Selina Concise Solutions for ICSE Class 7 Geography Chapter 7 Types of Rocks

THINK AND ANSWER

Question: Why is diamond clear and hard while graphite is soft ? Why is coal hard and chunky ?
Answer:
Carbon atoms in diamond are bonded in a strong tetrahedron pattern making it hard and clear whereas carbon atoms in graphite are bonded in weak covalent bonds making it soft and black. Coal is a combustible sedimentary rock occurring in rock strata in layers or veins called coal beds. Because of pressure and elevated temperature the carbon becomes hard and chunky.
Diamonds and graphite are made of the same element, but the way their atoms connect determines their strength. Coal forms over millions of years from buried plants that turn into solid rock due to the weight of the earth above them.
Teacher's Tip: Think of diamond as a strong 3D cage and graphite as thin, slippery sheets of paper.
Exam Tip: Use the term "tetrahedron pattern" specifically when describing diamond's hardness for higher marks.

Discuss

Question: Discuss the importance of rocks for the contruction business.
Answer:
Rocks such as marble and granite are used in construction industries. They are cut into stones and are used for building houses, dams, roads, etc. Rocks also provide raw materials such as limestone and gypsum used in the manufacturing of cement.
Construction relies on rocks because they are durable and can withstand heavy weight and harsh weather. Many modern skyscrapers and bridges wouldn't exist without the strength provided by stone and concrete.
Teacher's Tip: Look around your house - the floors, walls, and even the sidewalk are all made from processed rocks!
Exam Tip: Mentioning specific examples like "limestone for cement" shows you understand how rocks are transformed into building materials.

VALUES & LIFE SKILLS

Question: Rocks and minerals play a very important role in our lives. Can you imagine modern life without minerals ? What problems would you face in their absence ?
Answer:
Life without rocks and minerals would be disastrous -
Rocks are of great value to us. Some of the main benefits of- rocks are listed below :
Soil is formed when rocks gradually break up and disintegrated by mechanical and chemical processes.
Rocks contain a great variety of minerals that yield metals such as iron, manganese, and copper, which are important raw materials for mineral-based manufacturing industries.
We shall face many problems if rocks and minerals get diminished.
The following problems may exist:
1. Human life will come to an end, as soil is essential for food production and if rocks diminished, it would lead to non-availability of soil.
2. Mineral-based manufacturing industries will also come to an end.
Minerals provide the essential metals for our electronics and the nutrients in soil that grow our food. Without them, we would lose everything from our kitchen utensils to the very ground that supports our agriculture.
Teacher's Tip: Remember that almost every metal object you touch started as a mineral found inside a rock.
Exam Tip: Always connect the disappearance of rocks to the loss of soil and food production to show the impact on human survival.

EXERCISES

1. Granite is an example of intrusive igneous rocks.
2. A category of rock which is formed by the alteration of the parent rock due to heat and pressure is called: metamorphic rocks.
3. Conglomerate is an example of sedimentary rocks. .
4. Sedimentary rocks are formed from sediments accumulated over long periods.
5. Sedimentary rock such as limestone change into marble.

B. Write True or False. Rewrite the false statements correctly :

Question 1: Minerals are organic substances.
Answer: False.
Correct : Minerals are inorganic substances.
Inorganic means they do not come from living things like plants or animals. They are naturally occurring solids formed by geological processes.
Teacher's Tip: If it never lived, it's likely inorganic!
Exam Tip: When correcting a "False" statement, always use the proper antonym like "inorganic" for "organic".

Question 2: Basalt is an example of an intrusive igneous rock.
Answer: False.
Correct : Basalt is an example of an extrusive igneous rock.
Extrusive rocks form on the surface after lava cools down quickly. Because it cools so fast, basalt has very tiny crystals that you can barely see.
Teacher's Tip: "Extrusive" starts with "Ex" - think of "Exit," as the lava exits the volcano.
Exam Tip: Clearly differentiate between "intrusive" (inside the earth) and "extrusive" (on the surface).

Question 3: Sedimentary rocks are formed by the cooling and solidification of molten rock called magma.
Answer: False.
Correct : Igneous rocks are formed by the cooling and solidification of molten rock called magma.
Magma is extremely hot liquid rock found deep underground. When it cools down and turns solid, it creates the first type of rock on our planet, known as igneous rock.
Teacher's Tip: "Igneous" comes from the word for "fire," just like the hot magma it forms from.
Exam Tip: Remember that magma leads to igneous rocks, while sediments lead to sedimentary rocks.

Question 4: Igneous and sedimentary rocks can change into metamorphic rocks.
Answer: True.
Intense heat and weight from the earth's layers can squeeze any existing rock into a brand new form. This transformation is what we call the process of metamorphism.
Teacher's Tip: Think of "Metamorphism" like a caterpillar changing into a butterfly - same material, totally new look!
Exam Tip: State "True" clearly and mention both "heat" and "pressure" as the causes of change if asked for more detail.

Question 5: Rocks are used in construction industries.
Answer: True.
From the gravel in roads to the marble in palaces, rocks are the foundation of our buildings. They are chosen for their strength and ability to last for hundreds of years.
Teacher's Tip: Rocks are the world's oldest and strongest building blocks.
Exam Tip: Even for a simple "True" answer, be ready to name two rocks like granite or limestone as examples.

C. Answer the following questions in brief:

Question 1: What is crust ?
Answer:
The outermost layer of the Earth is the crust. Crust is the thinnest layer of the earth.
The crust is the solid shell we live on, containing all the mountains and ocean floors. It is very thin compared to the rest of the planet, much like the skin of an apple.
Teacher's Tip: The crust is the only part of the Earth that humans have ever actually touched!
Exam Tip: Always describe the crust as both "outermost" and "thinnest" for a complete definition.

Question 2: Differentiate between SIAL and SIMA.
Answer:
SIAL
1. It is the upper layer of the earth.
2. it is made up of silica and aluminium.
SIMA
1. It is the lower layer of the earth.
2. It is made up of magnesium and silica.
SIAL forms the continents and is lighter, while SIMA forms the ocean floors and is much denser. Together, these two parts make up the solid crust of our planet.
Teacher's Tip: Look at the names: SI + AL (Silica + Aluminium) and SI + MA (Silica + Magnesium).
Exam Tip: Presenting this as a side-by-side comparison makes it easier for the teacher to grade.

Question 3: State two characteristics of the mantle.
Answer:
1. The mantle is semi-solid.
2. It is rich in iron and magnesium.
3. This layer is about 2,850 km wide. Its average density is between 3.5 and 5.5.
The mantle is the thick middle layer that sits between the crust and the core. Because it is semi-solid, it can flow very slowly, which helps move the continents above it.
Teacher's Tip: Think of the mantle as having a texture like thick honey or toothpaste.
Exam Tip: Mentioning the specific width of 2,850 km shows you have memorized the key facts perfectly.

Question 4: What is the difference between the inner and the outer core ?
Answer:
Inner core
1. Inner most layer of the Earth.
2. Solid ball of iron and nickel.
3. Has a radius of 1220 km.
Outer core
1. Surrounds the inner core.
2. Made of liquid iron and nickel.
3. Has a radius of Approx. 2300 km.
The main difference is their physical state; the inner core is a solid ball because of extreme pressure, while the outer core is liquid. Both are made of heavy metals that help create the Earth's magnetic field.
Teacher's Tip: High pressure keeps the center solid, even though it's the hottest part!
Exam Tip: Be sure to distinguish between "Solid" for the inner core and "Liquid" for the outer core.

Question 5: What are minerals?
Answer:
Minerals are natural inorganic substances having a crystalline form with definite physical and chemical properties. The most common minerals in the earth’s crust are silica, quartz, feldspar, mica, calcite, dolomite, gypsum, etc. There are about 2,000 minerals in the earth’s crust and these combine differently to form various kinds of rocks.
Minerals are the "ingredients" used to make rocks. Each mineral has a specific chemical recipe and crystal shape that never changes, like salt or quartz.
Teacher's Tip: A rock is like a cookie, and the minerals are the chocolate chips and flour inside it.
Exam Tip: Using the word "crystalline" is essential when defining a mineral scientifically.

Question 6: Give an example of an intrusive igneous rock.
Answer:
Instrusive rocks, which form large crystals at some depth in the earth’s crust example : granite, basalt.
Intrusive rocks form when magma gets trapped deep underground and cools very slowly over thousands of years. This slow cooling time allows huge, beautiful mineral crystals to grow within the rock.
Teacher's Tip: "Intrusive" = "In" the earth. Slow cooling makes big crystals!
Exam Tip: Granite is the most common example of an intrusive rock used in exams; remember its name!

Question 7: Classify the following rocks as igneous, sedimentary, and metamorphic: gneiss, coal, shale, granite, gypsum, marble, sandstone, basalt, limestone, schist
Answer:
Minerals - Rocks
Gneiss - Metamorphic
Coal - Sedimentary
Shale - Sedimentary
Granite - Igneous
Gypsum - Sedimentary
Marble - Metamorphic
Sandstone - Sedimentary
Basalt - Igneous
Limestone - Sedimentary
Schist - Metamorphic
Each rock is grouped by how it was born: from fire (Igneous), from water deposits (Sedimentary), or from pressure (Metamorphic). Understanding this classification helps geologists know the history of a specific area.
Teacher's Tip: Group them in your head first: Granite/Basalt are the "Fire Rocks."
Exam Tip: Make three columns with headers for Igneous, Sedimentary, and Metamorphic to keep your list organized.

D. Answer the following questions in one or two paragraphs :

Question 1: Explain the structure of the earth’s interior ?
Answer:
The earth’s interior is divided into three main layers - the crust is the outermost layer, the mantle is the intermediate layer, and the core is the innermost layer of the earth.
The crust is the thin surface layer where we live, while the mantle below is a thick region of semi-solid rock. At the very center is the core, which is split into a liquid outer layer and a solid inner ball of metal.
Teacher's Tip: Imagine the Earth is like a peach: the skin is the crust, the fruit is the mantle, and the pit is the core.
Exam Tip: Use the term "intermediate layer" specifically for the mantle to sound more professional.

Question 2: How are rocks different from minerals?
Answer:
Minerals are natural inorganic substances having a crystalline form with definite physical and chemical properties whereas rocks are made up of a combination of different minerals compacted together. There are about 2,000 minerals in the earth’s crust and these combine differently to form various kinds of rocks.
Minerals are pure substances with a single chemical formula, while rocks are mixtures of several minerals stuck together. You can find many different colored minerals inside a single piece of granite rock.
Teacher's Tip: Minerals are the building blocks, and rocks are the buildings made from them.
Exam Tip: Emphasize that rocks are "mixtures" or "combinations" while minerals are "definite substances".

Question 3: How are igneous rocks formed?
Answer:
Igneous rocks are formed by the cooling and solidification of molten rock called magma, which lies beneath the earth’s crust. These rocks are crystalline and compact. They do not occur in layers or strata nor do they contain fossils.
The cooling and solidification may take place at some depth within the earth or at the surface. The molten magma that reaches the surface of the earth is called lava. Thus, there are two types of igneous rocks classified on the basis of their place of origin - intrusive rocks, which form at some depth in the earth’s crust and extrusive rocks which form at or near the surface of the earth.
These rocks are the "parent rocks" of the Earth because they come from the original molten material of the planet. They are usually very hard and do not have the layers that you see in sedimentary rocks.
Teacher's Tip: Magma underground = Intrusive; Lava on surface = Extrusive.
Exam Tip: Mention that igneous rocks "do not contain fossils" because the heat of the magma would destroy them.

Question 4: How do acidic igneous rocks differ from basic igneous rocks ?
Answer:
Igneous rocks may also be classified as acidic or basic depending on their composition. The acidic rocks contain more than 65% silica and very low percentage of oxides. These are less dense and light coloured. For example, granite. The basic rocks contain low percentage of silica and high percentage of oxides. These are dense and dark coloured. For example, basalt.
Acidic rocks are usually light in color because of the high silica content, while basic rocks appear dark and heavy. This chemical difference affects how the rock behaves and how it looks when used in buildings.
Teacher's Tip: High Silica = Light color; Low Silica = Dark color.
Exam Tip: Remember the examples: Granite is Acidic, and Basalt is Basic.

Question 5: How are sedimentary rocks formed ?
Answer:
Sedimentary rocks are formed from sediments accumulated over long periods, usually under water on the floors of shallow seas, rivers and lakes. These rocks are non crystalline and are found in layers or strata and contain fossils. Conglomerate, sandstone, limestone, chalk, calcite, and dolomite are example of sedimentary rocks. Sedimentary rocks are divided into various types depending on how they are formed.
1. Mechanically formed sedimentary rocks : These rocks have been formed by the accumulation of materials derived from other rocks which have been cemented together. Examples - conglomerate (rounded fragments), breccia (angular fragments).
2. Organically formed sedimentary rocks : These rocks have been formed by the accumulation of the remains of living organisms. Examples - calcareous rocks such as limestone and chalk.
3. Chemically formed sedimentary rocks: These rocks have been commonly formed by the process of evaporation of water containing salts in solution. Examples - rock salt, gypsum, potash, nitrates, calcite, and dolomite.
These rocks are unique because they form in layers over time, trapping dead plants and animals that eventually turn into fossils. They are like a history book of the Earth's past life and environment.
Teacher's Tip: Sedimentary = Settling. Think of sand settling at the bottom of a jar of water.
Exam Tip: Mentioning "layers" and "fossils" is critical for describing sedimentary rocks.

Question 6: What are metamorphic rocks ?
Answer:
Metamorphic rocks are those rocks which are formed by the metamorphism of pre-existing rocks due to high temperature or pressure or both. Igneous and sedimentary rocks completely changes over long periods of time because of movements in the earth’s crust and volcanic or mountain building activity. Transformation of rocks due to high temperature is called thermal metamorphism. Alteration of parent rock due to pressure exerted on rocks from the earth’s movements is called dynamic metamorphism.
A metamorphic rock is a "changed" rock that has been baked or squeezed into a harder, denser form. For example, soft limestone can be transformed into hard, beautiful marble through this process.
Teacher's Tip: Thermal = Heat change; Dynamic = Pressure change.
Exam Tip: Explain both "Thermal" and "Dynamic" types of metamorphism to show a complete understanding.

E. Picture study

The photograph shows you a type of rock that is widely used in the construction industry.

Question 1: Name the type of rock.
Answer:
The type of rock is chemically formed sedimentary rock as limestone becomes marble. It is marble.
Marble is a highly valued rock because of its smoothness and beautiful patterns. Although the textbook classifies it here by its origin, it is most famously known as a metamorphic rock.
Teacher's Tip: Marble is what famous statues like the Taj Mahal are made of!
Exam Tip: Be careful to identify the rock as "marble" based on its appearance and its use in construction.

Question 2: How is it formed ?
Answer:
Sedimentary rocks are formed from sediment accumulated over long periods, usually under water on the floors of shallow seas, rivers, and lakes. These rocks are fromed by the process of evaporation of water containing in limestone which becomes marble.
The formation involves layers of material building up and being pressed together by the weight above. Over time, these materials harden into solid stone through natural chemical processes.
Teacher's Tip: Remember that "sediment" is just a fancy word for bits of sand, mud, and shells.
Exam Tip: Use the word "accumulation" to describe how the sediments gather together to form the rock.

LET’S DO SOMETHING

Question: With the help of your teacher, collect different types of rocks such as granite, sandstone, limestone, marble, slate, graphite, etc. and prepare a chart containing information about their colour, hardness, texture, and permeability.
Answer:
Do it Yourself.
Touching and seeing real rocks is the best way to understand how they differ in texture and weight. Comparing them on a chart helps you see the patterns in how they are formed and used.
Teacher's Tip: Use a magnifying glass to look for tiny crystals or layers in the rocks you collect!
Exam Tip: If you are making a chart, always include a column for "Permeability" to show if water can pass through the rock.