Rocks: Mineral Groups, Igneous, Sedimentary and Metamorphic Rocks, Distribution in India and the Rock Cycle, ISC Class 11

What Rocks is about

Weightage: CISCE gives no Class XI unit weightage for ISC Geography. Rocks is topic 3 of the Class XI syllabus (Formation of the Earth) and can be asked anywhere in the 70-mark Paper I (Theory, 3 hours). Class XI is examined by the school, so your school sets the paper and decides how many marks each topic carries.Source: CISCE ISC Geography (853) syllabus, examination year 2028, Class XI

Key concepts

Mineral groups responsible for rocks

A mineral is a naturally occurring inorganic substance with a definite chemical composition and an orderly internal (crystal) structure. Rocks are aggregates of one or more minerals.

The crust is dominated by eight elements, chiefly oxygen (nearly half by weight) and silicon (over a quarter), followed by aluminium, iron, calcium, sodium, potassium and magnesium.

1. Silicates: compounds of silicon and oxygen with other elements; by far the largest group, making up most of the crust.

• Feldspar: the most abundant mineral group; light-coloured; in granite.
• Quartz: hard, resistant silica; main mineral of sand and sandstone.
• Mica: splits into thin sheets; used in electrical insulation.
• Ferromagnesian silicates (pyroxene, amphibole, olivine): dark, heavy, in basalt.

2. Carbonates: contain the carbonate group; calcite (calcium carbonate) and dolomite (calcium-magnesium carbonate). They form limestone, chalk and marble and dissolve in weakly acidic water.

3. Sulphides: metals combined with sulphur; many are important ores: pyrite (iron), galena (lead), sphalerite (zinc), chalcopyrite (copper).

4. Metals: native metals found in pure form (gold, silver, copper, platinum) and metallic minerals in ores, often oxides such as haematite and magnetite (iron) and bauxite (aluminium).

Worked example

[4] Name four mineral groups that make up rocks, giving one example and one use of each. (Recall)

Try it first, then show the solutionSolution

1. Silicates: feldspar, quartz (glass, sand) - 1 mark.
2. Carbonates: calcite (limestone for cement) - 1 mark.
3. Sulphides: galena (lead ore) or pyrite - 1 mark.
4. Metals: native gold, copper; iron ores (haematite) - 1 mark.

Answer: Silicates, the largest group, include feldspar and quartz, which is used for glass and sand; carbonates include calcite, which forms limestone used for cement; sulphides include galena, the main ore of lead; and metals include native gold and copper as well as metallic ores such as haematite, the source of iron.

Igneous rocks

Formation: from the cooling and solidification of molten magma or lava. They are called primary rocks because all other rocks are ultimately derived from them.

Types
1. Intrusive (plutonic): cool slowly deep inside the crust; large crystals; e.g., granite, gabbro. Hypabyssal rocks cool at moderate depth in dykes and sills (e.g., dolerite).
2. Extrusive (volcanic): cool quickly at the surface; small crystals or glassy texture; e.g., basalt, pumice, obsidian.
3. By chemistry: acid igneous rocks (rich in silica, light-coloured, e.g., granite) and basic igneous rocks (less silica, dark and heavy, e.g., basalt, gabbro).

Characteristics: crystalline; hard and compact; no layers (non-stratified); no fossils; generally resistant to erosion; weathering of basalt gives fertile soil.

Distribution in India

• Basalt: the Deccan Traps of Maharashtra, Gujarat and western Madhya Pradesh.
• Granite: much of the Peninsular plateau, such as Karnataka, Tamil Nadu, Andhra Pradesh, Telangana, Jharkhand (Chotanagpur) and parts of Rajasthan.

Economic importance

• Building and monument stone (granite).
• Source of metallic minerals (iron, copper, gold are often associated with igneous and old crystalline rocks).
• Weathered basalt forms the fertile black (regur) soil of the Deccan, good for cotton.

Worked example

[4] Distinguish between intrusive and extrusive igneous rocks with examples from India. (Analysis)

Try it first, then show the solutionSolution

1. Intrusive: cool slowly inside the crust; extrusive: cool quickly at the surface - 1 mark.
2. Intrusive: large crystals; extrusive: small crystals or glassy - 1 mark.
3. Intrusive example: granite of Karnataka and Tamil Nadu - 1 mark.
4. Extrusive example: basalt of the Deccan Traps (Maharashtra) - 1 mark.

Answer: Intrusive igneous rocks cool slowly deep inside the crust and develop large crystals, as in the granite of Karnataka and Tamil Nadu. Extrusive igneous rocks cool quickly at the surface and have small crystals or a glassy texture, as in the basalt of the Deccan Traps in Maharashtra.

Sedimentary rocks

Formation: rocks are broken down by weathering and erosion; the fragments are carried by rivers, wind, ice or sea and deposited in layers; over time the layers are compacted and cemented (lithification).

Types
1. Mechanically formed (clastic): from rock fragments: sandstone (sand), conglomerate (rounded pebbles), shale (clay), loess (wind-blown dust).
2. Organically formed: from remains of plants and animals: limestone and chalk (shells and skeletons), coal (plant remains).
3. Chemically formed: from minerals precipitated from water: rock salt, gypsum, some limestones.

Characteristics: layered (stratified); may contain fossils; usually softer and more porous than igneous rocks; they cover most of the earth's land surface but make up only a small fraction of the crust's volume.

Distribution in India

• Alluvial deposits of the Indo-Gangetic plains.
• Vindhyan sandstone and limestone of Madhya Pradesh and Rajasthan.
• Gondwana rocks containing coal in the Damodar, Mahanadi and Godavari valleys (Jharia and Raniganj coalfields).
• Limestone in Madhya Pradesh, Chhattisgarh, Rajasthan and Andhra Pradesh.

Economic importance

• Coal, petroleum and natural gas are found in sedimentary rocks.
• Limestone is the raw material for cement and is used in steel-making.
• Sandstone is a building stone (the red sandstone of Mughal monuments).
• Porous sandstones hold groundwater.

Worked example

[4] Classify sedimentary rocks on the basis of formation, with one example of each, and state their economic importance. (Understanding)

Try it first, then show the solutionSolution

1. Mechanically formed: sandstone, shale - 1 mark.
2. Organically formed: limestone, coal - 1 mark.
3. Chemically formed: rock salt, gypsum - 1 mark.
4. Importance: coal and petroleum; limestone for cement; sandstone for building - 1 mark.

Answer: Sedimentary rocks are mechanically formed from rock fragments (sandstone, shale), organically formed from plant and animal remains (limestone, coal) or chemically formed by precipitation (rock salt, gypsum). They are economically important because they contain coal, petroleum and natural gas, supply limestone for cement and sandstone for building, and hold groundwater.

Metamorphic rocks

Formation: existing igneous or sedimentary rocks are changed in form, texture and mineral composition by great heat, pressure or both, without melting completely.

Types of metamorphism
1. Contact (thermal) metamorphism: rocks near intruding magma are baked by heat.
2. Regional (dynamic) metamorphism: rocks over large areas are changed by heat and pressure during mountain building.

Examples of change

• Limestone to marble.
• Sandstone to quartzite.
• Shale to slate (and with more heat and pressure, schist).
• Granite to gneiss.
• Coal, under intense heat and pressure, towards graphite.

Characteristics: harder and more compact than the original rock; minerals often re-arranged in bands or layers (foliation), as in slate and gneiss; fossils are usually destroyed.

Distribution in India

• Marble at Makrana (Rajasthan), used in the Taj Mahal.
• Slate in Himachal Pradesh (Kangra) and the Himalayan foothills.
• Gneiss, schist and quartzite in the old Peninsular rocks and the Aravallis.
• The ancient Dharwar rocks of Karnataka contain metamorphic rocks rich in minerals such as iron ore and gold (Kolar).

Economic importance: marble and slate are valuable building and decorative stones; quartzite is used in construction and refractories; many metallic ores occur in metamorphic regions.

Worked example

[4] Explain how metamorphic rocks are formed and give three examples of the change with an Indian location. (Understanding)

Try it first, then show the solutionSolution

1. Existing rocks changed by heat and pressure without melting - 1 mark.
2. Contact (heat of magma) and regional (mountain building) metamorphism - 1 mark.
3. Limestone to marble (Makrana); sandstone to quartzite - 1 mark.
4. Shale to slate (Kangra); granite to gneiss - 1 mark.

Answer: Metamorphic rocks form when existing rocks are changed by great heat, pressure or both, without fully melting, either near intruding magma (contact metamorphism) or over wide areas during mountain building (regional metamorphism). Limestone becomes marble, as at Makrana in Rajasthan; sandstone becomes quartzite; shale becomes slate, as at Kangra in Himachal Pradesh; and granite becomes gneiss.

The rock cycle

The rock cycle is the continuous process by which rocks are formed, broken down and changed from one type into another over geological time.

Stages
1. Magma cools and solidifies to form igneous rocks.
2. Igneous rocks exposed at the surface are weathered and eroded; the sediments are transported and deposited.
3. Deposited sediments are compacted and cemented into sedimentary rocks.
4. Igneous or sedimentary rocks buried deep or caught in mountain building are changed by heat and pressure into metamorphic rocks.
5. Metamorphic rocks may melt at great depth to form magma again, which cools to form new igneous rocks.
6. Short cuts are possible: any rock can be uplifted, weathered and turned into sediment; igneous rocks can be metamorphosed directly; sedimentary rocks can melt.

Driving forces: the earth's internal heat (melting, metamorphism, uplift through plate movements) and external processes powered by the sun (weathering, erosion, deposition).

Significance: it shows that the earth's materials are constantly recycled; no rock is permanent.

Worked example

[4] Explain the rock cycle with the help of a flow sequence. (Understanding)

Try it first, then show the solutionSolution

1. Magma cools to form igneous rocks - 1 mark.
2. Weathering, erosion and deposition form sediments, compacted into sedimentary rocks - 1 mark.
3. Heat and pressure change rocks into metamorphic rocks - 1 mark.
4. Melting forms magma again; short cuts between all types - 1 mark.

Answer: Magma cools to form igneous rocks; these are weathered and eroded, and the sediments are deposited and compacted into sedimentary rocks; heat and pressure change igneous or sedimentary rocks into metamorphic rocks; and deep melting turns metamorphic rocks back into magma. Any rock can also be uplifted and weathered again, so all three types are constantly recycled.

Rocks in real life

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

Topics in this chapter

• Mineral Groups and Igneous Rocks
• Sedimentary and Metamorphic Rocks and the Rock Cycle

Rocks: key facts

Mineral groups

$Silicates,carbonates,sulphides,metals$

Feldspar, calcite, galena, gold.

Igneous

Intrusive (granite) and extrusive (basalt); acid and basic

Deccan basalt; Peninsular granite.

Sedimentary

Mechanical (sandstone), organic (limestone, coal), chemical (rock salt)

Vindhyan, Gondwana, alluvium.

Metamorphic changes

Limestone to marble; sandstone to quartzite; shale to slate; granite to gneiss

Makrana, Kangra.

Metamorphism types

Contact (heat) and regional (heat and pressure)

Foliation in slate and gneiss.

Rock cycle

$Magma->igneous->sediments->sedimentary->metamorphic->magma$

Short cuts possible.

How the school exam asks Rocks

The question patterns to expect, and the method that works for each.

Tricks to remember Rocks

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

• Rock families: 'Fire, Layers, Change'

  Igneous (Latin 'ignis' = fire), Sedimentary (layers), Metamorphic (change of form).

  Use it when: Origin of names.

• Metamorphic pairs: 'L-M, S-Q, S-S, G-G'

  Limestone-Marble, Sandstone-Quartzite, Shale-Slate, Granite-Gneiss.

  Use it when: Examples of change.

• India map of rocks

  Deccan = basalt; Peninsula = granite and gneiss; Vindhya = sandstone; Damodar = coal; Makrana = marble.

  Use it when: Distribution answers.

Rocks: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 3 + 1 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Rocks

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

• ✗Mistake: Calling marble igneous

  Students guess from hardness.

  ✓Do this: Marble is metamorphosed limestone.

  Usually costs: 1 mark.

• ✗Mistake: Distribution without places

  Answers say 'in many parts of India'.

  ✓Do this: Name the region: Deccan basalt, Damodar coal, Makrana marble.

  Usually costs: 1 mark per rock type.

• ✗Mistake: Rock cycle drawn without processes

  Arrows alone do not explain.

  ✓Do this: Label each arrow: cooling, weathering, deposition, heat and pressure, melting.

  Usually costs: 1-2 marks.

• ✗Mistake: Confusing minerals and rocks

  Quartz and sandstone are mixed up.

  Wrong working

  Quartz is a sedimentary rock.

  Right working

  Quartz is a mineral; sandstone is a rock made mostly of quartz.

  ✓Do this: Minerals make rocks.

  Usually costs: 1 mark.

• ✗Mistake: Saying sedimentary rocks have crystals like granite

  Mixing features.

  ✓Do this: Sedimentary: layered, often fragmental; igneous: crystalline.

  Usually costs: 1 mark.

Practice questions on Rocks

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

1. Q1 · 1 mark · Very short answerExam level

   What is a mineral?

   Hint

   Natural, inorganic, definite.

   Answer

   A naturally occurring inorganic substance with a definite chemical composition and crystal structure.

2. Q2 · 1 mark · Very short answerExam level

   Name the most abundant group of minerals in the earth's crust.

   Hint

   Silicon and oxygen.

   Answer

   Silicates.

3. Q3 · 1 mark · MCQExam level

   Coal is an example of:
   (a) Chemically formed sedimentary rock
   (b) Organically formed sedimentary rock
   (c) Metamorphic rock
   (d) Igneous rock

   Hint

   Plant remains.

   Answer

   (b) Organically formed sedimentary rock.

4. Q4 · 1 mark · Very short answerHarder exam level

   Which rock forms when sandstone is metamorphosed?

   Hint

   Very hard.

   Answer

   Quartzite.

5. Q5 · 4 marks · Short answerHarder exam level

   [4] Explain the economic importance of igneous rocks.

   Hint

   Stone, minerals, soil.

   Answer

   (1) Granite is a durable building and monument stone. (2) Many metallic minerals (iron, copper, gold) occur in igneous and old crystalline rocks. (3) Basalt is used as road metal and building stone. (4) Weathered basalt forms fertile black soil of the Deccan, suited to cotton.

6. Q6 · 4 marks · Short answerHarder exam level

   [4] Distinguish between contact and regional metamorphism.

   Hint

   Cause, area, examples.

   Answer

   (1) Contact metamorphism is caused mainly by heat from intruding magma. (2) It affects a narrow zone around the intrusion. (3) Regional metamorphism is caused by heat and pressure during mountain building. (4) It affects large areas and often produces foliated rocks such as slate, schist and gneiss.

7. Q7 · 4 marks · Short answerHarder exam level

   [4] Describe the four mineral groups that form rocks.

   Hint

   One example each.

   Answer

   (1) Silicates: silicon-oxygen compounds; feldspar, quartz, mica. (2) Carbonates: calcite and dolomite, forming limestone and marble. (3) Sulphides: metals with sulphur; galena, pyrite. (4) Metals: native gold, copper; metallic ores like haematite.

8. Q8 · 4 marks · Short answerHOTS / competency

   [4] Why is the Damodar valley rich in coal while the Deccan plateau is not?

   Hint

   Rock type and origin.

   Answer

   (1) Coal forms from buried plant matter in sedimentary basins. (2) The Damodar valley contains Gondwana sedimentary rocks deposited in ancient basins with forests. (3) The western Deccan is covered by basalt lava flows, which are igneous. (4) Igneous rocks cannot contain coal, so coal is found in Gondwana basins, not in the basalt.

9. Q9 · 4 marks · Short answerHOTS / competency

   [4] 'Sedimentary rocks cover most of the land surface but form a small part of the crust.' Explain.

   Hint

   Thin surface layers.

   Answer

   (1) Sedimentary rocks form at the surface from weathered material. (2) They spread as thin layers over large areas of land and sea floor. (3) Below them lie thick igneous and metamorphic rocks forming most of the crust's volume. (4) So they dominate what we see but not the crust as a whole.

10. Q10 · 7 marks · Long answerHarder exam level

    [7] (a) Explain the formation and types of sedimentary rocks. [4]
    (b) Explain the rock cycle. [3]

    Hint

    (a) process plus three types; (b) sequence.

    Answer

    (a) (1) Weathering, erosion, transport and deposition of sediments in layers. (2) Compaction and cementation (lithification). (3) Mechanically formed (sandstone, shale) and organically formed (limestone, coal). (4) Chemically formed (rock salt, gypsum); layered, may contain fossils. (b) (1) Magma cools into igneous rock; weathering forms sediments that become sedimentary rock. (2) Heat and pressure produce metamorphic rock. (3) Melting returns rock to magma; short cuts possible; the cycle is continuous.

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. 1.What are rocks made of?Show answer

   One or more minerals.

2. 2.Name a native metal.Show answer

   Gold (or silver, copper, platinum).

3. 3.Which rock forms the Deccan Traps?Show answer

   Basalt.

4. 4.Where is marble quarried in Rajasthan for famous monuments?Show answer

   Makrana.

5. 5.What is lithification?Show answer

   Compaction and cementation of sediments into rock.

6. 6.What does shale become on metamorphism?Show answer

   Slate.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Minerals: silicates (feldspar, quartz, mica), carbonates (calcite), sulphides (galena, pyrite), metals (gold; haematite).
• ☐Igneous: cooled magma; intrusive granite, extrusive basalt; crystalline, no fossils; Deccan, Peninsular plateau.
• ☐Sedimentary: layered, fossils; mechanical, organic, chemical; alluvium, Vindhyan, Gondwana coal.
• ☐Metamorphic: heat and pressure; contact and regional; marble, quartzite, slate, gneiss; Makrana, Kangra.
• ☐Uses: building stone, cement, coal and oil, ores, soils, groundwater.
• ☐Rock cycle: igneous -> sedimentary -> metamorphic -> magma, with short cuts.

Rocks: frequently asked questions