Ch-3: Interior of the Earth
1. Sources of Information
- Direct Sources: Mining (Surface rocks), Deep Ocean Drilling (Kola, Arctic), Volcanic Eruptions (Magma).
- Indirect Sources: Meteors, Gravitation (g), Magnetic Field, Seismic Activity (Most important).
2. Earthquake Waves
Energy releases at Focus (Hypocentre) & reaches surface at Epicentre.
A. Body Waves (Travel through earth):
- P-Waves (Primary): Faster, like Sound waves. Travel through Solid, Liquid & Gas.
- S-Waves (Secondary): Slower. Travel ONLY through Solids. (Crucial for proving Outer Core is liquid).
B. Surface Waves: Last to report on seismograph. Most destructive.
Shadow Zones
Specific areas where waves are not reported.
- P-Wave Shadow: 105° to 145° band.
- S-Wave Shadow: Beyond 105° (Larger area, 40% of earth).
3. Structure of the Earth
- Crust: Brittle. Oceanic (5km) is thinner than Continental (30km).
- Mantle: Up to 2900km. Upper part = Asthenosphere (Weak, Magma source). Crust + Upper Mantle = Lithosphere.
- Core: Heavy materials (NiFe - Nickel & Iron). Outer Core (Liquid), Inner Core (Solid).
4. Volcanoes & Landforms
Types: Shield (Hawaiian, fluid), Composite (Explosive), Caldera (Collapsed), Flood Basalt (Deccan Traps), Mid-Ocean Ridge.
Intrusive Forms (Magma cooled inside)
- Batholiths: Large granitic domes.
- Lacoliths: Dome with level base.
- Sills/Sheets: Horizontal deposits.
- Dykes: Wall-like vertical structures.
NCERT Solutions
Q1. Multiple Choice Questions
(i) More destructive waves?
Ans: (c) Surface waves
(ii) Direct source of information?
Ans: (b) Volcanoes
(iii) Deccan Trap caused by?
Ans: (b) Flood basalt eruptions.
(iv) Lithosphere describes?
Ans: (b) Crust and upper mantle
Q2. Short Answers (30 words)
(i) What are Body Waves?
Body waves are generated due to the release of energy at the focus. They move in all directions travelling through the body of the earth. Types: P and S waves.
(ii) Direct Sources?
Deep earth mining (surface rocks), deep ocean drilling projects (like Kola), and volcanic eruptions (analysis of magma) are direct sources.
(iii) Why Shadow Zone?
Because P and S waves follow properties of reflection and refraction. S-waves cannot pass through liquid (outer core), creating a large shadow zone beyond 105°.
Long Answers & Activity
Q3. Long Answers (150 words)
(i) Effects of Propagation of Waves
Earthquake waves cause vibration in the rocks they pass through.
P-Waves: They vibrate parallel to the direction of the wave. This creates density differences leading to stretching and squeezing of the material.
S-Waves: They vibrate perpendicular to the direction of propagation (vertical plane). This creates troughs and crests in the material.
Surface Waves: These are considered most damaging as they cause displacement of rocks and collapse of structures.
(ii) Intrusive Forms
When magma cools within the crustal portions, it assumes different forms called Intrusive Forms.
1. Batholiths: Large body of magmatic material (granitic) cooled at deep depths.
2. Lacoliths: Large dome-shaped bodies with a level base, connected by a pipe-like conduit.
3. Lapolith: Saucer-shaped body concave to the sky.
4. Phacolith: Wavy mass at base of synclines or top of anticlines.
5. Sills/Dykes: Horizontal sheets are Sills; Vertical wall-like structures are Dykes.
Suggested Activity
Model of Earth's Interior:
Using clay or a ball, cut out a section to show layers.
1. Crust: Thin outer layer (Brown).
2. Mantle: Thick middle layer (Red/Orange).
3. Core: Inner ball (Yellow/White) for molten/solid NiFe.