Question 1

Question 8Assertion (A): Igneous rocks are called primary rocks.Reason (R): They were the first to be formed.Both A and R are true and R is the correct explanation of A.Both A and R are true but R does not explain A.A is true but R is false.A is false but R is true.

Accepted Answer

Both A and R are true and R is the correct explanation of A.Explanation — Since the igneous rocks were the first to be formed, they are called primary rocks. They form the basis of formation of other types of rocks.

Question 2

Question 14Study the diagram and identify A, B and C.A: action of agents of weathering B: deposition-sedimentation C: action of heat and pressureA: deposition-sedimentation B: action of agents of weathering C: action of heat and pressureA: deposition-sedimentation B: action of heat and pressure C: action of agents of weatheringA: action of heat and pressure B: action of agents of weathering C: deposition-sedimentation

Accepted Answer

A: action of agents of weathering B: deposition-sedimentation C: action of heat and pressure

Question 3

Question 17Your sister had gone camping in Uttarakhand. She was fascinated with the smooth striated rocks she had collected. What type of rocks had she brought home with her?Igneous rocksSedimentary rocksMetamorphic rocksBoth (1) and (3)

Accepted Answer

Metamorphic rocks

Question 4

Question 18Which of the following is NOT an organically formed rock?GypsumPeatChalkLimestone

Accepted Answer

Gypsum

Question 5

Question 5Acid Igneous Rocks and Basic Igneous Rocks.

Accepted Answer

Acid Igneous RocksBasic Igneous RocksThese rocks have silica content between 65% to 85 %. They lack in iron and magnesium.These rocks have higher percentage of oxides of denser elements and silica content varies between 40% to 60%.For example- granite.For example- basalt

Question 6

Question 13Name the types of Sedimentary rocks based on agents of formation.

Accepted Answer

The types of Sedimentary rocks based on agents of formation are-Riverine rocksLacustrine rocksGlacial rocksAeolian rocksMarine rocks

Question 7

Question 15What are known as metamorphic rocks? Give two examples.

Accepted Answer

Metamorphic rocks are formed by complete change of texture and composition of pre-existing rocks through volcanism and diastrophism. For example, slate is formed from clay and graphite from coal.

Question 8

Question 16Briefly define Mechanical Metamorphism.

Accepted Answer

Mechanical metamorphism is the mechanical transformation of pre-existing rocks, in which texture of rocks is changed by crushing and rubbing processes associated with heat and compression created by mechanical action.

Question 9

Question 1(d)Study the picture. Identify the rocks formed in case of A, B and C. Give reasons to support your answer.

Accepted Answer

A — Rock A is Extrusive Igneous rock. This is because it is located on the surface of the Earth, where lava has flowed out through fissure and solidified.B — Rock B is a Dyke. This is because magma when forced upwards fills vertical cracks or fissures in existing rocks. It then hardens there to form dykes.C — Rock C is a Sill. This is because magma flows between layers of rocks horizontally and then hardens there. This layer of intrusive rock is called Sill.

Question 10

Question 2(b)Explain the formation of sedimentary rocks on the basis of agents of formation.

Accepted Answer

There are five main types of sedimentary rocks on the basis of agents of formation:Riverine Rocks — These are formed by the sediments deposited by running water or river.Lacustrine Rocks — These are formed by the deposition of silt etc on the bed of lakes.Glacial Rocks — These are formed by the debris carried on and deposited by glaciers.Aeolean Rocks — These are formed by the deposition of sand particles accumulated by wind.Marine Rocks — These are formed by the deposition of sediments at the ocean floor. These are of two types- Calcarious sedimentary rocks and Carbonaceous sedimentary rocks.

Question 11

Question 2(c)Give a reason for each of the following:(i) Mechanically formed sedimentary rocks are also termed as stratified rocks.(ii) Rocks like peat, lignite and anthracite are called carbonaceous rocks.(iii) Fossils are present in sedimentary rocks.

Accepted Answer

(i) Mechanically formed sedimentary rocks are also termed as stratified rocks because such rocks are formed in layers.(ii) Rocks like peat, lignite and anthracite are mainly made out of carbon composition (almost 60-90%). Due to this majority of carbon composition in these rocks, these rocks are known as carbonaceous rocks.(iii) Fossils are found in sedimentary rocks, where gentler pressure and lower temperature allows preservation of past life-forms. Fossils become a part of sedimentary rocks when sediments such as mud, sand, shells and pebbles cover plant and animal organisms and preserve their characteristics through time.

Question 12

Question 3(a)State the chief characteristics of metamorphic rocks.

Accepted Answer

The chief characteristics of metamorphic rocks are-They are harder and more compact than their original form.Most of them are impermeable.They do not have fossils.They are formed due to change in texture and composition of the pre-existing rocks.New minerals are formed during the process of metamorphosis.

Question 13

Question 3(b)State the economic significance of rocks.

Accepted Answer

The economic significance of rocks are-Rocks are of great resource value, some directly and some as constituents of minerals.Soils are derived from weathering of rocks.Almost all types of building materials used for paving roads, floors or building walls of houses or various other structures including bridges come from rocks.Rocks are a source of precious metals like gold, silver, platinum etc.Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles.

Question 14

Question 3(c)What is Rock Cycle? How does it keep the earth young?

Accepted Answer

Rocks undergo cyclic transformation. The continuous process of transformation of old rocks into new ones is known as rock cycle. The cycle may also pass through the process of formation of sedimentary, metamorphic and igneous rocks.To keep the earth young, rocks melt again resulting in the formation of igneous rock. This disintegrated material again forms sedimentary rock, which takes hundreds and thousands of years.

Question 15

Question 3(d)Draw a well labelled diagram of Rock Cycle.

Accepted Answer

Please refer to the full solution above.

Question 16

Question 1The stone tools were quite significant in the growth and development of human civilisation. Are the rocks still important to humans today? Give reasons to support your answer.

Accepted Answer

Yes, rocks are still important to humans today for several reasons:Rocks (such as granite, sandstone, limestone, marble) are still used in construction to build houses, roads, bridges, and other structures.Rocks and minerals are important for industrial manufacturing, such as for making glass, cement, ceramics, and various types of metals.Some rocks like peat, lignite, coal etc., are used as fuel for power generation.Rocks have practical applications in technology, such as for semiconductors and batteries used in electronic devices.Rocks are a source of precious metals like gold, silver, platinum, etc. They are used in making jewellery and art, and some are used in medicine and cosmetics.Soils are derived from weathering of rocks.Apart from minerals and fossil fuels, even rock wastes have now been used in manufacturing various articles.Thus, rocks still play a crucial role in human civilization, both in our daily lives and in various industries.

Question 17

Question 2Do you think the minerals in a rock provide an important clue about how the rock was formed? Give reasons to support your answer.

Accepted Answer

Yes, the minerals in a rock provide an essential clue about how the rock was formed. Different minerals form under different conditions and the combination of minerals in a rock can help identify the type of rock and its formation process. Let us consider some examples:Igneous rocks are formed from the solidification of molten material, and their minerals are usually interlocking crystals that form as the material cools.Sedimentary rocks are formed from the accumulation and cementation of sediments, and their minerals are typically cemented together by minerals such as quartz, calcite, or clay.Metamorphic rocks form from pre-existing rocks that are changed by heat and pressure, resulting in the rearrangement of minerals into new forms.Thus, the presence, absence, or combination of minerals in a rock provides important clues about the rock's origin, formation process, and the geological history of the area where it was found.

Question 18

Question 3'The rock cycle has no beginning and no end.' Justify this statement with examples.

Accepted Answer

The continuous process of transformation of old rocks into new ones is known as rock cycle. The cycle may also pass through the process of formation of sedimentary, metamorphic and igneous rocks.The igneous rocks are the primary source of all other rocks. They are formed on the outer surface of the earth due to volcanic activities. Rocks are disintegrated and eroded by climatic factors. The disintegrated material is removed and transported to lower levels by the forces of running water, winds, glaciers and ocean currents. In the basins, lakes or ocean beds the sediments accumulate.The accumulations undergo lithification through the processes of evaporation, compaction and cementation.Once the loose sediment is transformed into sedimentary rock, it is carried in depressions or on the sea floor further down under the earth. There may be further disintegration and recrystallisation into igneous rock and then it may be transformed into metamorphic rock.In case the rock melts it may again result in formation of igneous rock. The disintegrated material may again form sedimentary rock. In completing any of these changes hundreds and thousands of years are involved.In the operation of the rock cycle, the earth is kept young and movements of adjustments specially through horizontal flow or convection currents maintain the balance between various elements of the earth like oceans and continents.

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