Rocks - Questions & Answers

1. An aggregate of minerals, with no definite chemical composition, that forms more or less a definite unit of the earth's crust:
(a) Rock (b) Mineral (c) Crust (d) Metamorphosis
Answer: (a) Rock

2. The magma when forced upwards fills vertical cracks or fissures in existing rocks and it then hardens there to form ________.
(a) Necks (b) Sills (c) Dykes (d) Laccoliths
Answer: (c) Dykes

3. With reference to Intrusive Igneous Rocks which of the following is incorrect?
(a) Batholiths – deep-seated large deposits of igneous rocks like graphite.
(b) Hypobyssal Rocks – small rock deposits.
(c) Neck – plug at the top of a volcano.
(d) Dykes – large deposits of cooled magma on the earth's surface.
Answer: (a) Batholiths – deep-seated large deposits of igneous rocks like graphite.

4. The process that turns loose sediments into rocks:
(a) Compaction (b) Lithification (c) Cementation (d) None of the above
Answer: (b) Lithification

5. The bonding together of compacted sediments by natural bonding materials such as calcium compounds, silica and iron:
(a) Compaction (b) Lithification (c) Cementation (d) None of the above
Answer: (c) Cementation

6. Transported sediments get deposited in layers leading to increased pressure and temperature on the lower layers. Select the correct sequence of processes involved in the lithification of these sediments.
(a) Compaction – Evaporation – Cementation.
(b) Evaporation – Compaction – Cementation.
(c) Evaporation – Cementation – Compaction.
(d) Evaporation – Dehydration – Cementation.
Answer: (b) Evaporation – Compaction – Cementation.

7. Sam was working under a geologist. He lost his balance and dropped the bags containing igneous rocks and sedimentary rocks and the rocks got mixed. How would he separate them?
(a) Igneous rocks are hard.
(b) Igneous rocks are crystalline.
(c) Sedimentary rocks have strata showing layers.
(d) Sedimentary rocks are soft.
Answer: (c) Sedimentary rocks have strata showing layers.

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

9. The rocks like peat, lignite, bituminous and anthracite are examples of:
(a) Igneous rocks (b) Mechanical rocks (c) Metallic rocks (d) Carbonaceous rocks
Answer: (d) Carbonaceous rocks

10. The following pairs are the types of rocks on the basis of agents of their formation. Which of the following is incorrect?
(a) Lacustrine : lake beds
(b) Aeolian : sand particles
(c) Glacial: ocean floor
(d) Riverine : flowing waters of rivers.
Answer: (c) Glacial: ocean floor

11. The alteration of the composition or structure of a rock by heat, pressure or other natural agency:
(a) Hypobyssal (b) Metamorphism (c) Compaction (d) Cementation
Answer: (b) Metamorphism

12. Assertion (A): Igneous rocks are of interest to builders and sculptors.
Reason (R): Igneous rocks are usually compact.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.

13. The transformation of rocks into metamorphic rocks takes place as a result of intense heat and pressure. Given below are the types of metamorphism and the processes involved. Which one is incorrect?
(a) Thermal Metamorphism: high temperature.
(b) Dynamic Metamorphism: pressure at great depths in the crust.
(c) Regional Metamorphism : large scale earth movement.
(d) Contact Metamorphism : high pressure below ocean bed.
Answer: (d) Contact Metamorphism : high pressure below ocean bed.

14. Study the diagram and identify A, B and C.
(a) A: action of agents of weathering B: deposition-sedimentation C: action of heat and pressure
(b) A: deposition-sedimentation B: action of agents of weathering C: action of heat and pressure
(c) A: deposition-sedimentation B: action of heat and pressure C: action of agents of weathering
(d) A: action of heat and pressure B: action of agents of weathering C: deposition-sedimentation
Answer: (a) A: action of agents of weathering B: deposition-sedimentation C: action of heat and pressure

15. The continuous process of transformation of old rocks into new ones:
(a) Metamorphism (b) Rock flow (c) Rock cycle (d) Compaction
Answer: (c) Rock cycle

16. ________ are deep-seated intrusion of igneous rocks.
(a) Sills (b) Dykes (c) Laccoliths (d) Batholiths
Answer: (d) Batholiths

17. Your sister had gone camping in Uttrakhand. She was fascinated with the smooth striated rocks she had collected. What type of rocks had she brought home with her?
(a) Igneous rocks (b) Sedimentary rocks (c) Metamorphic rocks (d) Both (a) and (c)
Answer: (c) Metamorphic rocks

18. Which of the following is NOT an organically formed rock?
(a) Gypsum (b) Peat (c) Chalk (d) Limestone
Answer: (a) Gypsum

19. Limestone is a mono-mineralic rock (i.e. a rock mainly made of a single mineral) of calcite. Glacier ice is actually a mono-mineralic rock. It is composed of which mineral?
(a) Water (b) Carbon (c) Silica (d) Iron
Answer: (a) Water

1. State two points of distinction between rocks and minerals.
Answer: 1. Rocks are aggregates of minerals, while minerals are solid inorganic substances occurring naturally. 2. A rock has no definite chemical composition, whereas minerals have a definite chemical composition.

2. Name three types of rocks.
Answer: The three main types of rocks are Igneous, Sedimentary, and Metamorphic rocks.

3. Give two differences between Extrusive Igneous and Intrusive Igneous rocks.
Answer: 1. Extrusive Igneous rocks are formed by the cooling of molten magma on the earth's surface, while Intrusive Igneous rocks cool slowly within the earth's crust. 2. Extrusive rocks are smooth, fine-grained, and crystalline, whereas Intrusive rocks are coarse-textured with large crystals.

4. Name any two characteristics of Igneous Rocks.
Answer: 1. They are hard and compact. 2. They do not have layers and are mostly devoid of fossils.

5. State two characteristics of Basic Igneous Rocks.
Answer: 1. They have a higher percentage of denser elements and their silica content varies between 40 to 60 percent. 2. They are heavy and dark in colour (e.g., Basalt).

6. Name two important rock forms made by Igneous Rocks.
Answer: Batholiths and Dykes.

7. What are Sills? Give an example.
Answer: Sills are formed when magma flows between layers of rocks horizontally and hardens there. An example is a horizontal intrusive sheet found between rock strata.

8. Which rocks are associated with ores of metals and which rocks are associated with fossil fuels?
Answer: Igneous and metamorphic rocks are generally associated with ores of metals, while Sedimentary rocks are associated with fossil fuels.

9. Mention any two characteristics of Sedimentary Rocks.
Answer: 1. They are the most widespread rocks on the surface of the earth. 2. They are generally soft and formed in distinct strata or layers, often containing fossils.

10. Name the rocks which are most widespread on the earth. Give two examples.
Answer: Sedimentary rocks are the most widespread. Two examples are Sandstone and Limestone.

11. Name the three stages of lithification of Sedimentary rocks.
Answer: The three stages of lithification are Evaporation, Compaction, and Cementation.

12. Name the types of Sedimentary rocks based on agents of formation.
Answer: They are classified as Riverine Rocks, Lacustrine Rocks, Glacial Rocks, Aeolian Rocks, and Marine Rocks.

13. Which agents are responsible for deposition of sediments?
Answer: The various agents responsible for the deposition of sediments include flowing water (rivers/streams), glaciers, wind, and ocean currents.

14. What are known as metamorphic rocks? Give two examples.
Answer: Metamorphic rocks are formed when pre-existing igneous or sedimentary rocks undergo an alteration in composition or structure due to intense heat, pressure, or other natural agencies. Two examples are Marble and Slate.

15. Briefly define Regional Metamorphism.
Answer: Regional Metamorphism is the process where igneous and sedimentary rocks buried deep inside the crust are transformed by the pressure of overlying rocks and intense heat caused by large-scale earth movements, such as mountain building.

1. Plutonic and Volcanic rocks.
Answer: Plutonic rocks (Intrusive Igneous) cool slowly deep within the earth's crust, resulting in coarse-textured rocks with large crystals (e.g., granite). Volcanic rocks (Extrusive Igneous) cool rapidly on the earth's surface, forming smooth and fine-grained rocks (e.g., basalt).

2. Thermal and Dynamic Metamorphism.
Answer: Thermal Metamorphism occurs primarily due to the influence of high temperatures, transforming rocks like clay to slate. Dynamic Metamorphism takes place mainly due to extreme pressure at great depths, causing minerals to arrange in bands known as foliation.

3. Sills and Dykes.
Answer: A Sill is formed when magma flows and solidifies horizontally between the layers of rocks. A Dyke is formed when magma is forced upwards and hardens vertically in cracks or fissures within existing rocks.

4. Calcarious and Carbonacious rocks.
Answer: Calcarious sedimentary rocks are formed from the skeletal remains and shells of sea organisms and contain large proportions of lime (e.g., limestone). Carbonaceous sedimentary rocks are formed from buried plant and vegetative matter over a long period and have a very high percentage of carbon (e.g., coal, peat).

5. Acid Igneous Rocks and Basic Igneous Rocks.
Answer: Acid Igneous Rocks have a high silica content (65 to 85 percent) and generally lack iron and magnesium (e.g., Granite). Basic Igneous Rocks have a lower silica content (40 to 60 percent), contain a higher percentage of denser elements, and are heavy and dark in colour (e.g., Basalt).

1. (a) Distinguish between rocks and minerals.
Answer: Rocks are naturally forming aggregates of one or more minerals that form the solid earth's crust, but they do not have a definite chemical composition. Minerals are naturally occurring inorganic solids with a definite chemical composition and crystalline structure.

(b) Describe how igneous rocks are formed. State their main characteristics.
Answer: Igneous rocks are formed due to the cooling, solidification, and crystallisation of hot molten material (magma) found in the earth's interior. Their main characteristics include: they are hard, compact, granular, and crystalline; they do not have layers; they are devoid of fossils; and they are less affected by chemical weathering because water cannot percolate easily through them.

(c) Give a reason for each of the following:
(i) Igneous rocks are also called the primary rocks.
Answer: They are called primary rocks because they were the first rocks to be formed on the earth and they form the fundamental basis for the formation of all other types of rocks.
(ii) Igneous rocks are used by builders and sculptors.
Answer: They are used by builders and sculptors because they are extremely hard, smooth, very compact, and highly durable.
(iii) Sedimentary rocks are called secondary rocks.
Answer: They are called secondary rocks because they are formed from the broken fragments and weathered materials of pre-existing primary (igneous) or metamorphic parent rocks.

2. (a) How are sedimentary rocks formed?
Answer: Sedimentary rocks are formed when pre-existing exposed rocks are weathered and broken down into smaller fragments. These sediments are transported by agents like water, wind, and glaciers, and then deposited in layers into basins, lakes, or oceans. Over time, these accumulated layers are transformed into solid rock through the lithification processes of evaporation, compaction, and cementation.

(b) Explain the formation of sedimentary rocks on the basis of agents of formation.
Answer: Sedimentary rocks form through various agents: Riverine rocks are formed by alluvial deposits from flowing streams. Lacustrine rocks form on lake beds from successive deposition. Glacial rocks form from debris and moraines left by melting glaciers. Aeolian rocks are created from sand particles transported and deposited by winds. Marine rocks form at the ocean floor from deposited shells, skeletons, and sea plants.

(c) Give a reason for each of the following:
(i) Mechanically formed sedimentary rocks are also termed as stratified rocks.
Answer: They are called stratified rocks because the sediments are continuously deposited one over the other layer by layer over long periods, creating distinct layers or strata.
(ii) Rocks like peat, lignite and anthracite are called carbonaceous rocks.
Answer: They are called carbonaceous rocks because they are organically formed from buried vegetative and plant matter, resulting in a very high percentage of carbon.
(iii) Fossils are present in sedimentary rocks.
Answer: Fossils are found in sedimentary rocks because these rocks are formed by the gradual accumulation of sediments which can quickly bury and preserve the remains of dead plants and animals without subjecting them to the extreme heat that would otherwise destroy them.

(d) Study the picture. Identify the rocks formed in case of A, B and C. Give reasons to support your answer.
Answer: A represents an Extrusive Igneous Rock, as the magma has reached the earth's surface as a lava flow and will cool there.
B represents a Sill (an Intrusive Igneous Rock), as it shows magma that has flowed and hardened horizontally between the rock layers.
C represents a Dyke (an Intrusive Igneous Rock), as it shows magma that has been forced upwards to fill a vertical crack or fissure where it hardens.

3. (a) State the chief characteristics of metamorphic rocks.
Answer: Their chief characteristics are: they are harder and more compact than their original forms; most are impermeable and do not allow water to percolate; they do not contain fossils; and new minerals are often formed during the process of metamorphism.

(b) State the economic significance of rocks.
Answer: Rocks are highly significant economically because soils essential for agriculture are derived from their weathering. Almost all building materials used for housing and infrastructure come from rocks. They are the primary source of precious metals (gold, silver, platinum) and fossil fuels (coal, petroleum). Additionally, rock wastes are heavily utilized in manufacturing various industrial items.

(c) What is Rock Cycle? How does it keep the earth young?
Answer: The Rock Cycle is the continuous, endless process of transformation of old rocks into new ones. It keeps the earth "young" because rocks are continuously broken down, eroded, transported, deeply buried, melted, and renewed. These movements and cyclical transformations maintain a dynamic balance among the various elements of the earth's crust.

(d) Draw a well labelled diagram of Rock Cycle.
Answer: [Note for student: Please draw the standard Rock Cycle diagram as illustrated in Figure 6.12 of the chapter, showing the cyclic flows between Igneous, Sedimentary, and Metamorphic rocks through processes like melting, metamorphism, and lithification.]

1. The 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.
Answer: Yes, rocks remain fundamentally important to human survival and civilization today. They weather into nutrient-rich soil required for all agriculture. We heavily rely on rocks like limestone and granite for construction and infrastructure. Furthermore, modern industries and power generation are utterly dependent on fossil fuels (coal) and metallic ores extracted directly from different types of rock formations.

2. Do you think the minerals in a rock provide an important clue about how the rock was formed? Give reasons to support your answer.
Answer: Yes, minerals provide vital clues regarding a rock's origin. For instance, the presence of exclusively silicate minerals in large, coarse crystals indicates that the rock cooled very slowly from magma deep underground (intrusive igneous). Finding minerals arranged in distinct bands (foliation) reveals that the rock was subjected to extreme dynamic pressure (metamorphic), whereas an abundance of carbon-based minerals indicates the rock was formed from ancient, preserved organic plant matter (sedimentary).

3. The rock cycle has no beginning and no end. Justify this statement with examples.
Answer: The rock cycle is continuous and cyclical. For example, magma cools to form primary igneous rocks. Weathering breaks these igneous rocks into sediments, which lithify into sedimentary rocks. Extreme heat and pressure can then alter these sedimentary rocks into metamorphic rocks. Finally, intense heat deep within the earth can melt metamorphic rocks entirely back into magma, restarting the entire process. Any rock type can transform into any other, ensuring the loop never truly ends.

1. Rock collection is a great hobby. Collect rock samples from different places and name them.
Answer: [Practical activity for the student to perform independently.]

2. Go around your area and survey the rocks. Write down your observations.
Answer: [Practical activity for the student to perform independently.]