PrepYodhaClass Notes · Geography
Geography · World · 04

Interior of the Earth & Rocks

The Earth's interior is divided into three major layers — the crust, the mantle, and the core — each with distinct composition, depth, and physical state. Understanding these layers helps explain why earthquakes occur, why volcanoes erupt, and how tectonic plates move. The Earth's crust (also called the Lithosphere) is made of rocks, which themselves are classified based on how they form — igneous, sedimentary, or metamorphic. The scientific study of rocks is called Petrology.

🪨 10 topics🎯 94+ points📝 self-test
📘
Topic 01

Layers of the Earth — Overview

📄
HANDWRITTEN PDF NOTES
Interior of the Earth & Rocks — downloadable PDF
Open PDF ↗Download
⭐
Key Point
The Earth's interior is divided into Crust, Upper Mantle, Lower Mantle, Outer Core, and Inner Core.
  • The interior is understood through seismic waves, since direct observation below 30 km is not possible.
  • The interior gets progressively hotter and denser with depth.
LayerApproximate DepthStateKey Composition
Crust (Continental)0–30 kmSolidSIAL (Silica + Alumina)
Crust (Oceanic)0–5 kmSolidSIMA (Silica + Magnesium)
Upper Mantle (Asthenosphere)30–700 kmSemi-solid / PlasticSilica, Magnesium, Iron
Lower Mantle700–2,900 kmSolid but ductileSilica, Magnesium, Iron
Outer Core2,900–5,150 kmLiquidNickel + Iron (NIFE)
Inner Core5,150–6,371 kmSolidNickel + Iron (NIFE)
📝 Quick self-test 2 MCQs · 2 fill-ups

In which order are the Earth's interior layers arranged from outside to centre?

  1. Crust, Mantle, Core, Asthenosphere
  2. Crust, Upper Mantle, Lower Mantle, Outer Core, Inner Core
  3. Core, Mantle, Crust
  4. Crust, Outer Core, Inner Core, Mantle
✔ B. Crust, Upper Mantle, Lower Mantle, Outer Core, Inner Core — The interior is divided into Crust, Upper Mantle, Lower Mantle, Outer Core and Inner Core.

What is the composition of the Outer Core (NIFE)?

  1. Silica + Alumina
  2. Silica + Magnesium
  3. Nickel + Iron
  4. Magnesium + Iron
✔ C. Nickel + Iron — The Outer Core is composed of Nickel and Iron (NIFE).

The Earth's interior is studied through waves, since direct observation is not possible.

✔ seismic

The continental crust is made of SIAL, which stands for Silica plus .

✔ Alumina
🪨
Topic 02

The Crust

⭐
Key Point
The crust is the outermost layer of the Earth, also called the skin of the Earth, ranging from 5 km to 30 km in thickness.
  • The crust forms only about 1% of Earth's total volume.
  • The thickness of the crust is greater below continents than below the ocean floor.
Continental vs Oceanic Crust
  • Continental crust is made up of SIAL (Silica + Alumina). It is thicker and less dense.
  • Oceanic crust is made up of SIMA (Silica + Magnesium). It is thinner and denser.
  • The boundary between the upper crust and the lower crust is called the Conrad Boundary.
📝 Quick self-test 2 MCQs · 2 fill-ups

What percentage of Earth's total volume does the crust form?

  1. 1%
  2. 10%
  3. 50%
  4. 84%
✔ A. 1% — The crust forms only about 1% of Earth's total volume.

Continental crust is made up of which combination?

  1. SIMA (Silica + Magnesium)
  2. SIAL (Silica + Alumina)
  3. NIFE (Nickel + Iron)
  4. Silica + Iron
✔ B. SIAL (Silica + Alumina) — Continental crust is made up of SIAL (Silica + Alumina) and is thicker and less dense.

The crust is also called the skin of the Earth and ranges from 5 km to km in thickness.

✔ 30

The boundary between the upper crust and the lower crust is called the Boundary.

✔ Conrad
🪨
Topic 03

The Mantle

⭐
Key Point
The mantle lies between the crust and the outer core, and is approximately 2,900 km thick — making up about 84% of Earth's total volume.
  • The mantle is composed of silica, magnesium, and iron.
  • The mantle is generally in a solid state, though the upper part (asthenosphere) is semi-molten and plastic.
  • The mantle is divided into the Upper Mantle and the Lower Mantle.
Asthenosphere
  • The upper part of the mantle is called the Asthenosphere.
  • It is semi-molten and plastic in nature, which allows tectonic plates to move over it.
  • The asthenosphere flows and moves the plates of the Earth — driving plate tectonics and volcanic activity.
📝 Quick self-test 2 MCQs · 2 fill-ups

Approximately what percentage of Earth's total volume does the mantle make up?

  1. 1%
  2. 16%
  3. 84%
  4. 100%
✔ C. 84% — The mantle is about 2,900 km thick and makes up about 84% of Earth's total volume.

Which layer allows tectonic plates to move over it because it is semi-molten and plastic?

  1. Inner Core
  2. Asthenosphere
  3. Lower Mantle
  4. Conrad Boundary
✔ B. Asthenosphere — The asthenosphere (upper mantle) is semi-molten and plastic, allowing tectonic plate movement.

The mantle is composed of silica, magnesium and .

✔ iron

The upper part of the mantle is called the .

✔ Asthenosphere
🪨
Topic 04

The Core

⭐
Key Point
The core is the innermost and hottest layer of the Earth, lying below the mantle.
  • The core is composed mainly of Nickel (Ni) and Iron (Fe), hence it is also called NIFE or Barysphere.
  • The core is divided into two parts: the solid Inner Core and the liquid Outer Core.
  • Nickel makes the core strong; Iron (a heavy metal) makes it very dense.
Outer Core
  • Lies from 2,900 km to 5,150 km depth.
  • State: Liquid — composed of molten Nickel and Iron.
  • The movement of liquid iron in the outer core generates Earth's magnetic field.
Inner Core
  • Lies from 5,150 km to 6,371 km (Earth's centre).
  • State: Solid — extreme pressure keeps it solid despite very high temperatures.
📝 Quick self-test 2 MCQs · 2 fill-ups

The core is also called NIFE or:

  1. Lithosphere
  2. Asthenosphere
  3. Barysphere
  4. Biosphere
✔ C. Barysphere — The core is called NIFE or Barysphere, composed mainly of Nickel and Iron.

What is the physical state of the Inner Core?

  1. Liquid
  2. Solid
  3. Gaseous
  4. Plastic
✔ B. Solid — The Inner Core is solid because extreme pressure keeps it solid despite high temperatures.

The movement of liquid iron in the outer core generates Earth's field.

✔ magnetic

The Outer Core lies from 2,900 km to km depth.

✔ 5,150
📘
Topic 05

Discontinuities

⭐
Key Point
Discontinuities are the boundary zones between different layers of the Earth, identified by sudden changes in seismic wave velocity.
DiscontinuitySeparatesDepth
Conrad DiscontinuityUpper Crust (SIAL) from Lower Crust (SIMA)Within crust
Mohorovicic (Moho) DiscontinuityCrust from Mantle~30 km (continental)
Repetti DiscontinuityUpper Mantle from Lower Mantle~700 km
Gutenberg DiscontinuityMantle from Outer Core~2,900 km
Lehmann DiscontinuityOuter Core from Inner Core~5,150 km
  • The Moho discontinuity (Mohorovicic) is the most important — it separates the crust from the mantle.
📝 Quick self-test 2 MCQs · 2 fill-ups

Which discontinuity separates the crust from the mantle?

  1. Conrad Discontinuity
  2. Mohorovicic (Moho) Discontinuity
  3. Gutenberg Discontinuity
  4. Lehmann Discontinuity
✔ B. Mohorovicic (Moho) Discontinuity — The Moho (Mohorovicic) Discontinuity separates the crust from the mantle.

The Gutenberg Discontinuity separates:

  1. Upper crust from lower crust
  2. Crust from mantle
  3. Mantle from Outer Core
  4. Outer Core from Inner Core
✔ C. Mantle from Outer Core — The Gutenberg Discontinuity separates the mantle from the outer core at about 2,900 km.

The Discontinuity separates the Outer Core from the Inner Core at about 5,150 km.

✔ Lehmann

The Repetti Discontinuity separates the Upper Mantle from the Lower Mantle at about km.

✔ 700
🪨
Topic 06

Rocks — Classification

⭐
Key Point
A rock is an aggregate of one or more minerals found on the Earth's crust (Lithosphere).
  • A rock can be hard and compact like Granite, or soft as clay, or loose as sand.
  • The scientific study of rocks is called Petrology.
  • Based on their formation, rocks are classified into three types:
TypeHow FormedExamples
Igneous RocksSolidification of magma or lavaGranite, Basalt, Pegmatite
Sedimentary RocksDeposition and compaction of sedimentsSandstone, Limestone, Coal, Gypsum
Metamorphic RocksChange in igneous/sedimentary rocks due to heat and pressureMarble, Quartzite, Slate, Diamond
📝 Quick self-test 2 MCQs · 2 fill-ups

The scientific study of rocks is called:

  1. Geology
  2. Petrology
  3. Seismology
  4. Mineralogy
✔ B. Petrology — The scientific study of rocks is called Petrology.

Rocks are classified into three types based on their:

  1. Colour
  2. Hardness
  3. Formation
  4. Location
✔ C. Formation — Based on their formation, rocks are classified into igneous, sedimentary and metamorphic types.

A rock is an aggregate of one or more found on the Earth's crust.

✔ minerals

Marble and Quartzite are examples of rocks.

✔ Metamorphic
🪨
Topic 07

Igneous Rocks

  • All igneous rocks are formed from magma or lava.
  • Igneous rocks do not contain fossils.
  • They are associated with volcanic activities.
Types of Igneous Rocks
  • Intrusive (Plutonic) Rocks — magma cools slowly beneath the Earth's surface; coarse-grained. Example: Granite.
  • Extrusive (Volcanic) Rocks — lava cools quickly on the Earth's surface; fine-grained. Example: Basalt.

Examples: Granite, Pegmatite, Basalt, etc.

⭐
Key Point
Igneous Rocks are formed by the solidification of molten magma — they are called Primary Rocks or Parent Rocks because all other rocks are ultimately formed from igneous rocks.
📝 Quick self-test 2 MCQs · 2 fill-ups

Igneous rocks are also called Primary Rocks or:

  1. Fossil Rocks
  2. Parent Rocks
  3. Layered Rocks
  4. Changed Rocks
✔ B. Parent Rocks — Igneous rocks are called Primary Rocks or Parent Rocks because all other rocks form from them.

Which is an example of an extrusive (volcanic) igneous rock that cools quickly on the surface?

  1. Granite
  2. Basalt
  3. Marble
  4. Limestone
✔ B. Basalt — Basalt is an extrusive rock that cools quickly on the surface and is fine-grained.

Igneous rocks are formed by the solidification of molten .

✔ magma

Igneous rocks do not contain .

✔ fossils
🪨
Topic 08

Sedimentary Rocks

⭐
Key Point
Sedimentary Rocks are formed by the aggregation and compaction of sediments derived from older rocks, plants, and animals — they contain fossils.
  • The bodies of animals and plants that fall on deposits get embedded in the layers and form fossils.
  • Formation process: Weathering & Erosion → Deposition → Compaction & Cementation → Sedimentary Rock.
Examples of Sedimentary Rocks
RockHow Formed
SandstoneFormed from sand grains
LimestoneFormed from remains of marine organisms
ChalkA soft limestone formed from tiny shells
GypsumFormed from evaporation of water containing minerals
CoalFormed from dead plant material in swampy areas
ConglomerateFormed from rounded pebbles and fragments
  • Coal is a sedimentary rock but an important source of energy.
📝 Quick self-test 2 MCQs · 2 fill-ups

Which type of rock contains fossils?

  1. Igneous
  2. Sedimentary
  3. Metamorphic
  4. Volcanic
✔ B. Sedimentary — Sedimentary rocks contain fossils as bodies of plants and animals get embedded in layers.

Which sedimentary rock is formed from the remains of marine organisms?

  1. Sandstone
  2. Limestone
  3. Coal
  4. Conglomerate
✔ B. Limestone — Limestone is formed from the remains of marine organisms.

Coal is formed from dead material in swampy areas.

✔ plant

The formation process of sedimentary rock ends with compaction and .

✔ cementation
🪨
Topic 09

Metamorphic Rocks

⭐
Key Point
Metamorphic Rocks are the changed form of igneous and sedimentary rocks, formed when existing rocks are subjected to extreme heat, pressure, and chemical action.
  • When igneous or sedimentary rocks undergo complete change in form and character under heat and pressure, they become metamorphic rocks.
Examples of Metamorphic Rocks
Original RockBecomes (Metamorphic)
LimestoneMarble
SandstoneQuartzite
ShaleSlate
Carbon (coal/graphite)Diamond
  • Marble — formed from limestone; used in buildings and statues.
  • Quartzite — formed from sandstone; very hard and durable.
  • Slate — formed from shale; used in roofing, flooring, and writing slates.
  • Diamond — formed from carbon under high pressure and temperature.
📝 Quick self-test 2 MCQs · 2 fill-ups

Metamorphic rocks are formed when existing rocks are subjected to:

  1. Cooling of magma
  2. Extreme heat, pressure and chemical action
  3. Deposition of sediments
  4. Evaporation of water
✔ B. Extreme heat, pressure and chemical action — Metamorphic rocks form when rocks undergo extreme heat, pressure and chemical action.

Limestone changes into which metamorphic rock?

  1. Slate
  2. Quartzite
  3. Marble
  4. Diamond
✔ C. Marble — Limestone becomes Marble through metamorphism.

Sandstone becomes the metamorphic rock .

✔ Quartzite

Shale becomes the metamorphic rock , used in roofing and flooring.

✔ Slate
🪨
Topic 10

The Rock Cycle

`` flowchart LR A[Magma / Lava] -->|Cooling & Solidification| B[Igneous Rock] B -->|Weathering, Erosion, Deposition, Compaction| C[Sedimentary Rock] C -->|Heat & Pressure| D[Metamorphic Rock] D -->|Heat & Pressure| C B -->|Heat & Pressure| D D -->|Melting| A C -->|Melting| A B -->|Melting| A ``

⭐
Key Point
The Rock Cycle shows how the three rock types are interconverted through geological processes over long periods.
  • Igneous rocks are the starting point (Primary Rocks).
  • All rock types can be melted back into magma, completing the cycle.
  • The cycle has no fixed start or end — it is continuous.
📝 Quick self-test 2 MCQs · 2 fill-ups

The Rock Cycle shows how the three rock types are:

  1. Destroyed permanently
  2. Interconverted through geological processes
  3. Formed only once
  4. Kept separate forever
✔ B. Interconverted through geological processes — The Rock Cycle shows how rock types are interconverted through geological processes.

Which rock type is the starting point of the Rock Cycle?

  1. Sedimentary
  2. Metamorphic
  3. Igneous
  4. Marble
✔ C. Igneous — Igneous rocks are the starting point (Primary Rocks) of the Rock Cycle.

All rock types can be melted back into , completing the cycle.

✔ magma

The Rock Cycle has no fixed start or end because it is .

✔ continuous
🎯
Recap

Quick Revision

  • The Earth's interior: Crust → Upper Mantle → Lower Mantle → Outer Core → Inner Core.
  • Crust thickness: 5 km (oceanic) to 30 km (continental); forms only 1% of Earth's volume.
  • SIAL = Silica + Alumina (continental crust); SIMA = Silica + Magnesium (oceanic crust).
  • Conrad Boundary separates upper crust (SIAL) from lower crust (SIMA).
  • Moho Discontinuity (at ~30 km) separates crust from mantle.
  • Asthenosphere = upper mantle; semi-molten, plastic — allows tectonic plate movement.
  • Mantle = 2,900 km thick; 84% of Earth's volume; composed of silica, magnesium, iron.
  • Core = NIFE (Nickel + Iron); Outer Core is liquid (2,900–5,150 km); Inner Core is solid (5,150–6,371 km).
  • Gutenberg Discontinuity at 2,900 km separates mantle from outer core.
  • Petrology = scientific study of rocks; Rock = aggregate of minerals.
  • Igneous rocks = Primary/Parent rocks; no fossils; e.g. Granite (intrusive), Basalt (extrusive).
  • Sedimentary rocks = contain fossils; e.g. Sandstone, Limestone, Coal, Gypsum, Chalk.
  • Metamorphic rocks = heat + pressure; e.g. Marble (from limestone), Quartzite (from sandstone), Slate (from shale).
  • Rock Cycle: Igneous → Sedimentary → Metamorphic → back to Magma (continuous process).

Test Yourself

Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.