Rocks | ICSE Class 9 Geography Notes

This note covers minerals and rocks, differences between them, igneous, sedimentary and metamorphic rocks, their formation and characteristics, intrusive and extrusive rocks, familiar rock examples, fossils and the rock cycle.

What are rocks, and how do they differ from minerals?

Definition: A rock is a natural mass of mineral matter forming part of the earth's crust. The crust is the earth's outermost layer. A mineral is a naturally occurring substance with certain physical properties and a definite chemical composition.

Chemical composition means what a substance is made of chemically. Physical properties are observable or measurable features, such as colour and hardness, meaning resistance to scratching. These terms describe the substances within rocks; they do not name separate rock groups.

How are minerals related to a rock?

Rocks are combinations of minerals. Some rocks consist of a single mineral, with limestone as an example. However, the majority of rocks contain several minerals in varying proportions. Therefore, the relationship is between the mineral material and the rock made from it.

The distinction does not mean that every rock must contain many different minerals. Equally, the single-mineral example does not justify describing all rocks as single-mineral substances. Keep the words some and majority attached to the correct statements.

Texture means the character of a rock's grains, including their size. A grain is a small particle within the rock. Differences in texture matter when comparing rocks formed under different cooling conditions, particularly granite and basalt.

What does a rock's appearance tell us?

Minerals occur in varied forms, from hard diamond to soft talc. This variation helps explain why mineral matter should not be treated as though it all has the same physical properties.

Begin a description by separating the questions “What material is present?” and “How did the rock form?” Mineral composition addresses the first question. The three main rock groups address the second. Both kinds of description are useful, but they organise information in different ways.

How thick is the crust in which rocks occur?

The crust is the earth's thinnest layer. Its thickness differs between continental masses and ocean floors.

The crust forms a small share of the earth's volume. These percentages compare the three layers by volume.

How are the three main types of rocks classified?

The three major groups are igneous, sedimentary and metamorphic rocks. Their formation provides the central distinction. Igneous rocks form when molten, or melted, material cools and becomes solid. Sedimentary rocks form from sediments, meaning small rock particles, that are deposited, or laid down. Metamorphic rocks form when existing rocks change under great heat and pressure.

Molten means melted. Molten rock material inside the earth is called magma; when it reaches the surface it is called lava. Sediments are small particles produced by the breaking down of rocks. Their accumulation provides material for sedimentary rock formation.

Solidification is the change from molten material to solid rock. Compression means pressing material together. Fossils are remains of dead plants and animals trapped in rock layers. These words describe different parts of the formation story and should be connected to the appropriate group.

Which process belongs to each group?

Granite and basalt belong together because both result from cooling molten material. Their textures differ, but this does not place them in different major groups. They are different types within the igneous group.

Similarly, sandstone and marble should not be grouped together merely because both are familiar building stones. Sandstone is sedimentary, while marble is metamorphic. Their use does not replace their formation as the basis of classification.

Why should the process accompany the name?

A rock name gives an example; a process explains the classification. Link granite to cooling magma, sandstone to compressed and hardened sand grains, and marble to changed limestone. This connects a named rock with the reason it belongs to its group.

The groups are also connected over time. Material belonging to one rock type can become part of another under suitable conditions. This connection is the basis of the rock cycle, rather than a reason to ignore the differences between the groups.

How do igneous rocks form from magma and lava?

Igneous rocks form when molten magma cools and becomes solid. They are also called primary rocks, an alternative name for this group. Their defining process is cooling and solidification, whether the molten material remains within the crust or reaches the surface.

What happens during cooling?

1. Molten rock material is present as magma within the earth.
2. Sometimes it remains deep within the crust; alternatively, it reaches the surface as lava.
3. The molten material loses heat and cools in its surroundings.
4. It becomes solid, producing an igneous rock whose grain size is linked to its cooling conditions.

The location of cooling distinguishes the two types: intrusive igneous rocks form inside the crust, while extrusive igneous rocks form from lava on the surface. These terms refer to where the rock forms, not to its later use.

When lava reaches the surface, it cools rapidly. The resulting extrusive rock has a very fine-grained structure, meaning that its grains are very small. Basalt is an example of an extrusive igneous rock.

Sometimes magma cools deep inside the crust. It cools slowly there, producing large grains. Granite is an example of an intrusive igneous rock. The contrast links the place of cooling, the rate of cooling and the resulting grain size.

How should the formation be explained?

A complete explanation should begin with molten material and end with solid rock. For basalt, include the lava reaching the surface and cooling rapidly. For granite, include magma remaining deep within the crust and cooling slowly.

Draw a line for the earth's surface. Below it, label magma cooling slowly to form granite with large grains. At the surface, label lava cooling rapidly to form basalt with very fine grains.

The drawing compares two formation settings. It need not show magma first becoming granite and then becoming basalt, because these are alternative cooling routes. Each route ends with an igneous rock formed under the conditions shown beside it.

Keep magma and lava distinct in the explanation. They refer to molten rock material in different settings. Both connect to the same central process: cooling followed by solidification.

How do intrusive and extrusive igneous rocks differ?

Intrusive and extrusive rocks are subdivisions of igneous rocks. Both form by cooling molten material, but they differ in the location and speed of cooling and in grain size. Granite and basalt make this comparison concrete.

What is the comparison between granite and basalt?

Read each column as a connected explanation. In the granite column, magma cools slowly deep inside the crust and forms large grains. In the basalt column, lava cools rapidly at the surface and forms a very fine-grained rock.

Do not separate the texture from the process that explains it. Saying “granite has large grains” identifies a characteristic. Adding its slow cooling within the crust explains how that characteristic is connected with its formation.

Which familiar examples help connect the ideas?

The Deccan plateau is made up of basalt rocks. This connects a named Indian landform with extrusive igneous rock. The rock's classification comes from the rapid cooling of lava at the surface, rather than from the name of the landform.

On an outline map of India, locate and label the Deccan plateau. Add the annotation “Basalt: extrusive igneous rock formed by rapidly cooling lava”.

Granite grinding stones are used to prepare paste or powder from spices and grains. This is a use of a named intrusive rock. The grinding stone's use and the original formation of its rock are separate parts of its description.

These examples can be linked in a short comparison: basalt is the extrusive example associated with the Deccan plateau, while granite is the intrusive example used for grinding stones. Retain the cooling and grain-size differences when explaining the two rocks.

Note: “Intrusive” does not mean sedimentary, and “extrusive” does not mean metamorphic. Both names belong within the igneous group and distinguish where molten rock material cooled.

A comparison is clearest when the same basis appears on both sides. Compare place with place, cooling rate with cooling rate, and grain size with grain size. This keeps the relationship between formation and characteristics visible throughout the explanation.

How do sediments become sedimentary rocks?

Sedimentary rocks form from sediments that have been deposited, compressed and hardened. A sediment is a small rock particle; a sedimentary rock is the rock formed from accumulated material. The change from loose particles to rock is central to this process.

What is the sequence of formation?

Rocks roll down, crack and hit one another, breaking into smaller fragments. Wind and water transport the resulting sediments. Transport means carrying the particles from one place to another, while deposition means their settling or accumulation.

1. Existing rocks break into smaller fragments, producing sediments.
2. Wind, water and other transporting agents carry the sediments.
3. The transported sediments are deposited and accumulate.
4. The loose sediments are compressed and hardened, forming layers of sedimentary rock.

Sandstone forms from grains of sand. It connects the starting particles with the name of the resulting rock. To explain its formation, include the transformation of deposited grains into a compressed and hardened rock, rather than stopping at the presence of sand.

The layered arrangement is a characteristic connected with sedimentary formation. Loose sediments accumulate before they are compressed and hardened into rock layers. The sequence therefore includes breaking, movement, deposition and the formation of solid rock.

What do fossils tell us about sedimentary rocks?

Sedimentary rocks may contain fossils of plants, animals and other organisms that lived in the past. Fossils connect the rock layers with remains of earlier life. The word “may” matters: the statement does not say that every sedimentary rock contains a fossil.

A fossil is different from an ordinary grain of sand. The fossil is a preserved remain of past life; the sand grain is a sediment particle. Both can be discussed in relation to sedimentary rocks, but they are not interchangeable terms.

Draw arrows from broken rock particles to transported sediments, then to deposited layers, and finally to compressed and hardened layers. Label the arrows with the processes they represent.

The Red Fort is made of red sandstone. This provides a familiar use of a sedimentary rock. Its building use does not change the explanation of sandstone's origin from sand grains or its position within the sedimentary group.

Keep the distinction between material and finished rock clear throughout: sand grains are the particles, while sandstone is the rock. Likewise, deposition is part of the formation sequence, while compression and hardening explain how loose sediments become rock.

On an outline map of India, label Gujarat, Assam and the western and eastern flanks of the peninsula. Annotate these regions: “Sedimentary rocks containing most of India's petroleum deposits”.

How are metamorphic rocks formed from existing rocks?

Metamorphic rocks form when existing igneous or sedimentary rocks change under great heat and pressure. Metamorphism is this change of an existing rock. The starting point is therefore a rock already present, rather than freshly deposited loose sediments.

Which examples show the change?

Clay changes into slate, and limestone changes into marble. Learn each starting material together with its result. Naming marble without identifying limestone leaves out the particular transformation that the example illustrates.

The Taj Mahal is made of white marble. This links the metamorphic example with a familiar building. Compare it with the red sandstone of the Red Fort: the buildings use different rocks belonging to different major rock groups.

How does this differ from igneous formation?

For igneous formation, the explanation centres on molten material cooling into solid rock. For metamorphic formation, it centres on an existing rock changing under heat and pressure. These are different process descriptions, even though both involve conditions affecting rock material.

The rock cycle also includes a later route in which rock melts to form magma. If that magma subsequently cools and solidifies, the resulting rock is igneous. Keep this melting-and-cooling route distinct from the limestone-to-marble transformation.

Note: Limestone is the starting rock in the limestone-to-marble example. Marble is the metamorphic result. Reversing the two names reverses the stated transformation.

To describe a metamorphic example clearly, identify the starting material, the conditions and the result. “Limestone changes under great heat and pressure to form marble” connects all three parts in a single explanation.

The same method applies to clay and slate. It also keeps metamorphism connected with the wider rock cycle: previously formed material changes, and its new rock type reflects the process it has undergone.

The examples demonstrate change between named materials. They do not mean that all rocks become marble or that every metamorphic rock begins as limestone. Use each example for the specific transformation it describes.

How does the rock cycle connect all three rock types?

Definition: The rock cycle is the process by which one type of rock changes into another under certain conditions in a cyclic manner. Its links include cooling, breakdown, transport, deposition, heat, pressure and melting.

The cycle connects rocks with the processes acting on their material. It shows that an igneous rock, sediment or metamorphic rock is part of a continuing set of transformations. Each change needs the appropriate conditions; the name of a rock alone does not cause the next change.

What is one connected route through the cycle?

1. Magma cools and solidifies, producing igneous rock.
2. The igneous rock breaks down into small particles, producing sediments.
3. The sediments are transported and deposited, then compressed and hardened into sedimentary rock.
4. Heat and pressure change existing igneous or sedimentary rock into metamorphic rock.
5. Metamorphic rock can melt under conditions of great heat and pressure, forming magma.
6. The magma can cool and solidify again, forming igneous rock and continuing the cycle.

This route is useful for connecting the processes, but the cycle also includes links between different parts. For example, igneous rock can change into metamorphic rock under heat and pressure. It need not be described as becoming sandstone before this change.

The drawing places magma beside a volcano at the top, igneous rock on the right, sediments at the lower right, sedimentary rock at the lower left and metamorphic rock on the left. Arrows connect these labelled parts in several directions.

See Fig. 2.4 in your NCERT textbook

How should the arrows be interpreted?

An arrow represents a change or process, rather than the name of another rock. Between magma and igneous rock, explain cooling and solidification. Between loose sediments and sedimentary rock, explain accumulation, compression and hardening.

Between an existing rock and metamorphic rock, explain heat and pressure. On the return towards magma, explain melting. Naming the process is essential because the same rock cycle contains both the production of molten material and its later solidification.

The diagram includes sediments and magma alongside the three major rock types. Sediments are particles, and magma is molten material. Their presence does not create two additional major rock groups; they show material involved in the transformations.

A rock-cycle explanation is strongest when each link includes its starting material, process and result. This avoids replacing the cycle with a bare list of names. It also makes clear why cooling, compression and metamorphism should not be used as interchangeable descriptions.

Retain the phrase under certain conditions. The cycle describes possible transformations when the required processes act. It does not require every specimen to pass through one identical sequence of changes.

How can formation clues and examples be used to compare rocks?

A useful comparison connects evidence about formation with a rock group and an example. Begin with the process supplied: cooling molten material, compressing and hardening sediments, or changing an existing rock under heat and pressure.

Which clue leads to which explanation?

The table connects evidence and conclusion. If the clue includes slow cooling within the crust, the relevant contrast is with rapid cooling at the surface. If the clue describes deposited grains becoming rock, the relevant process is sedimentary formation.

When the starting rock is named, preserve it in the explanation. Limestone becoming marble is a particular metamorphic example. Replacing limestone with an unnamed material loses information that helps explain the transformation.

How do the familiar examples fit together?

Basalt in the Deccan plateau illustrates an extrusive igneous rock. Granite grinding stones illustrate a use of intrusive igneous rock. Red sandstone in the Red Fort and white marble in the Taj Mahal connect sedimentary and metamorphic examples with buildings.

Hard rocks are used to make roads, houses and buildings. These uses show why rocks matter in everyday surroundings. However, the process of formation remains the basis for placing a rock in the igneous, sedimentary or metamorphic group.

A full comparison can therefore connect three kinds of information: the group, the formation process and a specific example. For intrusive and extrusive rocks, add the characteristic grain size associated with the different cooling rates.

Finally, keep the strength of each clue accurate. Sedimentary rocks may contain fossils, so fossil presence is not a feature that must occur in every specimen. The majority of rocks contain several minerals, but the single-mineral example of limestone remains part of the comparison.

Glossary

• Rock — A natural mass of mineral matter that forms part of the earth's crust.
• Mineral — A naturally occurring substance having certain physical properties and a definite chemical composition.
• Texture — The character of a rock's grains, including the size of those grains.
• Magma — Molten rock material within the earth that can cool and form igneous rock.
• Lava — Molten rock material that has emerged from the earth's interior onto its surface.
• Igneous rock — Rock formed through the cooling and solidification of molten magma or lava.
• Intrusive rock — Igneous rock formed when magma cools slowly deep within the earth's crust.
• Extrusive rock — Igneous rock formed when lava cools rapidly on the earth's surface.
• Sediment — A small particle produced by rock breakdown that can be transported and deposited.
• Deposition — The settling or accumulation of material after it has been transported.
• Sedimentary rock — Rock formed when accumulated sediments are compressed and hardened into layers.
• Fossil — A remain of a dead plant or animal trapped within layers of rock.
• Metamorphic rock — Rock produced when an existing rock changes under great heat and pressure.
• Solidification — The change of molten rock material into solid rock as it cools.
• Rock cycle — The cyclic transformation of one rock type into another under certain conditions.

Common errors and misconceptions

• Misconception: Every rock contains many minerals. Correct: The majority contain several minerals in varying proportions, but some, such as limestone, consist of a single mineral.
• Misconception: Magma and lava describe different rock groups. Correct: They describe molten material within the earth and at its surface respectively; cooling either can produce igneous rock.
• Misconception: Granite forms by rapid cooling at the surface. Correct: Granite is intrusive; magma cools slowly deep within the crust and forms large grains.
• Misconception: Basalt is intrusive because it forms part of the Deccan plateau. Correct: Basalt is extrusive and forms from lava cooling rapidly at the surface.
• Misconception: Every sedimentary rock contains fossils. Correct: Sedimentary rocks may contain fossils; fossil presence is not stated as universal.
• Misconception: Marble changes into limestone under heat and pressure. Correct: The stated metamorphic transformation is limestone into marble; clay changes into slate.
• Misconception: Melting and cooling produce metamorphic rock. Correct: Cooling molten material produces igneous rock; metamorphic formation involves the change of existing rock under heat and pressure.
• Misconception: The rock cycle requires every rock to follow one fixed route. Correct: Different transformations occur under certain conditions, including an igneous-to-metamorphic link.

Exam-style questions with model answers

Q1. A mineral is a naturally occurring substance with definite chemical composition and physical properties. A rock is a natural mass of mineral matter in the crust; some contain one mineral and the majority contain several. Use this information to distinguish minerals from rocks in two points. [2 marks]

1. A mineral is a naturally occurring substance identified by its definite chemical composition and physical properties.
2. A rock is a natural mass of mineral matter; it may contain one mineral or several minerals.

Q2. Granite forms when magma cools slowly deep inside the crust, producing large grains. Basalt forms when lava cools rapidly at the surface, producing very fine grains. Intrusive rocks form inside the crust and extrusive rocks at the surface. Compare these rocks by type and location, cooling rate and grain size. [3 marks]

1. Granite is an intrusive igneous rock formed deep within the crust, whereas basalt is an extrusive igneous rock formed at the surface.
2. Granite forms from slowly cooling magma, while basalt forms from rapidly cooling lava. Their cooling rates therefore differ.
3. Granite has large grains, whereas basalt has very fine grains, connecting their textures with their different cooling conditions.

Q3. Rocks break into small sediments; wind and water transport them; the sediments are deposited; compression and hardening form layers of sedimentary rock. Explain this formation sequence in four stages. [4 marks]

1. Existing rocks break into smaller fragments. These small particles are sediments and provide the material for the later rock.
2. Wind and water transport the sediments, carrying the fragments away from the place where the original rock broke down.
3. The transported sediments are deposited and accumulate. This supplies the loose material from which rock layers can form.
4. Compression presses the accumulated sediments together, and hardening turns them into layers of sedimentary rock, completing the stated sequence.

Q4. Under great heat and pressure, clay changes into slate and limestone into marble. These changed rocks are metamorphic. By contrast, igneous rocks form when molten material cools and solidifies. State the two transformations and explain the formation difference. [3 marks]

1. Clay changes into slate under great heat and pressure. Slate is the metamorphic result of the first transformation given.
2. Limestone changes into marble under great heat and pressure. Marble is the metamorphic result of the second transformation given.
3. Metamorphic formation changes existing material under heat and pressure, whereas igneous formation involves molten material cooling and solidifying into rock.

Q5. Use these supplied links to explain one route through the rock cycle: magma cools into igneous rock; breakdown produces sediments; transport, deposition, compression and hardening produce sedimentary rock; great heat and pressure produce metamorphic rock; melting produces magma; cooling can produce igneous rock again. Give six stages. [6 marks]

1. Magma cools and solidifies to form igneous rock. This begins the stated route with molten material becoming a solid rock.
2. The igneous rock breaks down into small particles called sediments. Rock material is now present as fragments rather than the original rock.
3. Transport and deposition move and accumulate these sediments. Compression and hardening then convert the loose material into sedimentary rock.
4. Great heat and pressure change existing rock into metamorphic rock. In this route, the sedimentary rock undergoes this transformation.
5. Melting changes metamorphic rock into molten magma. This returns the material to the molten state included at the beginning.
6. The magma can cool and solidify again into igneous rock. This completes the stated cyclic route through the supplied processes.

Q6. The Deccan plateau is made of basalt, an extrusive igneous rock. Granite, an intrusive igneous rock, is used for grinding stones. The Red Fort uses red sandstone, a sedimentary rock. The Taj Mahal uses white marble, a metamorphic rock. Sedimentary rocks may contain fossils. Give four rock-example associations with their groups, then correct the claim that every sedimentary rock contains fossils. [5 marks]

1. The Deccan plateau is associated with basalt. Basalt belongs to the extrusive igneous group, as specified in the supplied information.
2. Grinding stones are associated with granite. Granite belongs to the intrusive igneous group, distinguishing it from the basalt example.
3. The Red Fort is associated with red sandstone. Sandstone belongs to the sedimentary group in the information provided.
4. The Taj Mahal is associated with white marble. Marble belongs to the metamorphic group, distinguishing it from sandstone.
5. The claim about every sedimentary rock is too strong. These rocks may contain fossils; fossil presence is not given as universal.

Key takeaways

• Rocks are natural masses of mineral matter; the majority contain several minerals, while some contain a single mineral.
• Igneous, sedimentary and metamorphic rocks are distinguished by formation through cooling, sediment accumulation and hardening, or change under heat and pressure.
• Granite forms through slow cooling within the crust and has large grains; basalt forms through rapid surface cooling and has very fine grains.
• Sedimentary formation connects rock breakdown, sediment transport, deposition, compression and hardening into a sequence that produces rock layers.
• Sedimentary rocks may contain fossils, but this does not mean that every sedimentary rock must contain preserved remains.
• Clay changes into slate and limestone into marble under great heat and pressure, illustrating named metamorphic transformations.
• The rock cycle connects rock types through processes acting under certain conditions, with magma and sediments forming part of the cycle.
• Connect each rock example with its formation: the use of a rock does not replace the process that determines its group.

Test yourself

What is the relationship between rocks and minerals?

Rocks are made of mineral matter. Some contain one mineral, but the majority contain several minerals in varying proportions.

What distinguishes magma from lava?

Magma is molten rock material within the earth; lava is that material after it reaches the surface.

Why does granite have large grains?

Granite forms from magma cooling slowly deep within the earth's crust, producing large grains.

How is basalt formed, and which Indian plateau is made of it?

Basalt forms when lava cools rapidly at the surface. The Deccan plateau is made up of basalt rocks.

How do loose sediments become sedimentary rock?

After transport and deposition, sediments are compressed and hardened into layers of sedimentary rock.

Which metamorphic changes link clay and limestone to new rocks?

Under great heat and pressure, clay changes into slate and limestone changes into marble.

Why is “all sedimentary rocks contain fossils” incorrect?

Sedimentary rocks may contain fossils. The word “may” does not make fossil presence a universal characteristic.

Which processes connect magma with igneous rock in the rock cycle?

Magma cools and solidifies to form igneous rock, connecting molten material with a solid rock type.