Notes For All Chapters – Geography Class 8
Interior of the Earth
Introduction
- The Earth was formed around 4.6 billion years ago.
- Initially, it was in a gaseous state, which later cooled and solidified.
- The Earth’s interior is not directly observable, but its composition is understood through:
- Volcanic eruptions (study of lava, gases, and rocks).
- Seismic waves (generated by earthquakes).
- Temperature, density, and gravitational force estimations.
Layers of the Earth
The Earth’s interior is divided into three main layers:
1. Crust (Outermost Layer)
Thin and solid outermost layer of the Earth.
Thickness:
- 30-35 km on average.
- 16-45 km under continents.
- Less than 10 km under oceans.
Temperature:
Increases with depth, reaching 5500°C – 6000°C at the core.
Sub-layers of the Crust:
Continental Crust (SIAL):
1. Made up of Silica (Si) and Aluminium (Al).
Density: 2.65 – 2.90 g/cm³.
Rocks: Granite is commonly found.
2. Oceanic Crust (SIMA):
- Made up of Silica (Si) and Magnesium (Mg).
- Density: 2.9 – 3.3 g/cm³.
- Rocks: Basalt and Gabbro are common.
Important Discontinuities:
- Conrad Discontinuity: Between continental and oceanic crust.
- Moho Discontinuity: Between the crust and the mantle.
2. Mantle (Middle Layer)
Located below the crust, extends up to 2,900 km depth.
Divided into two parts:
1. Upper Mantle (Asthenosphere):
Semi-liquid layer with magma chambers.
Temperature: 2200°C – 2500°C.
Responsible for earthquakes, volcanic eruptions, and mountain formation.
2. Lower Mantle:
- More solid due to high pressure.
- Density: 5.7 g/cm³.
Gutenberg Discontinuity: Boundary between mantle and core.
3. Core (Innermost Layer)
Extends from 2,900 km to 6,371 km.
Made up of Iron (Fe) and Nickel (Ni) → also called NIFE.
Divided into:
1. Outer Core (Liquid)
Depth: 2,900 km – 5,100 km.
Density: 9.8 g/cm³.
Temperature: Around 5000°C.
Seismic waves: Secondary waves (S-waves) cannot pass (proof of liquid nature).
Generates Earth’s magnetic field due to moving iron currents (geo-dynamo effect).
2. Inner Core (Solid)
- Depth: 5,150 km – 6,371 km.
- Density: 13.3 g/cm³.
- Temperature: Same as the Sun’s surface.
- Solid due to extreme pressure.
Seismic Waves and Earth’s Interior
- Primary (P) Waves: Travel through solids and liquids.
- Secondary (S) Waves: Travel only through solids (stopped by the outer core).
- Magnetosphere: Protects Earth from solar winds.
Key Takeaways
- Crust: Outermost, solid, made of silica, aluminium, and magnesium.
- Mantle: Semi-solid, rich in magma, causes earthquakes and volcanoes.
- Core: Iron-Nickel composition, outer core is liquid, inner core is solid.
- Discontinuities: Moho, Gutenberg, and Conrad separate layers.
- Seismic studies help us understand the Earth’s interior.
Conclusion
The study of the Earth’s interior helps us understand natural disasters like earthquakes and volcanic eruptions. Scientists use seismic waves and volcanic activity to gain insight into the composition and structure of the Earth’s layers.
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