GeographyISC Class 11 · Rocks

Mineral Groups and Igneous Rocks (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Rocks are aggregates of minerals, and the four groups that matter are silicates, carbonates, sulphides and metals, with silicates the largest. Igneous rocks are the primary rocks, formed when magma or lava cools: intrusive types such as granite have large crystals, extrusive types such as basalt have small crystals or a glassy look.

Understand Mineral Groups and Igneous Rocks

Minerals first. A mineral is a naturally occurring inorganic substance with a definite chemical composition and an orderly crystal structure; a rock is an aggregate of one or more minerals. The ISC syllabus names four groups.

Silicates: silicon and oxygen with other elements; the largest group and the bulk of the crust. Feldspar, quartz, mica and the dark ferromagnesian minerals (pyroxene, amphibole, olivine) belong here.
Carbonates: calcite and dolomite; they build limestone, chalk and marble and dissolve in weakly acidic water.
Sulphides: metals combined with sulphur, such as pyrite (iron), galena (lead) and chalcopyrite (copper); many are ores.
Metals: native metals such as gold, silver and copper, and metallic minerals such as haematite and magnetite (iron) and bauxite (aluminium).

Igneous rocks. They are called primary rocks because every other rock is ultimately made from them. They form when molten material cools and solidifies.

By place of cooling: intrusive (plutonic) rocks cool slowly deep inside the crust and develop large crystals (granite, gabbro); extrusive (volcanic) rocks cool quickly at the surface and have small crystals or glassy texture (basalt, pumice, obsidian). Hypabyssal rocks such as dolerite cool at moderate depth in dykes and sills.
By chemistry: acid igneous rocks are rich in silica, light-coloured and lighter (granite); basic igneous rocks have less silica and are dark and heavy (basalt, gabbro).
Features: crystalline, hard and compact, not layered, no fossils, generally resistant to erosion.

India. Basalt makes up the Deccan Traps of Maharashtra, Gujarat and western Madhya Pradesh; granite forms much of the Peninsular plateau, for example in Karnataka, Tamil Nadu, Andhra Pradesh, Telangana, Jharkhand and parts of Rajasthan.

Uses. Granite is a building and monument stone; igneous and old crystalline rocks are associated with metallic ores; weathered basalt gives the fertile black (regur) soil good for cotton.

Mineral Groups and Igneous Rocks: key facts

Four mineral groups: $S i l i c a t es ∣ C a r b o na t es ∣ S u l p hi d es ∣ M e t a l s$; Silicates are the largest group.
Igneous by place of cooling: Intrusive: slow cooling, large crystals, granite | Extrusive: fast cooling, small crystals, basalt
Igneous by chemistry: Acid: rich in silica, light, granite | Basic: less silica, dark, heavy, basalt

How to answer Mineral Groups and Igneous Rocks questions

Igneous questions are usually 3 to 4 marks: classify, distinguish, or give distribution and uses.

1Define the rock in one line: formed by cooling of magma or lava.
2Classify first by place of cooling, then by silica content.
3Give one Indian example for each type with its state or region.
4Add features or economic importance in the closing lines.

Mineral Groups and Igneous Rocks in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why crystal size differs

Suppose a lab melts the same mineral mixture and pours half into a thick insulated mould that cools over days, and half onto a cold metal plate that cools in seconds.

Which half would show larger crystals, and which kind of igneous rock does each copy?

Try it first, then show the solutionSolution

Slow cooling gives time for large crystals to grow, as in intrusive rocks such as granite.
Fast cooling leaves little time, giving tiny crystals or glass, as in extrusive rocks such as basalt.

Answer: The slow-cooled half has larger crystals (intrusive); the quick-cooled half has small crystals (extrusive).

Memory hook: Slow cooling grows big crystals.

Rock use

Suppose a town must choose a stone for a flight of steps used by thousands daily and another stone for a soil-improving farm field.

Which igneous rock suits each need?

Try it first, then show the solutionSolution

Granite is hard, compact and resistant, so it suits steps.
Weathered basalt forms fertile regur soil, so basalt country suits farming.

Answer: Granite for steps; weathered basalt for fertile soil.

Memory hook: Hard for steps, soft soil from basalt.

Tricks to remember Mineral Groups and Igneous Rocks

Short memory aids. Say each one aloud once, then use it on a question the same day.

Cooling place decides crystals
Inside = Intrusive = Large crystals; Outside = Extrusive = tiny crystals.
Use it when: Intrusive versus extrusive.
Acid is light, basic is heavy
Granite is the acid pale rock; basalt is the basic dark rock.
Use it when: Acid and basic igneous rocks.
SCaSM
Silicates, Carbonates, Sulphides, Metals: the four mineral groups in order of the syllabus.
Use it when: Listing groups.

Mineral Groups and Igneous Rocks: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Mineral groups · 2 marks · Short answerBoard level

Name the four mineral groups that make up rocks and give one example of each.

Try it first, then show the solutionSolution

Silicates: feldspar or quartz.
Carbonates: calcite or dolomite.
Sulphides: pyrite or galena.
Metals: native copper or gold, or haematite as an ore.

Answer: Silicates (feldspar), carbonates (calcite), sulphides (galena) and metals (gold, copper).

Example 2: Rock identification · 1 mark · MCQBoard level

Which of these is an intrusive igneous rock?
(a) Basalt
(b) Granite
(c) Pumice
(d) Obsidian

Try it first, then show the solutionSolution

Basalt, pumice and obsidian are extrusive.
Granite cools slowly inside the crust.

Answer: (b) Granite.

Example 3: Compare igneous types · 4 marks · Long answerHarder board level

Distinguish between intrusive and extrusive igneous rocks on three points and give an Indian example of each.

Try it first, then show the solutionSolution

Place and rate of cooling: slow inside the crust versus quick at the surface.
Crystal size: large versus small or glassy.
Examples of type: granite versus basalt.
Indian examples: granite of the Peninsular plateau such as Karnataka; basalt of the Deccan Traps in Maharashtra.

Answer: Intrusive rocks cool slowly, have large crystals and include granite of the Peninsula; extrusive rocks cool quickly, have small crystals or glass and include Deccan basalt.

Example 4: Reason on composition · 4 marks · Long answerHarder board level

Two igneous rocks, P and Q, are both crystalline. P is pale and rich in silica; Q is dark, heavy and has less silica. (a) Classify P and Q as acid or basic. (b) Name one rock matching each. (c) State which would more likely form the Deccan Traps.

Try it first, then show the solutionSolution

(a) P is acid; Q is basic.
(b) P: granite; Q: basalt or gabbro.
(c) Q type, because the Deccan Traps are basalt, a basic extrusive rock.

Answer: (a) P acid, Q basic (b) granite and basalt (c) Q type, basalt.

Example 5: Evaluate economic role · 5 marks · Case-basedHOTS / competency

Case (invented): A district has a ridge of granite on one side and a basalt plateau on the other. (a) Say which side suits quarrying for building stone and why. (b) Which side is likelier to have deep fertile soil, and why? (c) Explain why neither side will show fossil-bearing layers.

Try it first, then show the solutionSolution

(a) The granite ridge: hard, compact and resistant.
(b) The basalt plateau: weathered basalt gives black regur soil.
(c) Igneous rocks form from cooling molten material, not in layers of deposited sediment, and heat destroys organic remains.

Answer: (a) Granite for quarrying (b) basalt for fertile soil (c) igneous rocks are unlayered and form from molten material, so they lack fossils.

Where students lose marks in Mineral Groups and Igneous Rocks

The slips examiners see most often, and what to write instead.

Mistake: Writing that basalt is intrusive
Basalt is seen in big flows and is confused with granite.
Wrong working
Intrusive: granite and basalt
Right working
Intrusive: granite; extrusive: basalt
Do this: Basalt is extrusive and cools quickly at the surface; granite is the intrusive example.
Usually costs: 1 mark.
Mistake: Saying igneous rocks are layered
Confusion with sedimentary rocks.
Do this: State that igneous rocks are non-stratified and unfossiliferous.
Usually costs: 1 mark.
Mistake: Listing only silicates
The four groups are not memorised.
Do this: Always list silicates, carbonates, sulphides and metals with an example each.
Usually costs: 1 to 2 marks.

Exam tips

Name the four mineral groups with one example each whenever the syllabus phrase appears.
Classify igneous rocks in two ways: place of cooling and silica content.
Always give an Indian region for basalt and granite.
Do not write that igneous rocks contain fossils.

Practice questions on Mineral Groups and Igneous Rocks

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which mineral group is the largest in the earth's crust?
(a) Carbonates
(b) Sulphides
(c) Silicates
(d) Native metals
Hint
Silicon and oxygen.
Answer
(c) Silicates.
Q2 · 3 marks · Short answerHarder board level
Explain why igneous rocks are called primary rocks and why they contain no fossils.
Hint
Origin from magma; heat.
Answer
They form directly from cooled magma or lava, and every sedimentary and metamorphic rock is ultimately made from them. They lack fossils because they solidify from molten material at high temperature, which would destroy organic remains, and are not laid down in layers.
Q3 · 4 marks · Long answerHarder board level
Name three features of igneous rocks and show how each follows from the way they form.
Hint
Crystals, layers, fossils.
Answer
Crystalline, because minerals crystallise as the melt cools; non-stratified, because they are not deposited in layers; hard, compact and without fossils, because they solidify from hot molten material.
Q4 · 4 marks · Long answerHarder board level
Explain the distribution of igneous rocks in India and mention two economic uses.
Hint
Deccan basalt and Peninsular granite.
Answer
Basalt forms the Deccan Traps of Maharashtra, Gujarat and western Madhya Pradesh; granite forms much of the Peninsular plateau (for example Karnataka, Tamil Nadu, Jharkhand). Uses: granite as building and monument stone; weathered basalt gives fertile regur soil for cotton.
Q5 · 4 marks · Case-basedHOTS / competency
Granite and gabbro are both coarse-grained igneous rocks, yet one is pale and the other dark. (a) Explain why both are coarse-grained. (b) Explain the colour difference. (c) Classify each as acid or basic.
Hint
Think of cooling depth and silica.
Answer
(a) Both are intrusive, cooling slowly deep in the crust, so large crystals grow. (b) Granite is rich in silica and light-coloured; gabbro has less silica and dark ferromagnesian minerals. (c) Granite is acid; gabbro is basic.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Largest mineral group?Show answer
Silicates.
2.Example of an acid igneous rock?Show answer
Granite.
3.Example of an extrusive rock?Show answer
Basalt.
4.Where are the Deccan Traps found?Show answer
Maharashtra, Gujarat and western Madhya Pradesh.
5.Soil formed from weathered basalt?Show answer
Black (regur) soil.

Mineral Groups and Igneous Rocks: frequently asked questions

Why do intrusive rocks have bigger crystals?

They cool slowly, so crystals have time to grow.

Is obsidian crystalline?

It cools so fast that it is glassy rather than clearly crystalline.

Is quartz a silicate?

Yes, quartz is silica and belongs to the silicate group.