1. Basic Structure of the Earth
The Earth is made up of three main concentric layers:
- Crust
- Mantle
- Core (Outer Core + Inner Core)
हिंदी: पृथ्वी प्याज़ की तरह परतों में बंटी है – ऊपर से नीचे तक: Crust → Mantle → Core
इसे बार-बार दोहराने से exam में भूल नहीं होगी।
2. Crust (Earth’s Outer Skin)
- Outermost, thinnest layer.
- Thickness: about 5–10 km under oceans (oceanic crust) and 30–60 km under continents (continental crust).
- Oceanic crust – mainly basalt (denser).
- Continental crust – mainly granite (lighter).
हिंदी: जिस पर हम रहते हैं, घर बनते हैं, खेती होती है – सब Crust पर ही होता है।
3. Mantle (Middle Layer)
- Located below the crust, up to about 2900 km depth.
- Made of semi-solid, very hot rock (silicates).
- Upper part of mantle + crust forms the Lithosphere.
- Below the lithosphere is Asthenosphere – partially molten, where plates move.
4. Core (Inner Most Layer)
- Extends from about 2900 km to the centre (about 6370 km).
- Mainly made of iron and nickel (Fe + Ni) → sometimes called “NIFE”.
- Outer core – liquid; Inner core – solid (due to very high pressure).
- Movement in liquid outer core helps generate Earth’s magnetic field.
हिंदी: Core = Fe + Ni (Iron + Nickel) → NiFe. यही याद रखना है।
5. What are Rocks?
Rocks are natural solid materials forming the Earth’s crust.
They are made of one or more minerals.
हिंदी: Rock = चट्टान; Mineral = खनिज। खनिज मिलकर चट्टान बनाते हैं।
6. Main Types of Rocks
(A) Igneous Rocks
- Formed by solidification of molten magma or lava.
- Called “Primary” or “Parent” rocks.
- Examples: Basalt, Granite.
- Formed by deposition and compaction of sediments (sand, silt, clay etc.).
- Often arranged in layers / strata.
- May contain fossils.
- Examples: Sandstone, Limestone, Shale.
- Formed when existing rocks (igneous / sedimentary) change due to heat and pressure.
- Examples: Marble (from limestone), Slate (from shale), Quartzite (from sandstone).
हिंदी Short-cut:
- Igneous – आग से (magma/lava).
- Sedimentary – परतदार, नदी की मिट्टी, रेत आदि से बनी।
- Metamorphic – heat + pressure से बदली हुई rock।
7. Rock Cycle (Very Simple View)
- Magma cools → Igneous rock.
- Igneous rock breaks into sediments → deposited → Sedimentary rock.
- Igneous or Sedimentary rock under heat & pressure → Metamorphic rock.
- Metamorphic rock may melt → magma again.
हिंदी: Rock कभी स्थायी रूप से एक type की नहीं रहती, समय के साथ बदलती रहती है – इसी को Rock Cycle कहते हैं।
📘 Practice Questions (NDA Style)
1. The thinnest layer of the Earth is:
a) Crust b) Mantle c) Outer Core d) Inner Core
2. The core of the Earth is mainly made up of:
a) Silica b) Iron and Nickel c) Aluminium d) Magnesium
3. Which rocks are known as primary or parent rocks?
a) Sedimentary b) Igneous c) Metamorphic d) Fossil rocks
4. Marble is an example of:
a) Igneous rock b) Sedimentary rock c) Metamorphic rock d) Volcanic ash
5. Rocks arranged in layers with fossils are usually:
a) Igneous b) Sedimentary c) Metamorphic d) Plutonic
a) Crust b) Mantle c) Outer Core d) Inner Core
2. The core of the Earth is mainly made up of:
a) Silica b) Iron and Nickel c) Aluminium d) Magnesium
3. Which rocks are known as primary or parent rocks?
a) Sedimentary b) Igneous c) Metamorphic d) Fossil rocks
4. Marble is an example of:
a) Igneous rock b) Sedimentary rock c) Metamorphic rock d) Volcanic ash
5. Rocks arranged in layers with fossils are usually:
a) Igneous b) Sedimentary c) Metamorphic d) Plutonic
Answers:
1 – a) Crust
2 – b) Iron and Nickel
3 – b) Igneous
4 – c) Metamorphic rock
5 – b) Sedimentary
1 – a) Crust
2 – b) Iron and Nickel
3 – b) Igneous
4 – c) Metamorphic rock
5 – b) Sedimentary
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।