Maarg Manthan · Topic 2.8

Three Families of Rocks: Igneous, Sedimentary, Metamorphic

Geography › Geology – Minerals and Rocks · Topic 2.8

Rocks are grouped into three families by the way they form. Igneous rocks form when molten magma or lava cools and solidifies, as with granite and basalt. Sedimentary rocks form when loose sediments are laid down in layers and hardened, as with sandstone, limestone and shale, and they often hold fossils. Metamorphic rocks form when existing rocks are changed by heat, pressure or chemical action without melting, as with marble from limestone and slate from shale.

9 min read · UPSC Prelims · Geography · Geology – Minerals and Rocks

At a Glance

Igneous Cooled magma or lava
Sedimentary Layered deposits; about 75 per cent of the surface
Metamorphic Changed by heat and pressure
Fossils Found in sedimentary rocks
Link All three are joined by the rock cycle
Exam link UPSC Prelims, Geography: Geology – Minerals and Rocks

Where Does It Fit?

This topic follows the one on rocks as aggregates of minerals and the links between rocks, landforms and soils. It sets out the main classification of rocks, which is used in all later work on the cycle of rocks, weathering and resources. Previous topic: Black soil From basalt (Deccan Traps) Red soil From granite and gneiss…">Relationship Between Rocks and Soils. Next topic: The Rock Cycle.

What Are Igneous Rocks?

In short: igneous rocks form when molten rock cools and hardens.

A rock is a natural aggregate of one or more minerals, and the rocks of the Earth’s crust are of three major families: igneous, sedimentary and metamorphic. Igneous rocks are the oldest and are called primary or parent rocks, because the other two kinds are formed from them. The word igneous comes from the Latin for fire. They form from magma, molten material inside the Earth, or lava, the same material on the surface, as discussed in Types of Lava: Andesitic (Acidic) and Basaltic (Basic) Lava. Igneous rocks make up the bulk of the crust by volume, along with metamorphic rocks, though sedimentary rocks cover most of the surface.

How Are Igneous Rocks Classified?

They are classified by where they cool and by their silica content.

Group	Cooling	Texture	Examples
Intrusive (plutonic)	Slowly, deep underground	Coarse, large visible crystals	Granite, gabbro, diorite, peridotite
Hypabyssal	At moderate depth, in dykes and sills	Medium	Dolerite
Extrusive (volcanic)	Quickly, on the surface	Fine-grained or glassy	Basalt, andesite, rhyolite, obsidian, pumice

By silica	Silica content	Colour and density	Examples
Acidic (felsic)	More than about 65 per cent	Light-coloured, lighter	Granite, rhyolite
Intermediate	About 52 to 65 per cent	Medium	Diorite, andesite
Basic (mafic)	About 45 to 52 per cent	Dark, denser; rich in iron and magnesium	Gabbro, basalt
Ultrabasic	Less than 45 per cent	Very dark, very dense	Peridotite

Intrusive rocks, also called plutonic rocks, are coarse-grained because of slow cooling, while extrusive rocks are fine-grained because of fast cooling. Obsidian is a glassy rock formed when lava cools so fast that crystals cannot grow, and pumice is a light, frothy volcanic rock full of gas bubbles that can float on water.

What Are the Uses and Indian Examples of Igneous Rocks?

Igneous rocks yield valuable minerals: ores of iron, copper, nickel, chromium, lead, zinc, gold and platinum are associated with them, and diamonds come from kimberlite, an igneous rock that rises from the mantle in pipes. Granite is a major building and ornamental stone and is used for monuments and road metal, and basalt is a common crushed stone for roads and railway ballast. In India, igneous rocks include the basalts of the Deccan Traps over much of Maharashtra, Gujarat and Madhya Pradesh, whose weathering gives the black soil, as described in Plate Tectonics: Major and Minor Lithospheric Plates. The granites and gneisses of the Peninsular shield, such as those of Karnataka, Andhra Pradesh and Jharkhand, and the Archaean rocks, are of igneous origin, and are discussed in Geological Regions of India: Deccan Plateau, The Himalayas, Other Peninsular Mountain Ranges, Indo-Gangetic-Brahmaputra Plain. As an analytical point, the Himalayan granites are products of the continental collision, whereas those of the Peninsula are far older, so the same rock name can represent very different geological ages.

How Are Sedimentary Rocks Formed?

In short: loose sediments settle in layers, and over time they are pressed and cemented into rock.

Rocks at the surface are broken down by weathering and erosion, discussed in Exogenic Processes: Denudation, Weathering, Mass Movements, Erosion, Transportation, Deposition. The fragments, called sediments, are carried by rivers, wind, glaciers and waves and settle in low areas such as lakes, river valleys, deltas, shallow seas and deserts. Layer upon layer builds up, and the weight of the upper layers compresses the lower ones, squeezing out water, while minerals such as silica, calcite and iron oxide dissolved in groundwater bind the grains together, as cement. The whole process of change from loose sediment to rock is called lithification. Sedimentary rocks therefore form in layers, called strata or beds, with the oldest at the bottom. Sedimentary rocks cover about three-quarters of the land surface, though they make up only a small share of the volume of the crust.

What Are the Types of Sedimentary Rocks?

Type	Origin	Examples
Mechanically formed (clastic)	Cemented fragments of other rocks, graded by grain size	Conglomerate and breccia (coarse), sandstone (sand), shale and clay (fine), loess (wind-blown silt)
Chemically formed	Minerals precipitated from solution or evaporation of water	Limestone, rock salt, gypsum, potash, flint
Organically formed	Remains of plants and animals	Coal, peat, chalk, some limestone

Limestone is the most common chemical and organic sedimentary rock: it is made largely of calcium carbonate, from shells and skeletons of marine organisms or from precipitation, and it forms karst landscapes, as in Karst (Groundwater) Topography: Sinkholes, Dolines, Uvalas. Coal is formed from plant matter buried and compressed in swamps, as described in Coal: Nature, Origin, Constituents, Types, Coal Fields, Production, By-Products, Conservation. Rock salt and gypsum form when shallow seas or lakes dry up in arid climates.

Where Are Sedimentary Rocks Found in India?

System or region	Rocks and resources
Cuddapah and Vindhyan systems	Sandstone, limestone and shale; Vindhyan sandstone is used for monuments such as the Red Fort and Sanchi; building stone, diamonds in Panna
Gondwana system	Coal in Jharkhand, West Bengal, Odisha, Chhattisgarh and Madhya Pradesh; also sandstone
Marwar and Rajasthan region	Sandstone, limestone and gypsum
Tertiary rocks of the Himalayas and coasts	Petroleum bearing rocks of Assam, Gujarat and the Mumbai offshore; the Shiwalik sandstones and clays
Indo-Gangetic plain	Deep alluvium; the largest unconsolidated sediment body

The Vindhyan and Gondwana systems are explained in Geological Regions of India: Deccan Plateau, The Himalayas, Other Peninsular Mountain Ranges, Indo-Gangetic-Brahmaputra Plain. As an analytical point, most of India’s fuel and building-stone resources lie in sedimentary rocks, which makes sedimentary geology central to energy planning, as in Petroleum: Nature, Origin, Recovery, Exploration, Refining, Reserves, Producing Areas, Role of OPEC.

What Are Metamorphic Rocks?

In short: metamorphic rocks are old rocks that have been transformed in the solid state by heat and pressure.

The word metamorphic means changed in form. Metamorphism is the process by which existing rocks of any type, igneous, sedimentary or metamorphic, are altered in structure and mineral composition by heat, pressure and chemically active fluids, without melting. If the rock melts, it turns into magma, and the new rock would be igneous. The change takes place deep in the crust or at the margins of plates and intrusions. Metamorphic rocks often have larger crystals and a harder, denser texture than the parent rock. They make up a large share of the crust, along with igneous rocks, as in Internal Structure of the Earth: Crust, Mantle, Outer Core, Inner Core.

What Are the Types of Metamorphism?

Type	Cause	Where it occurs	Examples
Contact (thermal)	Heat from a nearby magma body	Narrow zone around an intrusion	Limestone to marble; sandstone to quartzite
Regional (dynamothermal)	High pressure and heat over a wide area	Mountain-building zones at convergent boundaries	Shale to slate, schist and gneiss
Dynamic (cataclastic)	Intense directed pressure and shearing	Along fault zones	Crushed and sheared rocks
Hydrothermal	Hot chemically active fluids	Around vents and cracks	Mineral veins

Regional metamorphism is the most widespread and is associated with the mountain-building zones described in Plate Tectonics: Major and Minor Lithospheric Plates.

What Are Foliated and Non-Foliated Rocks?

When metamorphism takes place under directed pressure, mineral grains are flattened and aligned in parallel bands or layers, which is called foliation. Foliated rocks split along these planes. A sequence of increasing metamorphism from shale is: slate, then phyllite, then schist, then gneiss. Non-foliated rocks are made of equal-sized crystals with no banding and generally form from a single mineral.

Parent rock	Metamorphic rock	Notes
Shale or clay	Slate, phyllite, schist	Slate splits into thin sheets and is used for roofing and blackboards
Granite	Gneiss	Banded rock of coarse grain
Limestone	Marble	Used for sculpture and buildings; Makrana marble of Rajasthan was used in the Taj Mahal
Sandstone	Quartzite	Very hard and resistant
Coal	Anthracite, graphite	Increasing carbon content
Basalt	Greenschist, amphibolite	Occurs in old mountain belts

What Are the Uses and Indian Examples?

Metamorphic rocks supply building and decorative stones such as marble and slate, and rich ore deposits, including iron, gold, copper and graphite, are found in metamorphic belts. In India, much of the Peninsular shield consists of metamorphic rocks of the Archaean and Dharwar systems, including gneisses and schists, and the Dharwar rocks hold gold, iron and manganese, as in Geological Regions of India: Deccan Plateau, The Himalayas, Other Peninsular Mountain Ranges, Indo-Gangetic-Brahmaputra Plain. Marble comes from Makrana in Rajasthan, and slates from parts of Himachal Pradesh and the Aravalli region.

How Do the Three Families Compare?

Feature	Igneous	Sedimentary	Metamorphic
Origin	Cooling of magma or lava	Deposition and hardening of sediments	Change of older rocks by heat and pressure
Texture	Crystalline, interlocking grains	Layers (strata) of grains	Foliated or crystalline
Fossils	None, since heat destroys them	Common	Rare, usually destroyed
Share of crust	Igneous and metamorphic together form about 95 per cent of the crust by volume	About 5 per cent of the volume but about 75 per cent of the land surface	See igneous
Examples	Granite, basalt, gabbro, obsidian	Sandstone, limestone, shale, conglomerate, coal	Marble, slate, schist, gneiss, quartzite
Resources	Metal ores, building stone	Coal, oil, gas, limestone, salt	Marble, slate, some ores

The three families are linked in the rock cycle, described in The Rock Cycle. The texture terms used above are explained in Rocks as Aggregates of Minerals; Petrology.

Why Is This Topic Important?

Classification Core Prelims theme
Conversions Which rock gives which
Resources Coal, oil and building stone

What Came Next?

The next topic joins the three families into one continuous cycle of change. Next topic: The Rock Cycle.

Key People and Terms

Igneous rock Formed from cooled magma or lava
Sedimentary rock Formed from layered deposits
Metamorphic rock Rock changed by heat and pressure
Strata Layers in sedimentary rocks
Foliation Layered alignment of minerals
Fossil Preserved remains of life

Exam Corner

Points to Remember

Igneous rocks are the primary rocks.
Fossils occur in sedimentary rocks.
Limestone becomes marble; shale becomes slate.
Granite becomes gneiss; sandstone becomes quartzite.
Coal is a sedimentary rock.

Do Not Confuse With

Intrusive and extrusive igneous rocks Cooled below the surface versus on the surface
Foliated and non-foliated Layered versus massive metamorphic rocks
Clastic, chemical and organic sedimentary rocks Fragments, precipitates and living remains

Memory Hook

Fire makes igneous, layers make sedimentary, squeeze and bake make metamorphic.

Mains Angle

Questions ask for the characteristics and uses of the three families. These points add depth.

Formation Three processes
Resources Fuel, ore and stone
Cycle Rocks are recycled

A Question You May Face

An original practice question, not a past paper question.

“Classify rocks according to their mode of formation and explain the characteristics and economic importance of each type.”

How to Answer

Classify Three families.
Features Texture and fossils.
Examples India and the world.
Use Resources.

GS Relevance

Prelims: three families, examples, features, Indian occurrence, and conversions such as limestone to marble. GS Paper 1: geomorphology and rocks.

Frequently Asked Questions

What are the three types of rocks?

The three types are igneous rocks, formed by the cooling of magma or lava; sedimentary rocks, formed by the deposition and hardening of sediments; and metamorphic rocks, formed when existing rocks are changed by heat and pressure. They are joined together in the rock cycle.

Why are igneous rocks called primary rocks?

Igneous rocks are called primary rocks because they were the first rocks to form when the molten surface of the early Earth cooled and solidified. All other rocks are derived from them, either by breakdown and deposition to form sedimentary rocks, or by change to form metamorphic rocks.

In which type of rock are fossils found?

Fossils are found mainly in sedimentary rocks, because the layers of sediment bury the remains of plants and animals gently without heat. In igneous rocks the high temperature destroys remains, and in metamorphic rocks heat and pressure usually deform or destroy fossils.

What rocks are formed from limestone and shale?

Under heat and pressure, limestone is changed into marble, and shale or clay is changed into slate, and then into schist. Sandstone becomes quartzite, and granite becomes gneiss. These are examples of metamorphism, in which the original rock is changed without melting.

What are foliated rocks?

Foliated rocks are metamorphic rocks in which the minerals are arranged in parallel layers or bands because of directed pressure. Slate, schist and gneiss are examples. Non-foliated metamorphic rocks, such as marble and quartzite, are massive, with no layering.

PYQ Practice — Statement Analysis

1 Fossils are commonly found in igneous rocks.

False

They are found in sedimentary rocks.

2 Marble is a metamorphic rock formed from limestone.

True

Heat and pressure recrystallise the calcite.

3 Granite is an extrusive igneous rock.