthquakes: Focus (origin) vs Epicentre (surface). Seismic waves: P (fastest), S (solids only), L (surface, most destructive).
• Tsunami = Japanese "harbour waves."
• Pacific Ring of Fire. Igneous rocks = Primary/Mother rocks.