Inside Our Earth — AP Class 7 Geography
1. About the Chapter
'We live ON the Earth. But what lies BENEATH our feet?' This chapter takes us on a journey INSIDE the Earth — from the thin crust to the hot core. We also study rocks — the BUILDING BLOCKS of the Earth's crust.
Key Themes
- Interior structure of the Earth
- Crust, mantle, core
- Types of rocks: igneous, sedimentary, metamorphic
- The rock cycle
- Minerals and their uses
- AP focus: Geology of Andhra Pradesh, Eastern Ghats rocks
2. Interior of the Earth
The Three Layers
| Layer | Thickness | Composition | Temperature |
|---|---|---|---|
| Crust | 5–100 km | Silica, alumina (SIAL) + Silica, magnesium (SIMA) | Increases with depth |
| Mantle | 2,900 km | Silicate minerals, iron, magnesium | 1,000–3,700°C |
| Core | 3,500 km radius | Nickel + Iron (NiFe) | 4,400–6,000°C |
Crust
'The CRUST is the THINNEST layer — like the SKIN of an apple compared to the whole fruit.'
| Type | Thickness | Found |
|---|---|---|
| Continental crust | 30–100 km | Under continents (lighter — granite) |
| Oceanic crust | 5–10 km | Under oceans (heavier — basalt) |
Mantle
- The THICKEST layer — extends to 2,900 km depth
- Partially molten rock called MAGMA
- Magma moves in CONVECTION CURRENTS — this causes PLATE TECTONICS
Core
- Outer core: Liquid (molten nickel-iron)
- Inner core: SOLID (immense pressure keeps it solid despite high temperature)
- 'The CORE is as HOT as the surface of the SUN'
3. Rocks — The Building Blocks
What is a Rock?
'A rock is a NATURAL, solid material made of one or MORE minerals.' Rocks form the Earth's crust.
Types of Rocks
| Type | How Formed | Examples | Characteristics |
|---|---|---|---|
| Igneous | Cooling of MAGMA/LAVA | Granite, Basalt, Pumice | HARD, no fossils, crystalline |
| Sedimentary | Deposition of sediments | Sandstone, Limestone, Shale | LAYERED, may contain FOSSILS |
| Metamorphic | Changed by heat/pressure | Marble, Slate, Quartzite | DENSE, banded or foliated |
4. Igneous Rocks — 'Fire-Born'
Formation
'Igneous' comes from the Latin word 'IGNIS' meaning FIRE. These rocks form when magma or lava COOLS and SOLIDIFIES.
| Type | Where it Forms | Cooling | Crystal Size | Example |
|---|---|---|---|---|
| Intrusive | INSIDE the Earth (magma) | SLOW | LARGE crystals | Granite |
| Extrusive | ON the Earth's surface (lava) | FAST | SMALL/NO crystals | Basalt, Pumice |
AP Example: Basalt in Rayalaseema
The Deccan Traps (basalt rock) EXTEND into parts of AP (Rayalaseema). 'Basalt is DARK, heavy, and forms when lava COOLS quickly.' This rock makes the soil RICH in minerals — good for cotton cultivation in parts of AP.
Pumice is the LIGHTEST rock — it FLOATS on water (full of gas bubbles).
5. Sedimentary Rocks — 'Water-Born'
Formation
'These rocks form from SEDIMENTS — sand, mud, shells — that settle in LAYERS over millions of years.'
Process: Weathering → Erosion → Deposition → Compaction → Cementation
Types of Sedimentary Rocks
| Rock | Made From | Uses |
|---|---|---|
| Sandstone | Sand grains | Building material |
| Limestone | Shells, coral skeletons | Cement, buildings |
| Shale | Clay/mud | Bricks, pottery |
| Conglomerate | Pebbles + sand cemented | Decorative |
Why Sedimentary Rocks are Special
- They are LAYERED (strata) — oldest at bottom, youngest at top
- They CONTAIN FOSSILS — the remains of ancient life
- 'Fossils in sedimentary rocks tell us the STORY of LIFE on Earth'
AP Example: Limestone in AP
AP has EXTENSIVE limestone deposits — especially in Kurnool, Guntur, and Prakasam districts. Limestone is used for CEMENT manufacturing. AP is a major cement-producing state because of these limestone deposits.
6. Metamorphic Rocks — 'Changed by Fire and Pressure'
Formation
'Metamorphic' means 'CHANGE OF FORM.' When existing rocks are subjected to HIGH HEAT and PRESSURE inside the Earth, they TRANSFORM.
How Rocks Change
| Original Rock | Metamorphic Rock | Change |
|---|---|---|
| Limestone (sedimentary) | Marble | Recrystallisation — marble is HARDER |
| Sandstone (sedimentary) | Quartzite | Very HARD — quartz grains fuse |
| Shale (sedimentary) | Slate | FOLIATED — splits into thin sheets |
| Granite (igneous) | Gneiss | BANDED — alternating light and dark layers |
Marble — the Favourite of Builders
'Marble is LIMESTONE that has been BAKED and PRESSURED into something BEAUTIFUL.' Used in the Taj Mahal (white marble from Makrana, Rajasthan). Marble is also found in some parts of AP.
7. The Rock Cycle
'The SAME Earth material cycles through ALL three rock types over MILLIONS of years.'
Igneous Rock
↓ (weathering + erosion)
Sediments
↓ (deposition + compaction)
Sedimentary Rock
↓ (heat + pressure)
Metamorphic Rock
↓ (melting)
Magma
↓ (cooling)
Igneous Rock (back to start)
'There is NO beginning and NO end — rocks TRANSFORM from one type to another continuously.'
8. Minerals in AP
What Andhra Pradesh is Rich In
| Mineral | District | Uses |
|---|---|---|
| Limestone | Kurnool, Guntur, Prakasam | Cement |
| Bauxite (aluminium) | Visakhapatnam (Araku valley) | Aluminium |
| Manganese | Srikakulam, Vizianagaram | Steel-making |
| Gold | Chittoor, Anantapur | Jewellery |
| Diamonds | Kollur (Guntur) — HISTORIC | Gems (Koh-i-Noor came from here!) |
| Graphite | Vishakhapatnam | Pencils, batteries |
| Barytes | Kadapa, Prakasam | Oil drilling |
AP: The Diamond Capital
'AP was the WORLD'S richest source of diamonds in medieval times.' The Kollur mines (Guntur district) produced:
- Koh-i-Noor (now in British Crown Jewels)
- Hope Diamond (now in US Smithsonian)
- Regent Diamond (in French Crown Jewels)
9. Comparison Table: Types of Rocks
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| Origin | Cooling of magma/lava | Deposition of sediments | Change by heat/pressure |
| Texture | Crystalline | Layered | Banded/foliated |
| Fossils | NO | YES | NO (destroyed) |
| Hardness | Hard | Soft to medium | Very hard |
| Example | Granite, Basalt | Sandstone, Limestone | Marble, Slate |
10. Key Facts
- Earth's radius: 6,371 km
- Crust: less than 1% of Earth's volume
- Mantle: 84% of Earth's volume
- Core: 15% of Earth's volume
- Deepest mine in India: Kolar Gold Fields (3 km deep) — now closed
- AP has the DEEPEST deposits of some minerals in India
11. Common Mistakes
- Thinking the crust is thick — It is VERY thin (like the skin of an apple).
- Confusing magma and lava — Magma is INSIDE Earth; Lava is OUTSIDE.
- Believing sedimentary rocks always have fossils — MOST do, but not all.
- Forgetting that metamorphic rocks were ONCE another rock type — Marble was once limestone.
- Thinking the rock cycle is one-directional — It is a CYCLE — rocks can go through it multiple times.
12. Exam Focus
2 Marks
Q: Name the three layers of the Earth. A: Crust, mantle, and core.
Q: Name two types of igneous rocks. A: Granite (intrusive) and basalt (extrusive).
4 Marks
Q: Explain the rock cycle. A: The rock cycle shows how rocks transform. Igneous rocks break down into sediments (weathering). These form sedimentary rocks (deposition). Under heat and pressure, sedimentary rocks become metamorphic. Metamorphic rocks can melt into magma, which cools to form igneous rocks again. 'Every rock can become ANY other rock — given enough TIME.'
6 Marks
Q: Describe the mineral resources of Andhra Pradesh and their economic importance. A: AP is rich in minerals. Limestone (Kurnool, Guntur) supports the cement industry. Bauxite (Visakhapatnam) provides aluminium ore. Manganese (Srikakulam) is used in steel. Historically, the Kollur mines produced the world's finest diamonds (Koh-i-Noor). Barytes (Kadapa) is used in oil drilling. These minerals form the BACKBONE of AP's industrial economy, supporting cement plants, aluminium smelters, and mining employment.
13. Quick Self-Test
-
How thick is the Earth's crust? Answer: 5–100 km
-
What is the mantle made of? Answer: Silicate minerals, iron, magnesium (semi-molten rock)
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What are the two main elements in the Earth's core? Answer: Nickel and iron (NiFe)
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What type of rock contains fossils? Answer: Sedimentary rock
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Name a metamorphic rock and its original rock. Answer: Marble (from limestone) / Slate (from shale) / Quartzite (from sandstone)
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What is the difference between magma and lava? Answer: Magma is inside the Earth; lava is on the surface.
-
Which AP district is famous for historic diamond mines? Answer: Guntur (Kollur mines)
-
What is the deepest layer of the Earth called? Answer: The core
14. Conclusion
'We live on the CRUST — a thin, fragile shell covering a VAST, hot interior.' Understanding the Earth's interior helps us understand volcanoes, earthquakes, and the formation of rocks and minerals. AP is a GEOLOGICAL treasure house — from the basalt of the Deccan to the limestones of Kurnool to the diamonds of Kollur.
Key takeaways:
- Earth has three layers: crust, mantle, core
- Rocks form in three major types: igneous, sedimentary, metamorphic
- The rock cycle shows continuous transformation
- AP is rich in limestone, bauxite, manganese, and historic diamonds
- Minerals are non-renewable — use them WISELY
