DAV Board Class 7 Social Science (We and Our World) | 100% Authentic Textbook Solutions

DAV Class 7 SST Chapter 3 The Surface and Interior of the Earth Solutions

Get complete, 100% authentic textbook solutions for DAV Class 7 Social Science (We and Our World – Geography) Chapter 3: The Surface and Interior of the Earth. This comprehensive study guide features detailed chapter summaries and core concepts (Earth’s layers, SIAL, SIMA, Mantle, Core/NIFE, Seismic Waves, and Rock Cycle), key scientific definitions, complete textbook exercise solutions (Section A: MCQs, Section B: Fill in the Blanks, Section C: Single Term Identification, Section D: Short Answer Questions, Section E: Detailed Explanations), Value Based Questions (VBQ), Higher Order Thinking Skills (HOTS), and practical project activities strictly adhering to the DAV Education Board curriculum.

1. Detailed Chapter Summary and Core Concepts

A. Earth’s Surface and Origin

• Formation: The planet Earth was formed approximately 4.6 billion years ago. It is the only known planet in the Solar System possessing conditions uniquely favorable for sustaining life.
• Varied Landforms: The surface of Earth is characterized by immense geographic diversity, comprising high mountain ranges, vast plateaus, fertile plains, and deep valleys.
• Datum of Elevation: All elevations on land and depths in oceans are measured from a standard baseline called mean sea level. The highest known elevation on Earth’s surface is Mount Everest (8,850 meters above mean sea level) in the Himalayas, while the deepest known depression is the Marina Trench (11,034 meters below mean sea level) in the Pacific Ocean.

B. Exploring Earth’s Interior and Seismic Waves

• Limitations of Direct Exploration: Because internal temperatures rise rapidly with increasing depth (approx. 1 degree Celsius for every 32 meters), direct human entry or deep drilling is impossible. The deepest commercial mines (deep gold mines in South Africa) reach only about 3.5 kilometers.
• Indirect Evidence & Seismology: Scientists gather knowledge about the interior through indirect evidence, particularly by analyzing volcanic ejecta and the propagation of seismic waves produced during earthquakes. Seismology is the specialized branch of science devoted to studying earthquakes and seismic waves.
• Types of Seismic Body Waves:
  • P-Waves (Primary Waves): Longitudinal compressional waves that travel at the fastest speed and can pass through both solid and liquid mediums.
  • S-Waves (Secondary Waves): Transverse shear waves that travel at a slower speed and can only pass through solid materials, being absorbed by liquid zones.

C. Concentric Layers of the Earth

The interior of the Earth is organized into three distinct concentric spherical layers:

D. Classification of Rocks and the Rock Cycle

A rock is an aggregate of one or more minerals forming part of Earth’s crust. Rocks are classified into three major groups based on their origin and mode of formation:

• 1. Igneous Rocks (Primary Rocks): Formed through the cooling and solidification of molten rock material (magma or lava).
  • Intrusive Igneous Rocks: Magma cools slowly deep beneath the crust, forming coarse, large-grained crystals (e.g., Granite).
  • Extrusive Igneous Rocks: Lava erupts onto Earth’s surface and cools rapidly in contact with air, forming fine-grained glassy rocks (e.g., Basalt).
• 2. Sedimentary Rocks (Stratified Rocks): Formed when weathered rock particles (sediments) transported by rivers, glaciers, and wind are deposited in horizontal layers on lake or ocean beds. Over millions of years, pressure compacts and cements them into hard rocks. They contain fossils and fossil fuels (coal, petroleum). Examples include Sandstone, Limestone, Shale, Clay, and Chalk.
• 3. Metamorphic Rocks: Formed when existing igneous or sedimentary rocks undergo radical recrystallization in texture and mineral structure under immense heat and pressure deep within Earth without melting. Examples: Granite changes to Gneiss, Limestone transforms into Marble, Shale turns into Slate, and Sandstone becomes Quartzite.
• The Rock Cycle: A continuous, cyclic geological process whereby rocks of one type transform into another over millions of years through melting, cooling, weathering, sedimentation, and metamorphism.

2. Key Terms and Definitions

3. Complete Textbook Exercise Solutions

Section A: Tick (✓) the Correct Option

1. Living organisms do not survive inside the earth due to:
(a) high temperature [✓]
(b) cool temperature
(c) presence of gases
(d) excessive water
Answer: (a) high temperature

2. Earthquakes and volcanoes are caused by:
(a) internal disturbance [✓]
(b) external disturbance
(c) over-population
(d) gradation
Answer: (a) internal disturbance

3. Respective layers of the earth from top to bottom are:
(a) crust, core, mantle
(b) crust, mantle, core [✓]
(c) core, crust, mantle
(d) core, mantle, crust
Answer: (b) crust, mantle, core

4. The full form of SIMA is:
(a) silica and mica
(b) silica and magnesium [✓]
(c) silica and magma
(d) silica and magnetite
Answer: (b) silica and magnesium

5. The original rock from which Gneiss formed is:
(a) limestone
(b) shale
(c) granite [✓]
(d) quartzite
Answer: (c) granite

Section B: Fill in the Blanks

• 1. Earth was formed about 4,600 million years ago.
• 2. The behaviour of seismic waves tells us about the earth’s interior.
• 3. The thickness of crust varies from place to place.
• 4. Rocks are classified into three groups on the basis of their mode of formation / origin.
• 5. The core of the earth is rich in iron and nickel.

Section C: Give a Single Term for Each Statement

1. It is the hardest substance.
Answer: Diamond

2. The molten rock material present inside the earth.
Answer: Magma

3. Rocks which are formed due to cooling and solidification of magma inside the earth.
Answer: Intrusive Igneous Rocks

4. Shaking and trembling of the earth’s surface.
Answer: Earthquake

5. A naturally occurring solid substance made up of a mixture of minerals.
Answer: Rock

Section D: Answer the Following Questions in Brief

Question 1. What is the special feature of sedimentary rocks?
Answer: The special feature of sedimentary rocks is that they are stratified (layered) in structure and uniquely contain fossils – the preserved organic remains of plants and animals embedded between layers. Over millions of years of burial and thermal compaction, these organic deposits transform into invaluable fossil fuels such as coal and petroleum.

Question 2. How are metamorphic rocks formed?
Answer: Metamorphic rocks are formed when pre-existing igneous or sedimentary rocks undergo profound transformation in their physical texture, mineral alignment, and hardness due to intense heat, chemical fluids, and tectonic pressure deep beneath the Earth’s crust, without completely melting.

Question 3. What is lava?
Answer: Lava is red-hot, molten rock material (magma) that forces its way out through vents and fissures in the Earth’s crust and flows freely over the land surface during volcanic eruptions.

Question 4. Why are igneous rocks called the primary rocks?
Answer: Igneous rocks are called primary rocks because they were the very first rocks to form on Earth upon the cooling and solidification of molten magma and lava. All other rock categories (sedimentary and metamorphic) are derived directly or indirectly from the breakdown and alteration of igneous rocks.

Question 5. What are sediments?
Answer: Sediments are broken rock fragments, mineral grains, gravel, silt, and sand produced by the weathering, erosion, and disintegration of surface rocks, which are subsequently transported and deposited by natural agents such as running water, moving glaciers, and wind.

Section E: Answer the Following Questions in Detail

Question 1. Differentiate between the P-Wave and the S-Wave.
Answer:

Question 2. Write two characteristics of each of the following: (a) Crust (b) Mantle (c) Core
Answer:

• (a) Crust:
  1. It is the outermost, solid, and thinnest layer of Earth, with an average thickness ranging from 60 to 100 kilometers.
  2. It is subdivided into continental crust rich in silica and aluminium (SIAL) and oceanic floor crust rich in silica and magnesium (SIMA).
• (b) Mantle:
  1. It is the intermediate shell positioned beneath the crust, extending down to a depth of approximately 2,900 kilometers.
  2. It consists of heavy, dense silicate rocks rich in iron and magnesium, harboring convective currents in its upper semi-molten layer.
• (c) Core:
  1. It is the innermost sphere centered around Earth’s axis, composed primarily of heavy metallic elements: Nickel and Iron (hence called NIFE).
  2. It is subjected to extreme temperatures (up to 5,000 degrees Celsius) and crushing gravitational pressure; its outer zone is liquid, while its inner core is compressed into a solid ball.

Question 3. What are fossil fuels? Name any two fossil fuels and specify their importance.
Answer:

• Definition: Fossil fuels are naturally occurring, combustible carbon-rich energy resources formed from the organic remains of prehistoric flora and fauna that were buried beneath sediments and subjected to geological heat and pressure over millions of years.
• Two Examples: Coal and Petroleum (Mineral Oil).
• Importance:
  1. Industrial & Thermal Power Generation: Coal serves as the foundational fuel for operating thermal power stations that generate global electricity, as well as a vital reducing agent in iron and steel smelting.
  2. Global Transportation & Petrochemicals: Petroleum provides refined liquid fuels (petrol, diesel, aviation turbine fuel) indispensable for modern road, rail, air, and marine transport. Furthermore, petrochemical by-products supply raw materials for synthetic fertilizers, plastics, medicines, and synthetic fibers.

Question 4. Why is the knowledge of the earth’s interior based on indirect observation?
Answer: Our scientific understanding of Earth’s internal constitution relies on indirect observation due to the following insurmountable physical barriers:

• Intense Internal Heat: The temperature within Earth escalates rapidly with depth (approx. 1 degree Celsius per 32 meters). At deep levels, rocks exist in semi-molten and molten states, rendering human penetration impossible.
• Drilling Limitations: The deepest exploratory boreholes reach only about 12 kilometers, and mining is limited to roughly 3.5 kilometers. This represents less than 0.2 percent of the 6,371 km radius distance to Earth’s center.
• Reliance on Scientific Proxy Data: Consequently, geophysicists decipher internal composition indirectly by recording changes in the velocity and refraction trajectories of seismic waves (P and S waves) passing through interior density boundaries, and by chemical analysis of volcanic magma discharged from deep underground.

Question 5. What are minerals? On what basis can the minerals be identified? Give examples.
Answer:

• Definition: A mineral is a naturally occurring, inorganic solid chemical compound or element possessing an orderly crystalline atomic structure and a definite chemical formula.
• Basis of Identification: Minerals are identified and categorized based on characteristic physical and optical properties:
  1. Color: The visual hue exhibited by the mineral.
  2. Hardness: The resistance to scratching, evaluated on Mohs scale (e.g., Talc is softest; Diamond is hardest).
  3. Luster: The appearance and quality of light reflected from the mineral’s surface (e.g., metallic, vitreous, pearly).
  4. Streak & Cleavage: The color of mineral powder on porcelain and the plane along which it naturally fractures.
• Examples:
  • Metallic Minerals: Iron, copper, gold, bauxite, lead, and silver.
  • Non-Metallic Minerals: Diamond, graphite, quartz, gypsum, mica, and common salt (halite).

4. Value Based Questions (VBQ) and HOTS

Value Based Question (VBQ)

Context: A high-magnitude earthquake in Nepal caused tremendous loss of human life and destroyed thousands of residential buildings. Most victims were trapped due to narrow access lanes, improper construction materials, and weak building codes. Despite grave hardships, local neighbors, civil defense teams, and international aid agencies rallied together to extend rescue support.

1. Is nature totally responsible for this misery or are we at a fault somewhere? Explain.
Answer: While nature triggers the tectonic subterranean energy release that initiates the earthquake, human error and negligence are largely responsible for magnifying the devastation into a catastrophe. Unregulated urban expansion, disregard for earthquake-resistant building norms, usage of substandard cement and iron, congestion along narrow unpaved alleys, and absence of civic evacuation drills turn an otherwise survivable natural event into massive loss of life. Hence, human short-sightedness bears substantial culpability.

2. Which human values were seen in action in all those who helped the earthquake victims? Mention any five with examples.
Answer:

• 1. Compassion and Empathy: Residents comforting grief-stricken neighbors and providing emotional strength amid ruins.
• 2. Cooperation and Solidarity: Multiple foreign rescue teams collaborating harmoniously with domestic volunteers to remove heavy concrete slabs.
• 3. Selflessness: Civilians risking their personal safety beneath precarious, crumbling structures to rescue trapped children.
• 4. Generosity: Communities across regions organizing urgent food grain, blanket, clean drinking water, and medicine collection drives.
• 5. Civic Responsibility: Medical personnel and emergency responders working tirelessly without pause to administer trauma care.

HOTS (Higher Order Thinking Skills)

Q1. Why is the inner core of the Earth in a solid state despite temperatures being higher than the melting point of metals?
Answer: Although temperatures in Earth’s core reach approximately 5,000 degrees Celsius (well exceeding the surface melting temperature of iron and nickel), the overwhelming gravitational weight of thousands of kilometers of overlying mantle and crust creates immense crushing pressure. This extreme pressure forces the metallic atomic lattices together so tightly that they cannot expand into a liquid state, constraining the inner core to remain a solid metallic sphere.

Q2. If all igneous rocks were to disappear, how would it impact the rock cycle?
Answer: Igneous rocks are designated as the primary rocks from which all planetary crustal materials originate. In their absence, there would be no parent rock mass available to undergo atmospheric weathering and physical erosion into sediments. Consequently, sedimentary rocks could never form, nor could secondary metamorphic rocks develop. The continuous geochemical rock cycle would grind to an irreversible halt.

5. Something To Do / Project Activities

A. 3D Model Making Activity: Earth’s Interior

• Objective: Construct an accurate cross-sectional spherical model illustrating Earth’s internal concentric layers.
• Materials: Modeling clay (red, orange, yellow, and blue), cardboard base, cutter, toothpicks, and label flags.
• Execution Steps:
  1. Roll a dense red sphere of clay representing the Inner Core.
  2. Cover it with an orange layer of clay representing the Outer Core.
  3. Wrap a thicker layer of yellow clay representing the Mantle.
  4. Enclose the exterior with a thin blue and green clay crust representing the Crust.
  5. Carefully slice out a 90-degree quadrant wedge to expose the internal concentric strata, inserting toothpick flags labeling: Crust (SIAL & SIMA), Mantle, Outer Core, and Inner Core (NIFE).

B. Rock Classification Master Table

Explore All DAV Class 7 Social Science Chapter Solutions: