Rocks & the Rock Cycle: Igneous, Sedimentary & Metamorphic (UPSC Geography)
Minerals and Rocks — the Building Blocks
🎯 Exam priority: Foundational. Tested occasionally but worth nailing — the three rock types and the rock cycle underpin soils, minerals, weathering and landforms. UPSC has asked "primary rocks" in Mains (2022) and rock permeability in Prelims (2025). Learn the types, one or two examples each, and the cycle, then move on.
Everything solid beneath your feet — the Himalayas, the Deccan, the marble of the Taj Mahal — is rock. And rock is not permanent: over geological time, one type is endlessly remade into another. Understand the three rock families and the cycle that links them, and the rest of geomorphology falls into place.
Minerals and rocks — the building blocks
A mineral is a naturally occurring substance with a definite chemical composition and orderly atomic structure (e.g. quartz, feldspar, mica). A rock is an aggregate of one or more minerals. Feldspar and quartz are the most common rock-forming minerals.
Rocks are grouped by how they form into three families: igneous (from cooling magma), sedimentary (from deposited sediments) and metamorphic (from transformed older rocks). Each grades into the others through the rock cycle.
Igneous Rocks — the 'Primary' Rocks
The story starts with fire — the first rocks to form from the molten Earth, and the parents of all the rest.
Igneous rocks — the "primary" rocks
Igneous rocks form from the cooling and solidification of molten magma or lava ("igneous" = from fire). They are called primary rocks because they were the first to form and all other rocks are ultimately derived from them. They are crystalline, hard, lack layering (strata) and contain no fossils.
Intrusive (plutonic) rocks cool slowly deep in the crust, so crystals grow large and coarse-grained — the classic example is granite (the core of many mountains and the Peninsular plateau).
Extrusive (volcanic) rocks cool fast at the surface, so crystals are tiny and the rock is fine-grained — the classic example is basalt, which makes up the vast Deccan Traps. (This intrusive-vs-extrusive split was the basis of the 2022 Mains question on "primary rocks".)
Acid vs basic: igneous rocks rich in silica (over ~65%) are acidic — light-coloured, viscous and hard, like granite; those poorer in silica but rich in iron and magnesium are basic — dark, dense and fluid, like basalt and gabbro. Very fast cooling can give the glassy obsidian or the frothy, float-on-water pumice.
Sedimentary Rocks — the Layered Record
Expose any rock to wind, water and weather and it crumbles. Those fragments, laid down and compacted, become the second rock family.
Sedimentary rocks — the layered record
Sedimentary rocks form from the deposition, compaction and cementing (lithification) of sediments — the weathered debris of older rocks, or organic and chemical matter. They are laid down in layers (strata), are usually softer, and are the only rocks that contain fossils. They cover about three-quarters of the land surface but form only a thin skin of the crust.
By origin: mechanically formed (clastic) from rock fragments — e.g. sandstone, shale, conglomerate; organically formed from living matter — e.g. limestone (shells/coral) and coal (plant remains); and chemically formed by precipitation — e.g. rock salt and gypsum.
Permeability matters: porous sedimentary rocks like chalk and sandstone let water pass through (permeable), while clay does not (impermeable) — a distinction the 2025 Prelims tested directly, and the key to aquifers and springs.
India's sedimentary record: the ancient Vindhyan rocks gave the red sandstone of Mughal monuments; Jodhpur (red sandstone) and Jaisalmer (yellow limestone) are built from it; and the Gondwana sedimentary basins hold almost all of India's coal. Layering (stratification) also lets geologists read Earth's history and date events from the fossils in each bed.
Metamorphic Rocks — Transformed by Heat & Pressure
Bury or squeeze any rock deep enough, heat it under pressure, and it is reborn as something harder and often more beautiful — the third family.
Metamorphic rocks — transformed by heat and pressure
Metamorphic rocks form when existing rocks are transformed by intense heat and/or pressure (metamorphism) without fully melting — if they melted, they would become magma and then igneous rock. The minerals recrystallise into a denser, harder rock.
Foliated metamorphic rocks have a banded or layered look from pressure aligning the minerals — e.g. gneiss, schist and slate. Non-foliated ones lack banding — e.g. marble and quartzite.
Classic transformations to remember: limestone → marble (India's famous Makrana marble of the Taj Mahal); sandstone → quartzite; shale → slate; granite → gneiss; coal → graphite (and, under extreme conditions, diamond).
Types of metamorphism: contact (thermal) metamorphism near hot magma (e.g. marble around an intrusion); regional metamorphism over large areas under the heat and pressure of mountain-building (gneiss, schist of the Himalayas and the Peninsular shield); and dynamic metamorphism along fault zones. Much of India's Peninsular basement is ancient gneiss and schist.
Family | How it forms | Key features | Examples |
Igneous | Cooling of magma/lava | Crystalline, no strata, no fossils | Granite, Basalt |
Sedimentary | Deposition + lithification | Layered (strata), can hold fossils | Sandstone, Limestone, Coal |
Metamorphic | Heat/pressure on old rock | Recrystallised, often foliated | Marble, Slate, Gneiss |
The Rock Cycle — and Why Rocks Matter
None of these families is a dead end. Given enough time, each turns into the others — the great recycling system of the solid Earth.
The Rock Cycle — and why rocks matter
The rock cycle is the continuous transformation of rock from one type to another. Igneous rock is weathered and eroded into sediments that become sedimentary rock; burial under heat and pressure turns that into metamorphic rock; deeper still, it melts back into magma, which cools to form igneous rock again — and the loop repeats endlessly.
Why it matters: rocks are the parent material of soils (basalt gives the Deccan's black cotton soil), the host of minerals and fossil fuels (coal and petroleum sit in sedimentary basins), the store of groundwater (permeable strata form aquifers), and our building stone (granite, marble, sandstone).
Why this matters for UPSC
Prelims tests rock-type identification, permeability and examples; Mains asks the characteristics and types of primary (igneous) rocks (2022). Always tie a rock to what it becomes (soil, mineral, aquifer) — that link is where the marks are.
Further Reading
Standard NCERT-level geography textbooks and reference books, plus a good atlas (any UPSC reading list).
Test Yourself: Practice Questions & PYQs
Test yourself on minerals, the three rock families, permeability and the rock cycle. Attempt each question before revealing the answer.
Practice Questions
Q1. Rocks formed by the cooling and solidification of magma or lava are called:
(a) sedimentary rocks
(b) igneous rocks
(c) metamorphic rocks
(d) clastic rocks
Show answer
Answer: (b) — Igneous rocks form from molten magma/lava and are called 'primary' rocks because all others derive from them. Sedimentary rocks form from sediments; metamorphic from transformation of older rocks.
Q2. Which one of the following is an example of an INTRUSIVE (plutonic) igneous rock?
(a) Basalt
(b) Granite
(c) Sandstone
(d) Marble
Show answer
Answer: (b) — Granite cools slowly deep in the crust, giving coarse crystals — an intrusive igneous rock. Basalt is extrusive (fine-grained); sandstone is sedimentary; marble is metamorphic.
Q3. Fossils are found almost exclusively in which type of rock?
(a) Plutonic rocks
(b) Sedimentary rocks
(c) Metamorphic rocks
(d) Igneous rocks
Show answer
Answer: (b) — Only sedimentary rocks, laid down in layers, preserve fossils. Igneous rocks form from melt (no fossils) and metamorphism usually destroys any fossils present.
Q4. The Deccan Traps of India are made chiefly of which rock?
(a) Granite
(b) Limestone
(c) Marble
(d) Basalt
Show answer
Answer: (d) — The Deccan Traps are vast flood-basalt flows — basalt is a fine-grained extrusive igneous rock that, on weathering, gives the black 'regur' cotton soil.
Q5. Which sequence correctly matches a parent rock with its metamorphic product?
(a) Coal → Limestone
(b) Granite → Basalt
(c) Sandstone → Shale
(d) Limestone → Marble
Show answer
Answer: (d) — Limestone metamorphoses into marble. (Sandstone → quartzite, shale → slate, granite → gneiss, coal → graphite.) Granite→basalt and coal→limestone are not metamorphic pairs.
Q6. Which one of the following rocks is IMPERMEABLE to water?
(a) Clay
(b) Chalk
(c) Sandstone
(d) Limestone
Show answer
Answer: (a) — Clay is impermeable — it does not let water pass. Chalk, sandstone and limestone are porous/permeable, which is why they form aquifers (the 2025 Prelims tested chalk vs clay).
Q7. Metamorphic rocks that show a banded, layered appearance due to pressure are described as:
(a) clastic
(b) stratified
(c) foliated
(d) extrusive
Show answer
Answer: (c) — Foliated metamorphic rocks (gneiss, schist, slate) show banding from aligned minerals. Non-foliated ones (marble, quartzite) do not. 'Stratified' describes sedimentary layering, not metamorphic foliation.
Q8. In the rock cycle, what must happen for a metamorphic rock to become an igneous rock?
(a) It must dissolve in rainwater
(b) It must be deposited in layers
(c) It must melt into magma and then cool
(d) It must be compacted by cementing
Show answer
Answer: (c) — A metamorphic rock must melt completely into magma and then cool and solidify to become igneous. Deposition and compaction make sedimentary rock, not igneous.
Mains Practice Questions
Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover.
Discuss the characteristics and types of primary (igneous) rocks. (UPSC Mains 2022)
Explain the rock cycle and how the three families of rocks transform into one another.
Distinguish between igneous, sedimentary and metamorphic rocks with suitable examples.
How do rocks influence soils, minerals and groundwater? Explain with examples.
UPSC Mains — Previous Years’ Questions on this topic (last 10 years)
These are the actual GS Mains questions UPSC has asked on this theme — real proof of how, and when, it is tested. Try to write a full answer to each.
2022: Discuss the characteristics and types of primary rocks.