Structure of the Earth
Overview
The Earth is composed of three main layers: the crust, the mantle, and the core. These layers differ in composition, temperature, density, and physical state. The structure is studied through seismic waves generated by earthquakes. This chapter also covers the three major rock types, the processes of volcanism, and the causes and measurement of earthquakes.
Internal Structure of the Earth
| Layer | Thickness | Composition | Temperature | State |
|---|
| Crust | 5–70 km | Granite (continental), Basalt (oceanic) | 200–1,000°C | Solid |
| Mantle | 2,900 km | Silicates of iron and magnesium | 1,000–3,700°C | Solid (but plastic/ductile in asthenosphere) |
| Outer core | 2,200 km | Iron and nickel (liquid) | 3,700–4,500°C | Liquid |
| Inner core | 1,220 km radius | Iron and nickel (solid) | ~5,500°C | Solid |
The Crust
| Feature | Continental Crust | Oceanic Crust |
|---|
| Thickness | 30–70 km | 5–10 km |
| Density | 2.7 g/cm³ | 3.0 g/cm³ |
| Composition | Granite (silica + aluminium) | Basalt (silica + magnesium) |
| Age | Up to 4 billion years | Up to 200 million years |
The Mantle
The mantle is the largest layer of the Earth (84% of volume). The upper part includes the asthenosphere — a partially molten, plastic layer on which tectonic plates move.
The Core
The core is extremely hot (~5,500°C at the centre — as hot as the Sun's surface). The outer core is liquid and generates Earth's magnetic field. The inner core is solid despite the temperature because of immense pressure.
Rocks and Their Types
The Rock Cycle
IGNEOUS → (weathering) → SEDIMENTARY → (heat/pressure) → METAMORPHIC → (melting) → MAGMA → (cooling) → IGNEOUS
Igneous Rocks
| Aspect | Details |
|---|
| Formation | Cooling and solidification of magma/lava |
| Texture | Crystalline (larger crystals = slower cooling) |
| Types | Intrusive (e.g., granite) and Extrusive (e.g., basalt) |
| Rock | Type | Use |
|---|
| Granite | Intrusive igneous | Building stone, kitchen countertops |
| Basalt | Extrusive igneous | Road metal, black soil formation |
| Pumice | Extrusive igneous | Abrasive, lightweight concrete |
| Obsidian | Extrusive igneous | Sharp tools (used in prehistoric times) |
Sedimentary Rocks
| Aspect | Details |
|---|
| Formation | Deposition, compaction, and cementation of sediments |
| Texture | Layered/stratified |
| Fossils | Often contain fossils (unlike igneous or metamorphic) |
| Rock | Type | Use |
|---|
| Sandstone | Clastic | Building material |
| Limestone | Organic | Cement, building stone |
| Coal | Organic | Fuel |
| Shale | Clastic | Pottery, bricks |
| Conglomerate | Clastic | Decorative stone |
| Aspect | Details |
|---|
| Formation | Existing rocks altered by heat and/or pressure |
| Types | Foliated (layered, e.g., schist) and Non-foliated (e.g., marble) |
| Parent Rock | Metamorphic Rock | Use |
|---|
| Limestone | Marble | Sculpture, buildings |
| Sandstone | Quartzite | Building material |
| Shale | Slate | Roofing |
| Granite | Gneiss | Decorative stone |
| Coal | Graphite | Pencils, lubricant |
Volcanoes
| Aspect | Details |
|---|
| Definition | An opening in the Earth's crust through which magma, gases, and ash escape |
| Location | Mostly along plate boundaries (Pacific Ring of Fire) |
Types of Volcanoes
| Type | Characteristics | Example |
|---|
| Active | Erupting or has erupted in recent history | Mount Etna, Kilauea |
| Dormant | Not currently active but could erupt again | Mount Vesuvius |
| Extinct | No longer capable of eruption | Mount Kilimanjaro |
Products of Volcanic Eruptions
| Product | Description |
|---|
| Lava flow | Molten rock on the surface (basaltic = fluid; andesitic = viscous) |
| Ash and tephra | Fragmented rock material ejected into the air |
| Volcanic gases | Water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide |
| Pyroclastic flow | Superheated gas and ash racing down the slope (deadliest) |
Famous Volcanic Eruptions
| Eruption | Year | Deaths | Notes |
|---|
| Mount Vesuvius | 79 CE | ~16,000 | Buried Pompeii and Herculaneum |
| Krakatoa | 1883 | ~36,000 | Heard 3,000 km away; caused tsunamis |
| Mount Pelée | 1902 | ~30,000 | Pyroclastic flow destroyed St. Pierre |
| Mount St. Helens | 1980 | 57 | Most deadly in US history |
Earthquakes
| Aspect | Details |
|---|
| Cause | Sudden release of energy along faults in the Earth's crust |
| Focus (Hypocentre) | Point underground where the earthquake originates |
| Epicentre | Point on the surface directly above the focus |
Seismic Waves
| Wave Type | Speed | Motion | Travel Through |
|---|
| P-wave (Primary) | Fastest (5–8 km/s) | Compressional (back-and-forth) | Solids, liquids, gases |
| S-wave (Secondary) | Slower (3–5 km/s) | Transverse (side-to-side) | Solids only |
| L-wave (Surface/Love) | Slowest | Horizontal rolling | Surface only — most destructive |
Richter Scale
The Richter scale measures the magnitude of an earthquake logarithmically. Each whole number increase represents a 10-fold increase in amplitude and approximately 31.6 times more energy release.
| Magnitude | Effects |
|---|
| < 2.0 | Not felt by people |
| 2.0–3.0 | Felt by some; minor |
| 3.0–5.0 | Felt by many; minor damage possible |
| 5.0–6.0 | Damage to weak buildings |
| 6.0–7.0 | Destructive; serious damage |
| 7.0–8.0 | Major earthquake; widespread damage |
| > 8.0 | Great earthquake; total destruction over large area |
Major Earthquakes
| Earthquake | Year | Magnitude | Deaths |
|---|
| Bhuj (Gujarat, India) | 2001 | 7.7 | 20,000+ |
| Indian Ocean tsunami (Sumatra) | 2004 | 9.1 | 230,000+ |
| Haiti | 2010 | 7.0 | 160,000+ |
| Nepal (Gorkha) | 2015 | 7.8 | 9,000+ |
Self-Test
-
Fill in the blank: The Earth's core is composed mainly of ______ and ______. (Answer: iron; nickel)
-
True or False: P-waves can travel through liquids. (Answer: True)
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Match: (a) Granite — Igneous; (b) Marble — Metamorphic; (c) Sandstone — Sedimentary. (Answer: All correct)
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Name the scale: Which scale is used to measure the magnitude of an earthquake? (Answer: Richter scale)
-
Explain: What is the difference between magma and lava? (Answer: Magma is molten rock beneath the Earth's surface; lava is magma that has reached the surface.)
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Critical thinking: Why do most volcanoes and earthquakes occur along plate boundaries? (Answer: Plate boundaries are zones of geological stress where plates collide, separate, or slide past each other, creating conditions for earthquakes and volcanic activity.)
Summary
The Earth's layered structure — crust, mantle, and core — is fundamental to understanding geological processes. The rock cycle shows how igneous, sedimentary, and metamorphic rocks are interconnected through weathering, heat, and pressure. Volcanoes and earthquakes are dramatic manifestations of the Earth's internal energy, occurring predominantly along plate boundaries. Understanding these phenomena is essential for hazard preparedness and for interpreting the Earth's geological history. For ICSE students, this chapter provides the foundation for physical geography and geology.
This chapter is aligned with the ICSE Class 9 2025–26 Geography syllabus prescribed by the Council for the Indian School Certificate Examinations (CISCE).
