Selina Textbook Solutions Types of Rocks - Geography Class 7 ICSE PDF Download

 Page 1


Types of Rocks 
THINK AND ANSWER 
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. 
Discuss 
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. 
VALUES & LIFE SKILLS 
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. 
 
Page 2


Types of Rocks 
THINK AND ANSWER 
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. 
Discuss 
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. 
VALUES & LIFE SKILLS 
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. 
 
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 : 
1. Minerals are organic substances. 
Answer. False. 
Correct : Minerals are inorganic substances. 
2. Basalt is an example of an intrusive igneous rock. 
Answer. False. 
Correct : Basalt is an example of an extrusive igneous rock. 
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. 
4. Igneous and sedimentary rocks can change into metamorphic rocks. 
Answer. True. 
5. Rocks are used in construction industries. 
Answer. True. 
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. 
Question 2. 
Differentiate between SIAL and SIMA. 
Answer: 
SIAL 
Page 3


Types of Rocks 
THINK AND ANSWER 
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. 
Discuss 
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. 
VALUES & LIFE SKILLS 
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. 
 
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 : 
1. Minerals are organic substances. 
Answer. False. 
Correct : Minerals are inorganic substances. 
2. Basalt is an example of an intrusive igneous rock. 
Answer. False. 
Correct : Basalt is an example of an extrusive igneous rock. 
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. 
4. Igneous and sedimentary rocks can change into metamorphic rocks. 
Answer. True. 
5. Rocks are used in construction industries. 
Answer. True. 
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. 
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. 
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. 
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. 
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. 
 
 
Page 4


Types of Rocks 
THINK AND ANSWER 
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. 
Discuss 
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. 
VALUES & LIFE SKILLS 
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. 
 
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 : 
1. Minerals are organic substances. 
Answer. False. 
Correct : Minerals are inorganic substances. 
2. Basalt is an example of an intrusive igneous rock. 
Answer. False. 
Correct : Basalt is an example of an extrusive igneous rock. 
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. 
4. Igneous and sedimentary rocks can change into metamorphic rocks. 
Answer. True. 
5. Rocks are used in construction industries. 
Answer. True. 
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. 
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. 
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. 
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. 
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. 
 
 
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. 
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 
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. 
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. 
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. 
Page 5


Types of Rocks 
THINK AND ANSWER 
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. 
Discuss 
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. 
VALUES & LIFE SKILLS 
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. 
 
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 : 
1. Minerals are organic substances. 
Answer. False. 
Correct : Minerals are inorganic substances. 
2. Basalt is an example of an intrusive igneous rock. 
Answer. False. 
Correct : Basalt is an example of an extrusive igneous rock. 
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. 
4. Igneous and sedimentary rocks can change into metamorphic rocks. 
Answer. True. 
5. Rocks are used in construction industries. 
Answer. True. 
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. 
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. 
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. 
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. 
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. 
 
 
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. 
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 
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. 
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. 
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. 
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 Arundeep’s Self-Help to Voyage-7 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. 
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. 
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.