📑 Topics on this page (Jump to Topic):
📌 1. Igneous Rocks
- Formed by cooling of magma/lava — called primary rocks or "parent rocks". They have no fossils and no layers.
- Intrusive (plutonic): cool slowly inside — large crystals, e.g. granite. Extrusive (volcanic): cool fast on the surface — small crystals, e.g. basalt.
- The Deccan Trap is made of basalt; its weathering forms black (regur) soil.
- Other examples: gabbro, diorite, pumice (can float on water), obsidian (volcanic glass). Intrusive forms: batholith, laccolith, sill, dyke.
📌 2. Sedimentary Rocks
- Formed when eroded particles settle in layers and are compacted (lithification); they are layered and contain fossils.
- They cover ≈ 75% of Earth's surface but only ≈ 5% of the crust by volume.
- Examples: sandstone, limestone, shale, conglomerate, coal (organic), rock salt/gypsum (chemical).
- Coal and petroleum are found in sedimentary rocks.
📌 3. Metamorphic Rocks
Formed when existing rocks change under great heat and pressure.
| Original rock | Metamorphic form |
|---|---|
| Limestone | Marble |
| Sandstone | Quartzite |
| Shale | Slate → schist |
| Granite | Gneiss |
| Coal | Graphite (→ diamond under extreme pressure) |
- The Taj Mahal is built of Makrana (Rajasthan) marble.
📌 4. The Rock Cycle
- Any rock can become sedimentary by weathering-erosion, metamorphic by heat-pressure, or melt into magma → igneous again — this continuous process is the rock cycle.
- Study of rocks = Petrology; of minerals = Mineralogy.
- Most abundant mineral group in the crust: feldspar, then quartz. On the Mohs scale the hardest is diamond (10), the softest talc (1).
⚠️ Exam Traps:
- Fossils occur only in sedimentary rocks, never in igneous.
- Marble is metamorphic, but its parent rock limestone is sedimentary.
- Granite is igneous (intrusive); its metamorphic form is gneiss.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which rocks are called "primary rocks"? Practice
Correct answer: (B)
Igneous rocks formed first from magma, so they are called primary.
Igneous rocks formed first from magma, so they are called primary.
Q2. Fossils are mainly found in which rocks? Practice
Correct answer: (C)
Organic remains get buried and preserved within sediment layers.
Organic remains get buried and preserved within sediment layers.
Q3. Metamorphism of limestone forms — Practice
Correct answer: (D)
Heat and pressure change limestone into marble.
Heat and pressure change limestone into marble.
Q4. The metamorphic form of shale is — Practice
Correct answer: (A)
Shale → slate; further metamorphism gives schist.
Shale → slate; further metamorphism gives schist.
Q5. The metamorphic form of sandstone is — Practice
Correct answer: (B)
Quartz grains of sandstone recrystallise into quartzite.
Quartz grains of sandstone recrystallise into quartzite.
Q6. Granite is which type of rock? Practice
Correct answer: (C)
Granite cools slowly underground, giving large crystals.
Granite cools slowly underground, giving large crystals.
Q7. The Deccan Trap is mainly made of — Practice
Correct answer: (D)
Lava from Cretaceous fissure eruptions solidified as basalt.
Lava from Cretaceous fissure eruptions solidified as basalt.
Q8. Coal is which type of rock? Practice
Correct answer: (A)
Coal is an organic sedimentary rock formed from buried plant remains.
Coal is an organic sedimentary rock formed from buried plant remains.
Q9. The Taj Mahal is built of marble from — Practice
Correct answer: (B)
White marble of Makrana (Rajasthan) is famous.
White marble of Makrana (Rajasthan) is famous.
Q10. Metamorphism of granite forms — Practice
Correct answer: (C)
Granite → gneiss, with banded minerals.
Granite → gneiss, with banded minerals.
Q11. Which rock can float on water? Practice
Correct answer: (D)
Pumice is full of gas holes, making it light enough to float.
Pumice is full of gas holes, making it light enough to float.
Q12. The softest mineral on the Mohs scale is — Practice
Correct answer: (A)
Talc = 1, diamond = 10.
Talc = 1, diamond = 10.
Q13. The study of rocks is called — Practice
Correct answer: (B)
Petrology = rocks; pedology = soils.
Petrology = rocks; pedology = soils.
Q14. Petroleum is mainly found in — Practice
Correct answer: (C)
Marine organic remains buried in sedimentary layers form oil and gas.
Marine organic remains buried in sedimentary layers form oil and gas.
Q15. Why are crystals small in extrusive igneous rocks? Practice
Correct answer: (D)
Rapid cooling gives crystals little time to grow.
Rapid cooling gives crystals little time to grow.
Q16. Which rocks cover the largest share of Earth's surface? Practice
Correct answer: (A)
Sedimentary rocks cover ≈ 75% of the surface though small in volume.
Sedimentary rocks cover ≈ 75% of the surface though small in volume.
Q17. Magma solidified in a vertical crack in the crust forms a — Practice
Correct answer: (B)
A dyke is vertical; a sill is a horizontal sheet.
A dyke is vertical; a sill is a horizontal sheet.
Q18. The most abundant mineral group in the crust is — Practice
Correct answer: (C)
Feldspars make up about half of the crust.
Feldspars make up about half of the crust.
Q19. Under heat and pressure coal changes into — Practice
Correct answer: (D)
Coal → graphite (metamorphosed carbon).
Coal → graphite (metamorphosed carbon).
Q20. The continuous process of one rock changing into another is the — Practice
Correct answer: (A)
Continuous change among igneous, sedimentary and metamorphic rocks is the rock cycle.
Continuous change among igneous, sedimentary and metamorphic rocks is the rock cycle.