WBBSE Chapter 3 Rocks Synopsis
Rocks are the homogeneous or heterogeneous aggregates of one or more minerals which are the complex of one or more inorganic elements.
Aggregates in which all elements remain in equal ratio everywhere are called homogeneous aggregates.
Aggregates in which all elements are formed in different ratios in different places are called heterogeneous aggregates.
The permeability of a rock is its ability or capacity to allow liquid and gaseous substances to pass through it.
The porosity of a rock is the ratio between the voids in a rock mass and the total volume of the rock.
Rocks are classified into three groups according to their origin-igneous rocks, sedimentary rocks and metamorphic rocks.
The rocks that form from cooled and solidified lava and magma on the earth’s crust or in the interior respectively, are called igneous rocks.
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Extrusive igneous rocks are rocks formed due to the cooling and solidification of lava on the earth’s surface.
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Intrusive igneous rocks are rocks formed when magma cools and solidifies below the earth’s surface.
When magma cools and solidifies just below the earth’s surface in the cracks, pores and hollow places the resultant rocks are called hypabyssal or sub-volcanic igneous rocks.
When magma cools and solidifies at a great depth within the earth the resultant rocks are called the plutonic or abyssal igneous rocks.
Granite is a hard, heavy, impermeable and erosion-resistant rock. The continental crust is mainly made of granite.
Basalt is a hard, heavy, erosion-resistant and permeable rock. The oceanic crust is mainly made up of basalt.
Sediments and debris derived by natural agents of erosion like rivers, glaciers, wind and sea waves through the processes of erosion and transportation are deposited regularly underwater.
These depositions are consolidated and compacted due to the continuous increase of weight and pressure of the overlying sediments to form sedimentary rocks.
Marine flora and fauna get buried within the beds of sediments during the formation of sedimentary rocks. Later, the solidified imprints of the dead remains of these organisms on rocks are called fossils.
Rocks formed due to cementing of disintegrated old rocks are called clastic rocks.
Chemically or organically formed sedimentary rocks are called non-clastic rocks.
Limestone is a calcareous sedimentary rock.
Sandstone is a sedimentary rock rich in sand.
Class 8 Geography Chapter 3 Rock Classification
Mudstone or shale is a soft sedimentary rock rich in clay.
Igneous and sedimentary rocks lose their physical and chemical characteristics over a long period and acquire new characteristics to form newly transformed rocks known as metamorphic rocks.
Marble is the metamorphic form of limestone.
Slate is the metamorphic form of mudstone or shale.
Gneiss is the metamorphic form of granite.
The rock cycle is the process by which rocks of one kind change into another kind. It is the complete cyclic process of the formation of rocks and their change from one type to the other.
Karst topography is the landscape formed from the dissolution of limestone as a result of flowing river or rainwater.
Elements or compounds that are crystalline, have well-defined atomic structures and chemical bondages and occur naturally in rocks are called minerals.
Some important minerals are quartz, feldspar, mica and gypsum.
Moh’s Scale of Hardness measures the hardness of minerals. It is graduated from 1-10.
The Chotanagpur plateau is known as the storehouse of minerals in India.
Natural agents like rivers, wind, rainfall, glaciers, sea waves, etc degrade rocks and minerals over a long period to form loose debris of rocks. This debris is called regolith.
Chapter 3 Topic A Characteristics And Classification Of Rocks Analytical Type Questions
Question 1. What is a rock? Classify different types of rocks.
Answer:
Rocks:
Rocks are the homogeneous or heterogeneous aggregates of one or more minerals which are the complex of one or more inorganic elements.
Geologists believe that the materials which form the earth’s crust can be called rocks in general. Hence, pebbles, stones, clay, gravels, sands, etc., are all included in the category of rocks. Some of the rock-forming minerals are quartz, feldspar, mica, olivine and pyroxene.
Classification:
According to the origin, rocks are classified into three categories
- Igneous rocks
- Sedimentary rocks and
- Metamorphic rocks.
1. Igneous rocks:
The rocks that form from cooled and solidified lava and magma on the earth’s crust or in the interior respectively, are called igneous rocks. These are also known as primary rocks as these were the first rocks to originate on Earth.
When the earth first cooled from a burning mass, the hot molten materials or magma slowly radiated heat to cool down and formed a solid, hard crust on the surface.
Later on, lava was ejected through craters or fissures and over time, it cooled to form igneous rocks.
Example-granite and basalt.
2. Sedimentary rocks:
Sediments and debris derived by natural agents of erosion like rivers, glaciers, wind and sea waves through the processes of erosion and transportation are deposited regularly underwater.
These depositions are consolidated and compacted due to the continuous increase of weight and pressure of the overlying sediments to form sedimentary rocks.
Example-sandstone, limestone and mudstone.
3. Metamorphic rocks:
Igneous and sedimentary rocks lose their physical and chemical characteristics over a long period and acquire new characteristics to form newly transformed rocks known as metamorphic rocks.
Example-Gneiss, marble, quartzite and slate.
Question 2. Classify igneous rocks according to their origin and structure and define them.
Answer:
Classification of Igneous Rocks According To Their Origin And Structure:-
Igneous rocks, according to origin and structure, are divided into two classes-extrusive igneous rocks and intrusive igneous rocks.
Understanding Rock Types for Class 8
Extrusive igneous rocks:
Rocks formed due to the cooling and solidification of lava on the earth’s surface are called extrusive igneous rocks. The grains or crystals formed in this rock are fine.
Example-Basalt, obsidian, etc.
Classification:
Based on their origin, extrusive igneous rocks are further divided into two types
Lava rocks:
When hot, molten magma reaches the surface of the earth through holes, cracks or fissures and gets solidified, it is called lava rock.
Example-Basalt.
Pyroclastic rocks:
As a volcano erupts, it also throws out lava that had previously solidified in and around the craters or vents along with cinder ash. All these together are ejected out with the erupting lava and they form rocks known as pyroclastic rocks.
Example-Tuff.
Intrusive igneous rocks:
Rocks formed when magma cools and solidifies below the earth’s surface are called intrusive igneous rocks. The grains or crystals are bigger than the extrusive rocks.
Example-Granite, are dolerite, etc.
Classification:
Intrusive igneous rocks are further sub-divided into two types
Abyssal rocks:
When magma cools and solidifies at a great depth within the earth the resultant rocks are called plutonic or abyssal igneous rocks. The size of the grains or crystals is the biggest.
Example-Granite.
Hypabyssal rocks:
When magma cools and solidifies just below the earth’s surface in the cracks, pores and hollow places the resultant Plutonic rocks are called hypabyssal or sub-volcanic igneous rocks.
The grains or crystals of these rocks are not as fine as the extrusive igneous rocks and neither are these as big as those of the abyssal rocks.
Example-Dolerite.
Question 3. What are sedimentary rocks? Classify different types of sedimentary rocks.
Answer:
Sedimentary rocks:
Sediments and debris derived by natural agents of erosion like rivers, glaciers, wind and sea waves through the processes of erosion and transportation are deposited regularly underwater.
These depositions are consolidated and compacted due to the continuous increase of weight and pressure of the overlying sediments to form sedimentary rocks.
WBBSE Class 8 Geography Characteristics of Rocks
Examples— are sandstone, limestone and mudstone.
Classification of sedimentary rocks:
Based on the origin of sediments, sedimentary rocks can be divided into two types
Clastic rocks:
Rocks formed due to cementing of disintegrated old rocks are called clastic rocks.
Example-Conglomerate, breccia. Clastic rocks are mechanically formed when sediments bind together due to the immense pressure of the overlying layers.
They can be further subdivided into the following
- Argillaeceous sedimentary rocks, with a grain diameter size of less than 0.06mm. Example-Mudstone.
- Arenaceous sedimentary rock, with a grain diameter size of 0.06 mm-2 mm. Example-Sandstone.
- Rudaeceous sedimentary rock, with a grain diameter size of more than 2 mm. Example-Conglomerate.
Non-clastic rocks:
Chemically or organically formed sedimentary rocks are called non-clastic rocks. Example-Limestone, rock salt.
Types of Rocks for Class 8 Geography
Sub-divisions of the non-clastic rocks are as follows
1. Chemically formed sedimentary rocks:
Running water contains chemical materials in suspension. When this water comes in contact with rocks on its way, the soluble materials from the rocks are removed. These materials are known as chemically derived or formed which gradually settle under water to form chemically formed sedimentary rocks.
Examples- Dolomite, rock salt, etc.
2. Organically formed sedimentary rocks:
Sediments derived from the disintegration and decomposition of plants and animals when deposited and compacted form organically formed sedimentary rocks.
Example-Limestone from marine organisms, and coal from plants.
Question 4. What information can be gathered from fossils?
Answer:
Information Gathered From Fossils:-
Fossils can rightly be called pages from history. A lot of information might be gathered from fossils.
Some of these are
- Finding the time to which the fossil belonged by testing the C14 isotope.
- Understanding the climatic condition of the time to which the fossil belonged by observing the shapes of leaves, branches, etc.
- Getting to know the species to which the organism belonged by knowing the age of the fossil.
- Finding the origin of an organism by studying the fossil.
- Getting to know about the condition of the place where the fossil has been found. A fossil also indicates the contemporary condition of the place (where it is found) at that age.
Question 5. What are metamorphic rocks? Classify different types or methods of metamorphism.
Answer:
Metamorphic rocks:
Igneous and sedimentary rocks lose their physical and chemical characteristics over a long time and acquire new characteristics to form newly transformed rocks known as metamorphic rocks.
Example-Granite changes to gneiss, limestone to marble and phyllite to schist.
Methods of metamorphism:
The types or methods of metamorphism are
Thermal metamorphism:
Rocks change their forms when they come in contact with the hot molten magma from the interior of the earth. This metamorphism occurs during volcanic activities.
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Example-Peat coal changes to graphite.
1. Local or contact metamorphism:
The change of rocks over a small area on the earth’s surface under the influence of heat is called local metamorphism. This method is also known as contact metamorphism because the rocks come in contact with the magma.
Example-Limestone to marble.
2. Pressure-induced metamorphism:
When pressure plays a dominant role in the alteration of the rock form to form new rocks the process is known as pressure-induced metamorphism.
Example- Mudstone to slate.
Regional metamorphism:
Large-scale mountain building or orogeny causes extensive areas to undergo temperature and pressure changes. Rocks thus formed over extensive areas mainly due to pressure changes are said to have formed by regional metamorphism.
Example-Shale or mudstone to slate.
3. Chemically induced metamorphism:
Geologists believe that groundwater or reactive gaseous substances sometimes react with rocks to change their present forms. The resultant rocks thus formed are due to the chemically induced processes.
Example andalusite to silimenite.
Question 6. Compare igneous, sedimentary and metamorphic rocks.
Answer:
Comparison Of Igneous, Sedimentary And Metamorphic Rocks:-
The comparison among igneous, sedimentary and metamorphic rocks are as follows
Chapter 3 Topic A Characteristics And Classification Of Rocks Short Analytical Type Questions
Question 1. How did igneous rocks form?
Answer: Formation Igneous Rocks:-
Geologists believe that the earth was formed from a fiery gaseous state. It gradually cooled down to a solid body and the hard cover on the surface further hardened and became solid to form rocks, called igneous or primary rocks.
Not only at the time of the earth’s inception, but even now lava flows out, cools and solidifies on the earth’s surface to form igneous rocks. These are extrusive igneous rocks.
Example-Basalt.
Similarly, when magma cannot come out onto the surface, it cools and solidifies inside the earth over a long period to form igneous rocks. These are intrusive igneous rocks.
WBBSE Class 8 Geography Study Guide on Rocks
Example- Granite.
Question 2. Discuss the characteristics of igneous rocks.
Answer:
Characteristics Of Igneous Rocks:-
The characteristics of igneous rocks are as follows
- Crystalline structure: All igneous rocks have a crystalline structure.
- Stratification: The absence of stratification is an important feature of these rocks.
- Mineral content: Igneous rocks contain various molten metallic substances. They are heavy because of their mineral content.
- Hardness and weight: These rocks are hard, heavy and compact.
- Fossils: The absence of fossils is another important feature of igneous rocks.
- Resistance to erosion: These rocks are less fragile and highly resistant to erosion.
- Nature: These rocks can be both acidic and basic or alkaline.
- Permeability: These are usually impermeable rocks. However, the presence of cracks and joints makes them permeable.
Question 3. Distinguish between hypabyssal and abyssal rocks.
Answer:
Differences Between Hypabyssal And Abyssal Rocks:-
The differences between hypabyssal and abyssal rocks are as follows
Question 4. What do you know about granite?
Answer:
Granite:-
It is a kind of abyssal intrusive igneous rock. The continental crust is mainly formed of granite. The characteristics of granite are as follows
1. Grain size:
The cooling process is very slow, so the grains formed are big, coarse and distinct. The diameter of the grains are more than 3mm.
2. Mineral content:
This rock is mainly composed of quartz, feldspar, mica and hornblende.
3. Resistance to erosion and hardness:
This rock is very hard and thus very resistant to erosion.
WBBSE Class 8 Geography Rock Types Overview
4. Colour:
Its colour varies from white, and grey to pink.
5. Permeability:
This is an impermeable rock, but the presence of cracks and joints may allow water to enter into it.
6. Landforms:
Landforms made of granite are usually dome-shaped.
Question 5. What do you know about basalt?
Answer:
Basalt:-
It is a kind of extrusive igneous rock. The oceanic crust is mainly composed of basalt.
The characteristics of basalt are as follows
Grain size: Molten materials from the earth’s interior flow over the surface of the earth and cool down rapidly after coming into contact with air so these rocks are fine-grained. The grain size is less than 1mm.
Mineral content: The chief mineral constituents are feldspar, olivine and pyroxine.
Resistance to erosion and hardness: This rock is very hard and thus very resistant to erosion. It is also a heavy rock.
Colour: It is grey to black.
Permeability: Due to the presence of many cracks and joints this rock is highly permeable.
Landforms: Flat-topped plateaus formed in regions composed of basalt.
Question 6. How are sedimentary rocks formed?
Answer:
Formation Of Sedimentary Rocks:-
The rock materials on the earth’s crust are constantly getting eroded by natural agents (running water, wind, glaciers, sea waves, etc.) and are also undergoing constant chemical reactions.
These eroded materials like pebbles, sands, silt, clay, etc., get deposited in layers on the beds of the water bodies. As water and the upper layers of silt exert pressure on the lowest layers, the sediments get compacted and hardened to form sedimentary rocks.
Examples- are limestone, sandstone, etc.
Question 7. What are the characteristic features of sedimentary rocks?
Answer:
Characteristic Features Of Sedimentary Rocks:-
The characteristic features of sedimentary rocks are as follows
Stratification:
Distinct layers are found in sedimentary rocks. Hence, these rocks are also known as stratified rocks. Mudcracks are found on these rocks.
Classification of Rocks for Class 8 Students
Hardness:
Sedimentary rocks get easily eroded due to a lack of hardness. These rocks are soft and fragile.
Porosity and permeability:
Sedimentary rocks are porous and highly permeable.
Fossils:
These are the only rocks with fossil content.
Mineral content:
Only sedimentary rocks bear deposits of coal, mineral oil, natural gas, etc.
Crystalline structure:
Sedimentary rocks do not have a crystalline structure or the presence of cracks or joints.
Resistance to erosion:
Although different rocks have different capacities to resist erosion, these rocks are less resistant on a general basis.
Question 8. Why do sedimentary rock layers bear mineral oil?
Answer:
Sedimentary Rock Layers Bear Mineral Oil:-
During the process of sedimentation, minute sea organisms, bacteria, plant cells, phytoplankton, etc. get trapped between the layers of sediments.
Gradually, the pressure of the upper layers of silt, internal heat of the earth and chemical reactions lead to the formation of petroleum after being transformed into the solution of hydrocarbons and other organic compounds.
Thus, sedimentary rocks have mineral oil-bearing rock strata. Petroleum is also called a type of hydrocarbon for excessive hydrogen and carbon content.
Question 9. Why is mineral oil not found in igneous rocks?
Answer:
Mineral Oil Not Found In Igneous Rocks:-
Mineral oil was formed when dead animal bodies got deposited between layers of sedimentary rocks and were transformed into a solution of hydrocarbon and another organic compound due to overlying weight, pressure and high temperature.
Igneous rock formation on the other hand involves hot molten magma and lava which destroys the dead bodies of all living organisms. Thus, mineral oil formation is never possible in the igneous rocks.
Question 10. What are fossils and why are they found in sedimentary rocks?
Answer:
Fossils:-
Marine flora and fauna get buried within beds of sediments during the formation of sedimentary rocks. Later, the solidified imprints of the dead remains of these organisms on rocks are called fossils.
The impressions or remnants of living organisms and plants left within the layers of sedimentary rocks are called fossils. Sedimentary rocks are formed when sediments deposited on the river or seabed get compacted under the influence of pressure from the overlying layers.
At the time of sediment deposition marine flora and fauna may get buried within the sediment layers. Later dead remains of these organisms leave their imprint solidified.
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Thus, fossils are formed and found in the layers of sediments after being sedimentary rocks.
Question 11. Why are fossils not found in igneous rocks?
Answer:
Fossils Are Not Found In Igneous Rocks:-
There are several reasons why igneous rocks do not contain fossils.
These are as follows
- During the time of formation of igneous rocks, the heat of the molten magma and lava destroys the presence or evidence of all organic matter buried in it. Hence, no impression of fossils is found here.
- Igneous rocks are not deposited in layers like the sedimentary rocks and so remnants of organic life cannot be found there.
- Igneous rocks are the parent rocks that existed even before the beginning of life on Earth. Hence, these cannot have any fossil content in them.
Question 12. Write a short note on different kinds of sedimentary rocks.
Answer:
Sedimentary rocks:
Sediments and debris derived by natural agents of erosion like rivers, glaciers, wind and sea waves through the processes of erosion and transportation are deposited regularly underwater.
These depositions are consolidated and compacted due tothe continuous increase of weight and pressure of the overlying sediments to form sedimentary rocks.
Example— sandstone, limestone and mudstone.
Classification of sedimentary rocks:
Based on the origin of sediments, sedimentary rocks can be divided into two types
Clastic rocks:
Rocks formed due to cementing of disintegrated old rocks are called clastic rocks.
Example-Conglomerate, breccia. Clastic rocks are mechanically formed when sediments bind together due to the immense pressure of the overlying layers.
They can be further subdivided into the following
- Argillaeceous sedimentary rocks, with a grain diameter size of less than 0.06mm. Example-Mudstone.
- Arenaceous sedimentary rock, with a grain diameter size 0of .06 mm-2 mm. Example-Sandstone.
- Rudaeceous sedimentary rock, with a grain diameter size of more than 2 mm. Example-Conglomerate.
Non-clastic rocks:
Chemically or organically formed sedimentary rocks are called non-clastic rocks. Example-Limestone, rock salt.
Sub-divisions of the non-clastic rocks are as follows
1. Chemically formed sedimentary rocks:
Running water contains chemical materials in suspension. When this water comes in contact with rocks on its way, the soluble materials from the rocks are removed. These materials are known as chemically derived or formed which gradually settle under water to form chemically formed sedimentary rocks.
Examples- Dolomite, rock salt, etc.
2. Organically formed sedimentary rocks:
Sediments derived from the disintegration and decomposition of plants and animals when deposited and compacted form organically formed sedimentary rocks.
Example-Limestone from marine organisms, and coal from plants.
Question 13. Write the differences between clastic and non-clastic rocks.
Answer:
The differences between clastic and non-clastic rocks are
Question 14. What do you know about limestone?
Answer:
Limestone:-
It is a calcareous sedimentary rock. The characteristic features of limestone are as follows
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Nature:
Limestone is organically formed. Calcium carbonate (CaCO3) or limestone is not soluble in water. But when it is transformed into calcium bicarbonate it readily dissolves in rainwater or acid solutions.
Colour:
Limestone may be white, grey, green and blackish.
Permeability:
It is highly permeable.
Hardness:
It is a type of soft rock and thus has very little resistance to erosion. Uses: It is used as a raw material in making cement and in the iron and steel industries.
Landforms:
Regions composed of limestone form karst landforms.
Question 15. What do you know about sandstone?
Answer:
Sandstone:-
It is a sedimentary rock rich in sand.
The characteristics of sandstone are as follows-
Nature:
It is a mechanically formed sedimentary rock. The soil in regions dominated by sandstone is saline and infertile.
- Resistance to erosion: It is resistant to erosion.
- Permeability and porosity: It is highly rocks are as follows porous and permeable.
- Colour: It is yellow, orange, red, pink, white or grey in colour.
- Uses: Many architectural wonders have been built out of sandstone, for example, the Red Fort, the temples in Udaygiri-Khandagiri.
Question 16. What do you know about shale?
Answer:
Shale:-
It is a sedimentary rock rich in clay.
The characteristics of sedimentary rock are as follows
- Presence of strata: Thin layers of stratification are seen in this rock.
- Grain size: It is a fine-grained rock.
- Porosity: It is highly porous.
- Hardness: It is soft and brittle and thus all construction work should be avoided in regions formed of shale.
- Colour: It is blackish grey in colour.
- Uses: It is used in making roof tiles of buildings as they can be separated along thin layers or foliation.
Question 17. What are metamorphic rocks?
Answer:
Metamorphic Rocks:
Igneous and sedimentary rocks lose their physical and chemical characteristics over characteristics to form newly transformed rocks for a long time and acquire new known as metamorphic rocks.
Example-
- Metamorphosis of igneous rock-Granite to gneiss.
- Metamorphosis of sedimentary rock- limestone to marble.
- Metamorphosis of metamorphic rock-Phyllite to schist.
Question 18. What are the different methods of metamorphism?
Answer:
Metamorphic rocks:
Igneous and sedimentary rocks lose their physical and chemical characteristics over a long period and acquire new characteristics to form newly transformed rocks known as metamorphic rocks.
Example-Granite changes to gneiss, limestone to marble and phyllite to schist.
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Methods of metamorphism:
The types or methods of metamorphism are
Thermal metamorphism:
Rocks change their forms when they come in contact with the hot molten magma from the interior of the earth. This metamorphism occurs during volcanic activities.
Example-Peat coal changes to graphite.
1. Local or contact metamorphism:
The change of rocks over a small area on the earth’s surface under the influence of heat is called local metamorphism. This method is also known as contact metamorphism because the rocks come in contact with the magma.
Example-Limestone to marble.
2. Pressure-induced metamorphism:
When pressure plays a dominant role in the alteration of the rock form to form new rocks the process is known as pressure-induced metamorphism.
Example- Mudstone to slate.
Regional metamorphism:
Large-scale mountain building or orogeny causes extensive areas to undergo temperature and pressure changes. Rocks thus formed over extensive areas mainly due to pressure changes are said to have formed by regional metamorphism.
Example-Shale or mudstone to slate.
3. Chemically induced metamorphism:
Geologists believe that groundwater or reactive gaseous substances sometimes react with rocks to change their present forms. The resultant rocks thus formed are due to the chemically induced processes.
Example andalusite to silimenite.
Question 19. What are the characteristics of metamorphic rocks?
Answer:
The characteristics of metamorphic
Resistant to erosion:
The constituent minerals become more compact and hard during the process of metamorphism which makes the resultant rock harder. So, the process of erosion is slower in metamorphic rocks than in igneous and sedimentary rocks.
Presence of bands:
During metamorphism, the minerals recrystallise and similar minerals tend to get together. As a result, bands appear in metamorphic rocks. Igneous rocks after metamorphism become more crystalline and more lustrous.
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Example- granite, when metamorphosed to gneiss, develops bands on it.
Stratification:
Igneous rocks do not have strata so when they are metamorphosed, there would be no stratification; but in the case of sedimentary rocks, which are always stratified, their layers are lost after metamorphosis due to tremendous pressure and heat.
Fossils:
Fossils in sedimentary rocks also disappear after metamorphosis under heat and pressure.
Question 20. Write a short note on the marble.
Answer:
Marble:-
It is the metamorphosed state of limestone.
The characteristics of marble are as follows
- Nature: Marble is a very smooth, glittery and attractive rock.
- Colour: It has a variety of colours like white, green, grey, yellow and blue.
- Erosion: Marble erodes rapidly when it comes into contact with acid. So, it has to be kept away from acidic solutions.
- Uses: Marble is soft and so can be cut and shaped according to requirement. Hence, it has been used in architecture and sculpture. The Taj Mahal, the Victoria Memorial, etc. have been built out of marble.
Question 21. Write a short note on the slate.
Answer:
Slate:-
It is the metamorphosed form of mudstone or shale.
The characteristics of the state are as follows
- Nature: It is smooth and arranged in thin layers that can be easily broken.
- Colour: It is bluish-grey to black.
- Uses: It is used in making roof tiles, and blackboards and is also used as a surface for writing.
Question 22. Briefly describe the rock gneiss.
Answer:
Rock Gneiss:-
It is the metamorphosed form of granite. It is also called banded gneiss’.
The characteristics of gneiss are as follows
- Nature: It is a hard rock and resistant rock.
- Colour: It can be greyish, blackish or whitish in shade.
- Presence of circular band: In gneiss, minerals are arranged in circular bands or layers. Thus, extracting a certain mineral from this is convenient.
- Uses: It is used in constructing roads and other construction works.
Question 23. Compare sedimentary rocks and metamorphic rocks.
Answer:
Chapter 3 Topic A Characteristics And Classification Of Rocks Short Answer Type Questions
Question 1. What are homogeneous aggregates?
Answer:
Homogeneous Aggregates:-
Aggregates in which all elements remain in equal ratio everywhere are called homogeneous aggregates.
Question 2. What are heterogeneous aggregates?
Answer:
Heterogeneous Aggregates:-
Aggregates in which all elements are formed in different ratios in different places are called heterogeneous aggregates.
Question 3. What are the permeability and porosity of a rock?
Answer:
Permeability And Porosity Of A Rock:-
The permeability of a rock is its ability or capacity to allow liquid and gaseous substances to pass through it. The porosity of a rock is the ratio between the voids in a rock mass and the total volume of the rock.
Question 4. Are porosity and permeability of rocks the same thing? Give a suitable example.
Answer: No, porosity and permeability of rocks are two different properties, but these are closely related. Porosity is the ratio between voids in the rock mass and the total volume of the rock.
Permeability means a rock’s capacity to allow liquid and gaseous matter to pass through Greater the porosity of a rock, the lesser its permeability and vice versa.
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Example- Mudstone is porous and thus is poorly permeable.
Question 5. Why are igneous rocks also called primary rocks?
Answer:
Igneous Rocks Also Called Primary Rocks:-
The rocks that were formed first on the earth were igneous rocks and sedimentary and metamorphic rocks came into existence in later stages from these igneous rocks. Hence, igneous rocks are also known as primary rocks.
Question 6. Why are sedimentary rocks also called stratified rocks?
Answer:
Sedimentary Rocks Also Called Stratified Rocks:-
Sedimentary rocks have been formed by the deposition of a large amount of silt on the beds of waterbodies in layers or strata. So, they are also called stratified rocks.
In sedimentary rocks, one layer is separated from another layer by a stratification plane.
Question 7. What is a stratification plane?
Answer:
Stratification Plane:-
Sedimentary rocks are generally formed of pebbles, stones, sand, clay, etc. During the formation of these rocks, comparatively heavier and older sediments get deposited below, while the lighter and newer ones settle above.
As a result, a stratification is noticed. Thus, the plane that separates one strata from the other, is called the stratification plane.
Question 8. Why are the sedimentary rocks porous?
Answer:
Sedimentary Rocks Porous:-
Sedimentary rocks are formed as a result of deposition of silt, clay, etc. on seabeds. As compared to igneous and metamorphic rocks there are pore spaces present in between the components of sedimentary rocks. Thus these rocks are porous.
Question 9. Why is coal considered to be a type of sedimentary rock?
Answer:
Coal Considered To Be A Type Of Sedimentary Rock:-
About 300-350 million years ago, as a result of earth movements, large forest tracts got buried deep under the ground. Due to the pressure and temperature of the earth’s interior, the plants and trees were stratified.
The carbon content of the stems of the trees gradually transformed into coal. Thus, coal is a type of organically formed sedimentary rock.
Question 10. What are clastic rocks?
Answer:
Clastic Rocks:-
The agents of erosion like rivers, winds, sea waves, etc. constantly break the rocks into pebbles, sand, silt and clay. The sedimentary rocks that are formed due to the cementing of disintegrated old rocks are called clastic rocks.
Example-Sandstone, mudstone, conglomerate.
Question 11. What are non-clastic rocks?
Answer:
Non-Clastic Rocks:-
Chemically or organically formed sedimentary rocks are called non-clastic rocks.
Example-Limestone, rock salt.
Question 12. What is a conglomerate?
Answer:
Conglomerate:-
Conglomerate is a kind of compact, mechanically formed, stony sedimentary rock. These are formed of comparatively bigger and rounded pebbles and coarse sand, which are deposited in coastal regions. These pebbles or sand grains are more than 2 mm in diameter.
Question 13. What are the different types of metamorphic rocks?
Answer:
Different Types Of Metamorphic Rocks:-
Metamorphic rocks can be classified in the following ways based on the origin of rocks
- Metamorphism of igneous rocks, for example, Granite to gneiss;
- Metamorphism of sedimentary rocks, Example-Mudstone to slate;
- Metamorphism of metamorphic rocks, Example-Slate to phyllite and then to schist.
Question 14. What is regional metamorphism?
Answer:
Regional Metamorphism:-
Large-scale mountain building or orogeny causes extensive areas to undergo temperature and pressure changes. Rocks thus formed over extensive areas mainly due to pressure changes are said to have formed by regional metamorphism.
Example-Slate.
Question 15. What is contact metamorphism?
Answer:
Contact Metamorphism:-
Rocks change their forms when they come in contact with the hot molten magma from the interior of the earth. This metamorphism occurs during volcanic activities over a small local area. This process is known as thermal local or contact metamorphism.
For example- Limestone changes into marble due to contact metamorphism.
Question 16. Why are crystals not formed in sedimentary rocks?
Answer:
Crystals Not Formed In Sedimentary Rocks:-
Molecules of water combine with different minerals to form crystals. When igneous rocks form, some amount of water remain in it taking the shape of veins.
Crystals are formed from the atoms of mineral water which evaporates while the rock cools. But in the case of sedimentary rocks, this water disappears due to the tremendous pressure of the overlying layers. As a result, crystals do not form in sedimentary rocks.
WBBSE Class 8 Geography Chapter 8 Question Answer
Question 17. From which type of rock is mineral extraction convenient and why?
Answer: Mineral extraction is convenient from metamorphic rocks. The position of minerals within a rock is altered due to excessive heat and pressure in the process of metamorphism.
Similar minerals tend to get together. Thus, the extraction of minerals becomes convenient.
Question 18. Try to know which rock is used to make a pencil lead and to which type of rock it belongs.
Answer: Pencil lead is made from graphite which is a metamorphosed form of peat coal.
Question 19. Why are the basic features of original rocks not found in their metamorphosed state?
Answer:
Basic Features Of Original Rocks Not Found In Their Metamor-Phosed State:-
Rocks metamorphose under the influence of great heat, pressure and even chemical reactions on the surface of the earth. As a result, the basic features of the original rock are lost in the process and the resultant rock formed has new characteristics altogether.
Chapter 3 Topic A Characteristics And Classification Of Rocks Multiple Choice Questions Choose The Correct Option
Question 1. When the porosity of a rock increases, its water retention capacity
- Decreases
- Increases
- Remains moderate
- None of these
Answer: 2. Increases
Question 2. The ratio between the space of a rock and its total volume is known as its-
- Porosity
- Cleavage
- Permeability
- Texture
Answer: 1. Porosity
Question 3. An example of primary rock is-
- Granite
- Shale
- Sandstone
- Limestone
Answer: 1. Granite
Question 4. Igneous rock is also called-
- Stratified rock
- Unstratified rock
- Metamorphic rock
- Incoherent rock
Answer: 2. Unstratified rock
Question 5. An example of a hypabyssal rock is-
- Basalt
- Dolerite
- Granite
- Gabbro
Answer: 2. Dolerite
Question 6. An example of an abyssal or plutonic rock is-
- Basalt
- Granite
- Dolerite
- Gneiss
Answer: 2. Granite
Question 7. The oceanic crust is mainly composed of-
- Granite
- Basalt
- Limestone
- Mudstone
Answer: 2. Basalt
WBBSE Class 8 Geography Chapter 8 Question Answer
Question 8. The mineral found in granite rock is-
- Quartz
- Aragonite
- Calcite
- None of these
Answer: 1. Quartz
Question 9. Which of the following is a sedimentary rock?
- Mudstone
- Graphite
- Augite
- Tuff
Answer: 1. Mudstone
Question 10. Chalk is a type of-
- Extrusive rock
- Sedimentary rock
- Lava
- Metamorphic rock
Answer: 2. Sedimentary rock
Question 11. Carbon deposits in the trunks of trees get stratified to primarily form-
- Coal
- Graphite
- Mineral oil
- Dolomite
Answer: 1. Coal
Question 12. Natural gas is available in
- Igneous rocks
- Metamorphic rocks
- Sedimentary rocks
- None of these
Answer: 3. Sedimentary rocks
Question 13. Fossils are found in
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
- None of these
Answer: 2. Sedimentary rocks
Question 14. A sedimentary rock formed from organic matter is-
- Coal
- Shale
- Gneiss
- Granite
Answer: 1. Coal
Question 15. Conglomerate is a type of-
- Igneous rock
- Sedimentary rocks
- Metamorphic rock
- None of these
Answer: 2. Sedimentary rocks
Question 16. A kind of sandy sedimentary rock is-
- Haematite
- Kimberlite
- Sandstone
- Phyllite
Answer: 3. Sandstone
Question 17. The permeability of sandstone is-
- Medium
- High
- Low very
- Low
Answer: 2. High
Question 18. Graphite is a/an-
- Igneous rock
- Metamorphic rock
- Sedimentary rocks
- None of these
Answer: 2. Metamorphic rock
WBBSE Class 8 Geography Chapter 8 Question Answer
Question 19. The rock formed due to zonal/regional metamorphism is-
- Phyllite
- Schist
- Slate
- Granite
Answer: 3. Slate
Question 20. The type of rock with the highest resistance to erosion is-
- A sedimentary rock
- Metamorphic rock
- Igneous rock
- Pyroclastic rock
Answer: 2. Metamorphic rock
Question 21. When igneous rocks metamorphose, their roughness-
- Decreases
- Increases
- Remains the same
- None of these
Answer: 1. Decreases
Question 22. Granite metamorphoses into-
- A hornblende
- Marble
- Gneiss
- Slate
Answer: 3. Gneiss
Question 23. The metamorphic form of basalt is-
- Amphibolite
- Hornblende
- Phyllite
- Gneiss
Answer: 1. Amphibolite
Question 24. The metamorphosed form of sandstone is-
- Gneiss
- Quartzite
- Chalk
- Marble
Answer: 2. Quartzite
Question 25. The metamorphic state of peat coal is-
- Bituminous
- Bentonite
- Graphite
- Peridotite
Answer: 3. Graphite
Question 26. An example of the metamorphosed form of a metamorphic rock is-
- Amphibolite
- Hornblende
- Phyllite
- Gneiss
Answer: 3. Phyllite
Question 27. Kimberlite is a type of-
- Extrusive rock
- Abyssal rock
- Hypabyssal rock
- Pyroclastic rock
Answer: 3. Hypabyssal rock
Question 28. The branch of science that deals with rocks is-
- Pedology
- Orology
- Petrology
- Cryptology
Answer: 3. Petrology
Question 29. An example of a non-crystalline rock
- Basalt
- Chalk
- None of these
- Graphite
Answer: 2. Chalk
Question 30. The moisture retention capacity of silt is-
- Low
- High
- Very Low
- Medium
Answer: 4. Medium
Question 31. A type of pyroclastic rock is-
- Tuff
- Graphite
- Ilmenite
- Basalt
Answer: 1. Tuff
Question 32. An example of an ultra-alkaline rock among the following is-
- Granite
- Peridotite
- Diorite
- Pegmatite
Answer: 2. Peridotite
WBBSE Chapter 3 Topic A Characteristics And Classification Of Rocks Very Short Answer Type Questions Fill In The Blanks
Question 1. When hot molten magma from the interior of the earth gushes out to the surface, it is called _________.
Answer: Lava
Question 2. Igneous rocks were the first rocks to form on the surface of the earth and so they are called _________ rocks.
Answer: Primary
Question 3. _________ rock is the parent rock of all other rock.
Answer: Igneous
Question 4. Basalt is a type of _________ igneous rock.
Answer: Extrusive
Question 5. Basalt is made up of fine grains while granite is made up of _________ grains.
Answer: Coarse
Question 6. Dolerite is a type of _________ igneous rock.
Answer: Hypabyssal
Question 7. Abyssal rocks are also called _________ igneous rocks.
Answer: Plutonic
Question 8. Crystals are chief characteristics of _________ rocks.
Answer: Igneous
Question 9. _________ is the chief constituent of the continental crust.
Answer: Granite
Question 10. Granite is a combination of quartz, hornblende, _________ and mica.
Answer: Feldspar
Question 11. Igneous rocks on the crust of the earth constantly _________ and transform into sand, gravel and pebbles.
Answer: Erode
Question 12. Stratification and formation of _________ are characteristics of sedimentary rocks.
Answer: Fossils
Question 13. Sedimentary rocks are more _________ igneous rocks.
Answer: Brittle
Question 14. An example of clastic rock is _________.
Answer: Breccia
Question 15. A combination of pebbles, cobbles, granules, etc forms _________.
Answer: Conglomerate
Question 16. Gneiss is an example of _________ rock.
Answer: Metamorphic
Question 17. Quartzite is a metamorphosed form of _________.
Answer: Sandstone
Question 18. Limestone gets metamorphosed into _________.
Answer: Marble
Question 19. Phyllite metamorphoses into _________.
Answer: Schist
Question 20. Limestone or _________ is generally not soluble in water.
Answer: Calcium Carbonate
Question 21. Igneous rocks containing more than 55% of silica are called _________ rock.
Answer: Acidic
Question 22. Tuff is an example of _________ rocks.
Answer: Pyroclastic
Question 23. Igneous rock is classified as _________ or basic depending on its silica content.
Answer: Acidic
Question 24. Dolomite is a type of _________ rocks.
Answer: Sedimentary
Question 25. The plane that separates one layer of silt from another in a sedimentary rock is called a _________ plane.
Answer: Stratification
Question 26. Rock containing excessive _________ is called an acidic rock.
Answer: Silica
Question 27. Based on chemical composition, igneous rocks can be subdivided into acidic, intermediate, _________ and ultrabasic rocks.
Answer: Alkaline
Question 28. Diorite and dolerite are _________ rocks.
Answer: Irneous
Question 29. Rhyolite is a type of _________ igneous rock.
Answer: Extrusive/Acidic
WBBSE Chapter 3 Topic A Characteristics And Classification Of Rocks Write True Or False
Question 1. Layers of silt harden to form sedimentary rocks.
Answer: True
Question 2. Another name for metamorphic rock is stratified rock.
Answer: False
Question 3. Both igneous and sedimentary rocks change to metamorphic rocks due to heat and pressure.
Answer: True
Question 4. The upper crust of the earth is made up of granite and the lower one of basalt.
Answer: True
Question 5. Igneous rock is the source of all other rocks.
Answer: True
Question 6. Fossils are found in sedimentary rocks.
Answer: True
Question 7. Igneous rocks are stratified.
Answer: False
Question 8. Porosity is the chief characteristic of sedimentary rocks.
Answer: True
Question 9. Coal is a type of igneous rock.
Answer: False
Question 10. Sandstone is a kind of stratified rock.
Answer: True
Question 11. Sedimentary rock layers contain a sufficient amount of metallic minerals.
Answer: False
Question 12. Coal, mineral oil, etc. are available in igneous rock layers.
Answer: False
Question 13. Limestone is composed of either calcite or aragonite.
Answer: True
Question 14. Another term for sedimentary rock is primary rock.
Answer: False
Question 15. Dolerite is a kind of abyssal rock.
Answer: False
Question 16. Permeability is more in igneous rocks than in other rocks.
Answer: False
Question 17. Granite is generally dark in colour.
Answer: False
Question 18. The amount of silica in acidic rocks is more than 65%.
Answer: True
Question 19. Porphyry is a kind of hypabyssal rock.
Answer: True
Question 20. Regions formed of basalt are dome-shaped.
Answer: False
Question 21. Sedimentary rocks are called unstratified rocks.
Answer: False
Question 22. If the permeability of a rock increases, its capacity to hold moisture will decrease.
Answer: True
Question 23. Porosity is one of the characteristics of igneous rocks.
Answer: False
Question 24. Only sedimentary rocks contain natural gas.
Answer: True
Question 25. An example of contact metamorphism is a marble.
Answer: True
Question 26. Peat transforms into a diamond.
Answer: False
Question 27. Water mixed with acid does not react chemically with marble.
Answer: False
WBBSE Chapter 3 Topic A Characteristics And Classification Of Rocks Match The Columns
1.
Answer: 1. B, 2. A, 3. E, 4. C, 5. F, 6. D
WBBSE Answer In One Or Two Words
Question 1. How many types of rocks are there?
Answer: Three.
Question 2. What is the capacity or ability to let liquid or gaseous substances pass through in a rock called?
Answer: Permeability.
Question 3. Name the igneous rock that comes out to the surface of the earth.
Answer: Extrusive igneous rock.
Question 4. Which type of rock mainly forms the continental crust?
Answer: Intrusive igneous rock (granite).
Question 5. In which rock are fossils found?
Answer: Sedimentary rock.
Question 6. Name à sedimentary rock that is made up of organic matter.
Answer: Coal.
Question 7. Which type of rock has been used to build the Red Fort?
Answer: Red sandstone.
Question 8. Which rock metamorphoses to form graphite?
Answer: Peat coal.
Question 9. What is a blackboard made of?
Answer: Slate.
Question 10. Give an example of acidic rock.
Answer: Granite.
Question 11. Which rock has a stratified plane?
Answer: Sedimentary rock.
Question 12. Which rock contains petroleum?
Answer: Sedimentary rock.