📌 Key Points
- The Earth's interior is like an onion, made up of several concentric layers: Crust, Mantle, and Core
- Crust is the outermost and thinnest layer: 35 km thick on continental masses and only 5 km on ocean floors
- Continental crust is called SIAL (Silica + Alumina); Oceanic crust is called SIMA (Silica + Magnesium)
- Mantle is the middle layer extending up to a depth of 2900 km below the crust; it is the thickest layer
- Core is the innermost layer with a radius of about 3500 km; made of Nickel and Iron, called NIFE
- The central core has very high temperature and pressure; temperature increases as we go deeper
- A rock is any natural mass of mineral matter that makes up the Earth's crust; it can vary in colour, size, and texture
- Three types of rocks: Igneous (from magma), Sedimentary (from sediments), and Metamorphic (from heat and pressure on existing rocks)
- Igneous rocks are also called primary rocks; formed when molten magma cools and solidifies
- Extrusive igneous rocks: lava cools rapidly on surface, fine-grained (e.g., Basalt - Deccan Plateau)
- Intrusive igneous rocks: magma cools slowly deep inside Earth, large-grained (e.g., Granite - grinding stones)
- Sedimentary rocks: formed from compressed and hardened sediments deposited by wind and water in layers
- Sedimentary rocks may contain fossils of plants, animals, and micro-organisms (e.g., Sandstone from sand grains)
- Metamorphic rocks: formed when igneous or sedimentary rocks change under great heat and pressure
- Metamorphic examples: Clay changes into Slate; Limestone changes into Marble
- Rocks are used for construction (buildings, roads) and are a source of minerals useful in daily life
- Rock Cycle: Magma cools to Igneous rocks, which weather into Sedimentary rocks, which under heat and pressure form Metamorphic rocks, which melt back to Magma
- The rock cycle is a continuous, never-ending process that takes hundreds and thousands of years
📘 Important Definitions
⚠️ Common Mistakes
✗ Wrong: Confusing SIAL and SIMA
✓ Correct: SIAL (Silica + Alumina) is the continental crust (35 km thick). SIMA (Silica + Magnesium) is the oceanic crust (5 km thick). Remember: AL = ALumina = Land (continental).
✗ Wrong: Mixing up mantle depth and core radius
✓ Correct: Mantle extends to 2900 km depth. Core has a radius of 3500 km. These are different measurements - depth vs radius.
✗ Wrong: Confusing extrusive and intrusive igneous rocks
✓ Correct: Extrusive = on surface, rapid cooling, fine grains (Basalt). Intrusive = inside Earth, slow cooling, large grains (Granite). Memory tip: EXtrusive = EXternal.
✗ Wrong: Saying all rocks can contain fossils
✓ Correct: Only sedimentary rocks contain fossils. Igneous rocks form from hot magma (destroys organic material) and metamorphic rocks form under extreme heat and pressure (destroys fossils).
✗ Wrong: Mixing up metamorphic transformation pairs
✓ Correct: Clay becomes Slate (not marble). Limestone becomes Marble (not slate). Remember: Clay-Slate (both short words), Limestone-Marble (both longer words).
✗ Wrong: Thinking the rock cycle is one-directional
✓ Correct: The rock cycle is continuous and multi-directional. Any rock type can transform into any other under the right conditions. Igneous can directly become metamorphic without becoming sedimentary first.
📝 Exam Focus
These questions are frequently asked in CBSE exams:
🎯 Last-Minute Recall
Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!