Rocks ICSE Class 9: Igneous, Sedimentary, Metamorphic and the Rock Cycle

What Rocks is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Rocks is topic 2(c), Structure of the Earth, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Minerals and rocks

A mineral is a naturally occurring inorganic substance with a definite chemical composition and, usually, a regular crystal structure. Examples: quartz, feldspar, mica, calcite. About eight elements (oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium) make up most of the crust's minerals.

A rock is a natural, solid aggregate of one or more minerals. Granite, for example, is made of quartz, feldspar and mica. Rocks have no fixed chemical composition.

Differences:

• A mineral has a definite chemical composition; a rock does not.
• A mineral is a single substance; a rock is a mixture of minerals.
• Minerals have definite physical properties such as colour, hardness and crystal form; rocks vary widely.

Worked example

(2 marks) Distinguish between a mineral and a rock, with one example of each.

Try it first, then show the solutionSolution

1. Mineral: natural inorganic substance with definite composition, e.g. quartz - 1 mark.
2. Rock: aggregate of one or more minerals without fixed composition, e.g. granite - 1 mark.

Answer: A mineral is a naturally occurring inorganic substance with a definite chemical composition, such as quartz. A rock is a solid aggregate of one or more minerals with no fixed composition, such as granite (quartz, feldspar and mica).

Igneous rocks

Formation: igneous rocks (from Latin ignis, fire) form when molten material cools and solidifies. They are called primary rocks because all other rocks are ultimately derived from them.

• Intrusive (plutonic) igneous rocks form when magma cools slowly deep below the surface; large crystals form. Example: granite. Rocks that cool at moderate depth in dykes and sills, such as dolerite, are sometimes called hypabyssal.
• Extrusive (volcanic) igneous rocks form when lava cools quickly on the surface; crystals are small or absent. Example: basalt (Deccan Traps); pumice and obsidian are other extrusive rocks.

Characteristics: crystalline; hard and compact; no layers; no fossils (formed from molten material); mostly impermeable; may contain valuable minerals such as iron, copper and gold.

Worked example

(3 marks) Distinguish between intrusive and extrusive igneous rocks.

Try it first, then show the solutionSolution

1. Intrusive: magma cools slowly below the surface; extrusive: lava cools quickly on the surface - 1 mark.
2. Intrusive rocks have large crystals; extrusive have small or no crystals - 1 mark.
3. Examples: granite (intrusive); basalt (extrusive) - 1 mark.

Answer: Intrusive igneous rocks form when magma cools slowly deep underground, giving large crystals, e.g. granite. Extrusive igneous rocks form when lava cools quickly on the surface, giving fine crystals, e.g. basalt.

Sedimentary rocks

Formation: rocks on the surface are broken down by weathering and erosion; the fragments (sediments) are carried by rivers, wind, ice or waves and deposited, usually in seas and lakes, in layers. Over time the layers are compressed by the weight above and cemented together (lithification) to form sedimentary rocks.

Types by origin:

• Mechanically formed (clastic), from fragments of other rocks: sandstone (sand), shale (clay or mud), conglomerate (rounded pebbles).
• Organically formed, from remains of plants and animals: limestone and chalk (shells and skeletons), coal (plant remains).
• Chemically formed, by precipitation from solution: rock salt, gypsum.

Characteristics: arranged in layers (strata); often contain fossils; usually softer than igneous rocks; often porous and permeable; non-crystalline (except chemical ones). They cover most of the land surface but form a small part of the crust's volume. Coal, petroleum and natural gas occur in sedimentary rocks.

Worked example

(3 marks) Explain the formation of sedimentary rocks and give one example of each type.

Try it first, then show the solutionSolution

1. Sediments from weathered rock are deposited in layers and compacted and cemented - 1 mark.
2. Mechanical: sandstone or shale; organic: limestone or coal - 1 mark.
3. Chemical: rock salt or gypsum - 1 mark.

Answer: Weathered fragments are carried and deposited in layers, usually in water; pressure from overlying layers compacts and cements them into rock. Examples: sandstone (mechanically formed), limestone or coal (organically formed) and rock salt (chemically formed).

Metamorphic rocks

Formation: metamorphic rocks (meaning 'changed form') form when existing igneous or sedimentary rocks are changed by great heat, great pressure, or both, without melting. This happens deep within the crust, near hot magma (contact metamorphism) or under the enormous pressure of mountain building (regional metamorphism).

Examples of change:

• limestone → marble
• sandstone → quartzite
• shale → slate
• slate → schist (with further change)
• granite → gneiss
• coal → graphite

Characteristics: harder and more compact than the original; minerals recrystallise and are often arranged in bands (foliation), as in gneiss and schist; usually no fossils (destroyed by heat and pressure); many are used in building (marble, slate).

Worked example

(3 marks) What are metamorphic rocks? Name the metamorphic forms of limestone, shale and granite.

Try it first, then show the solutionSolution

1. Rocks changed in form by great heat and pressure, without melting - 1 mark.
2. Limestone → marble; shale → slate - 1 mark.
3. Granite → gneiss - 1 mark.

Answer: Metamorphic rocks are existing rocks changed in form and structure by intense heat and pressure without melting. Limestone becomes marble, shale becomes slate and granite becomes gneiss.

Comparing the three types

Formation: igneous from cooling of molten material; sedimentary from deposition and cementing of sediments; metamorphic from alteration of existing rocks by heat and pressure.

Layers: sedimentary rocks have distinct layers; igneous do not; metamorphic may show bands (foliation).

Fossils: common in sedimentary rocks; absent in igneous; rare in metamorphic.

Crystals: igneous are crystalline; sedimentary mostly non-crystalline; metamorphic recrystallised.

Hardness: igneous and metamorphic are generally hard; sedimentary usually softer.

Economic value: igneous rocks contain metallic minerals; sedimentary rocks contain coal, petroleum and building stone; metamorphic rocks give marble and slate.

Worked example

(3 marks) Give three differences between igneous and sedimentary rocks.

Try it first, then show the solutionSolution

1. Igneous from cooling magma or lava; sedimentary from deposited sediments - 1 mark.
2. Igneous have no layers or fossils; sedimentary have layers and often fossils - 1 mark.
3. Igneous are crystalline and hard; sedimentary are usually softer and porous - 1 mark.

Answer: (1) Igneous rocks form from molten material; sedimentary rocks from deposited sediments. (2) Igneous rocks have no layers or fossils; sedimentary rocks are layered and often contain fossils. (3) Igneous rocks are crystalline and hard, while sedimentary rocks are usually softer and porous.

The rock cycle

The rock cycle is the continuous process by which rocks change from one type to another over long periods.
1. Magma cools and solidifies to form igneous rocks.
2. Igneous rocks exposed at the surface are weathered and eroded; the sediments are deposited, compacted and cemented into sedimentary rocks.
3. Igneous or sedimentary rocks buried deep or caught in mountain building are changed by heat and pressure into metamorphic rocks.
4. Metamorphic (or any) rocks may melt deep in the earth to form magma again, restarting the cycle.
Short cuts are possible: metamorphic rocks can be weathered to form sediments; sedimentary rocks can melt; igneous rocks can be metamorphosed directly.

Significance: the cycle shows that the crust is constantly recycled; no rock is permanent.

Worked example

(3 marks) What is the rock cycle? Explain how a sedimentary rock can become an igneous rock.

Try it first, then show the solutionSolution

1. Rock cycle: continuous change of rocks from one type to another - 1 mark.
2. Sedimentary rock buried deep is changed by heat and pressure (metamorphic) and may melt into magma - 1 mark.
3. The magma cools and solidifies to form igneous rock - 1 mark.

Answer: The rock cycle is the continuous transformation of rocks from one type to another. A sedimentary rock buried deep in the crust may be metamorphosed and then melted into magma; when this magma cools and solidifies it forms igneous rock.

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

• Igneous, Sedimentary and Metamorphic Rocks and the Rock Cycle

Rocks: key facts

Igneous

Cooling of magma/lava; intrusive (granite) and extrusive (basalt)

Primary rocks; no fossils.

Sedimentary

$Deposition+compaction+cementation$

Mechanical (sandstone, shale), organic (limestone, coal), chemical (rock salt, gypsum).

Metamorphic

Heat and/or pressure without melting

Contact and regional.

Metamorphic pairs

$Limestone\to marble;sandstone\to quartzite;shale\to slate;granite\to gneiss;coal\to graphite$

Slate → schist.

Rock cycle

$Magma\to igneous\to sediments\to sedimentary\to metamorphic\to magma$

Short cuts possible.

Mineral vs rock

Mineral: definite composition; rock: aggregate of minerals

Granite = quartz + feldspar + mica.

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.

• Fire, Settle, Change

  Igneous = fire; Sedimentary = settle; Metamorphic = change.

  Use it when: Three types.

• LiMe-SaQ-ShaSl-GrGn

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

  Use it when: Metamorphic pairs.

• MOC

  Sedimentary types: Mechanical, Organic, Chemical.

  Use it when: Sedimentary classification.

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 B 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 2 + 3 + 3 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: Wrong metamorphic pairs

  Five pairs to remember.

  Wrong working

  Sandstone changes into marble.

  Right working

  Sandstone → quartzite; limestone → marble.

  ✓Do this: Learn pairs by first letters: L-M, S-Q, Sh-Sl, G-G.

  Usually costs: 1 mark each.

• ✗Mistake: Calling coal an igneous or metamorphic rock

  Coal is hard and dark.

  ✓Do this: Coal is an organically formed sedimentary rock (graphite is its metamorphic form).

  Usually costs: 1 mark.

• ✗Mistake: Saying metamorphic rocks form by melting

  Confusion with igneous.

  ✓Do this: Metamorphism changes rocks without melting them.

  Usually costs: 1 mark.

• ✗Mistake: Defining a rock as 'a mineral'

  Words used loosely.

  ✓Do this: Rock = aggregate of minerals; mineral = definite composition.

  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 · MCQExam level

   Which of these is a chemically formed sedimentary rock?
   (a) Sandstone
   (b) Coal
   (c) Rock salt
   (d) Basalt

   Hint

   Precipitated from solution.

   Answer

   (c) Rock salt.

2. Q2 · 1 mark · MCQExam level

   Slate is formed from:
   (a) limestone
   (b) shale
   (c) granite
   (d) sandstone

   Hint

   Mud rock.

   Answer

   (b) shale.

3. Q3 · 1 mark · Very short answerHarder exam level

   Name the minerals that make up granite.

   Hint

   Three.

   Answer

   Quartz, feldspar and mica.

4. Q4 · 2 marks · Very short answerExam level

   Why are sedimentary rocks found in layers?

   Hint

   Deposition over time.

   Answer

   Because sediments are deposited one layer on top of another over long periods, each layer later compacted and cemented, giving strata.

5. Q5 · 2 marks · Very short answerHarder exam level

   Why do metamorphic rocks rarely contain fossils?

   Hint

   Heat and pressure.

   Answer

   The intense heat and pressure that change the original rock usually destroy or distort any fossils it contained.

6. Q6 · 2 marks · Very short answerHOTS / competency

   Why is coal found in sedimentary rocks and not igneous rocks?

   Hint

   Origin of coal.

   Answer

   Coal forms from buried plant remains compacted in layers, a sedimentary process; igneous rocks form from molten material, in which no plant matter could survive.

7. Q7 · 2 marks · Very short answerHarder exam level

   Basalt and granite are both igneous. Why does basalt have finer crystals?

   Hint

   Cooling speed.

   Answer

   Basalt forms from lava that cools quickly at the surface, leaving little time for crystals to grow; granite cools slowly underground, so its crystals grow large.

8. Q8 · 3 marks · Short answerExam level

   Explain the formation of metamorphic rocks with two examples.

   Hint

   Heat, pressure, pairs.

   Answer

   Existing rocks are subjected to great heat (near magma) or great pressure (during mountain building), which recrystallises their minerals without melting them. Examples: limestone becomes marble; shale becomes slate.

9. Q9 · 3 marks · Short answerHOTS / competency

   'The rock cycle shows that the earth's crust is constantly being recycled.' Explain with reference to two processes.

   Hint

   Weathering and melting.

   Answer

   Weathering and erosion break down existing rocks and their sediments form new sedimentary rocks; melting deep in the earth turns rocks back into magma, which forms new igneous rock. So material is continually reused rather than created anew.

10. Q10 · 3 marks · Short answerHarder exam level

    Distinguish between a mineral and a rock on three points.

    Hint

    Composition, nature, properties.

    Answer

    (1) A mineral has a definite chemical composition; a rock does not. (2) A mineral is a single substance; a rock is an aggregate of minerals. (3) A mineral has definite properties like crystal form and hardness; rocks vary widely. E.g. quartz (mineral), granite (rock).

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.Example of an extrusive igneous rock?Show answer

   Basalt.

2. 2.Metamorphic form of granite?Show answer

   Gneiss.

3. 3.Which rock type contains fossils?Show answer

   Sedimentary.

4. 4.Metamorphic form of limestone?Show answer

   Marble.

5. 5.Example of a mechanically formed sedimentary rock?Show answer

   Sandstone (or shale, conglomerate).

6. 6.What are igneous rocks also called?Show answer

   Primary rocks.

Quick revision checklist

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

• ☐Mineral: definite composition (quartz); rock: aggregate of minerals (granite).
• ☐Igneous: cooling magma/lava; intrusive granite (big crystals), extrusive basalt (fine); no layers or fossils.
• ☐Sedimentary: deposition, compaction, cementation; mechanical, organic, chemical; layers and fossils.
• ☐Metamorphic: heat/pressure without melting; marble, quartzite, slate, gneiss, graphite.
• ☐Rock cycle: igneous → sedimentary → metamorphic → magma → igneous (with short cuts).

Rocks: frequently asked questions