Types of Rocks: Igneous, Sedimentary, and Metamorphic

What Are the Three Rock Families?

How Each Family Forms

• Any natural mass of mineral matter that makes up the earth’s crust is a rock. Rocks are made of minerals, naturally occurring substances with a definite chemical composition, and many, such as coal, petroleum, and natural gas, are used as fuels.
• Igneous rock forms when magma or lava cools and solidifies. Granite forms slowly underground, with large crystals. Basalt forms quickly at the surface, with fine grains, and builds the Deccan plateau.
• Sedimentary rock forms when sediment, mineral grains, or organic matter accumulate in layers and get compacted and cemented together. Sandstone and limestone are common examples.
• Metamorphic rock forms when heat and pressure transform existing rock without melting it. Marble forms this way from limestone; slate forms from shale.

Intrusive vs Extrusive, Clastic vs Chemical

• Igneous rock that cools slowly underground is intrusive (or plutonic), with large, visible crystals, as in granite. Igneous rock that cools quickly at the surface is extrusive (or volcanic), with small, fine crystals, as in basalt.
• Clastic sedimentary rock forms from broken rock fragments, like sandstone from sand grains. Chemical or organic sedimentary rock forms from precipitated minerals or accumulated organic matter, like limestone from shell debris or coal from plant material.
• Sedimentary rock typically shows visible layering, called stratification, since each layer marks a separate depositional event. It is also the only rock type that commonly preserves fossils.
• Foliated metamorphic rock, like schist and gneiss, develops banded texture under directed pressure. Non-foliated metamorphic rock, like marble and quartzite, recrystallizes without developing bands.

Minerals, Metamorphism, and the Mohs Scale

Minerals: Properties, Hardness, and Classification

• A mineral is a naturally occurring substance with a fixed atomic structure and chemical makeup. Most combine two or more elements, though a few, like gold and sulphur, are pure single elements.
• Minerals are told apart by their cleavage (splitting along flat planes) or fracture (breaking irregularly instead). Lustre is their shine, and streak is the colour of their crushed powder.
• Ten reference minerals rank hardness on the Mohs scale, from softest to hardest: talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and diamond. A fingernail scores about 2.5, and glass or a knife blade scores about 5.5.
• Metallic minerals contain metal and split into three groups: precious metals like gold and silver, ferrous metals centred on iron, and non-ferrous metals like copper and zinc. Non-metallic minerals, like sulphur and phosphates, have no metal content at all.

Metamorphism: Contact, Regional, and Dynamic

• Dynamic metamorphism comes from mechanical breaking and crushing alone, with almost no chemical change. Rock grains get physically disrupted, not recrystallised.
• Contact metamorphism happens where rock touches hot intruding magma or lava directly. The heat recrystallises the rock, and new minerals from the magma itself often get added in. This is the process, not dynamic metamorphism, when intruding magma’s heat affects surrounding rock.
• Regional metamorphism affects much larger areas. It comes from tectonic shearing combined with high temperature, high pressure, or both, typically deep within mountain-building zones. It often lines up minerals into layers, called foliation.

Why This Matters: Economy and India’s Named Rocks

The Rock Cycle, Ores, Fuels, and Building Stone

• The three rock types are not fixed categories. Weathering and erosion break any rock into sediment, which can become sedimentary rock. Heat and pressure can turn any rock into metamorphic rock. Enough heat melts any rock into magma, which cools into new igneous rock.
• Igneous activity concentrates many metallic ores as magma cools and crystallizes. India’s copper and associated metal belts, and the Deccan Trap basalts themselves, both trace back to igneous processes.
• Sedimentary basins host most of the world’s coal, oil, and natural gas, since these fuels form from buried organic matter under sedimentary layers. Sedimentary rock also hosts major groundwater aquifers.
• Metamorphic rocks supply valued building and dimension stone. Marble is prized for monuments and flooring; slate is used for roofing tiles; both owe their properties to the metamorphic process itself.

India’s Rocks, Named

• The white marble of the Taj Mahal came from Makrana, Rajasthan, a metamorphic rock, formed from limestone recrystallising under heat and pressure.
• Every Ashokan pillar was carved from sandstone quarried at Chunar, in Uttar Pradesh’s Mirzapur district, a sedimentary rock, cemented from ancient sand grains.
• The red sandstone across Agra Fort, Fatehpur Sikri, and Lahore Fort came from the same region, the standard building stone of the early Mughal period.
• Tuff, a light, porous rock, forms from compacted volcanic ash, including ash reworked and redeposited by rivers as a sedimentary layer.

Previous Year Questions

Six Exams, One Rock Cycle

• CDS I 2021 — asked which of marble, halite, granite, or shale is an igneous rock. Marble is metamorphic, halite is a sedimentary evaporite, and shale is sedimentary. Granite alone forms from cooled magma. (Answer: Granite.)
• CDS I 2026, Q67 — on igneous rock facts. Most igneous minerals are silicates, which is correct. Deep cooling gives coarse grain, not fine. Basalt is extrusive, not intrusive. Lithification is a sedimentary process, not igneous.
• CDS I 2026, Q68 — ortho-metamorphic rocks genuinely come from igneous parent rock, which is true. But heat from intruding magma causes contact metamorphism, not “dynamic” metamorphism, a mislabelling trap.
• NDA-NA-1 2026, Q112 — quartzite is metamorphosed sandstone, heat and pressure fuse its quartz grains tightly together, unlike limestone (which becomes marble) or shale (which becomes slate).
• NDA-NA-1 2026, Q106 — loose sediment is turned into solid sedimentary rock through lithification, the two-step process of compaction and cementation.
• UPSC Mains 2022, GS Paper I, Q4 — Characteristics and Types of Primary Rocks.
• NDA & NA (II) 2017 — asked which of gabbro, granite, dolomite, and basalt is not an igneous rock. Dolomite is a sedimentary carbonate rock, while the other three all form from cooled magma or lava. (Answer: Dolomite.)
• NDA & NA (I) 2025 — asked what forms when volcanic ash is carried by running water and deposited as a sedimentary layer. (Answer: Tuff.)