Social Science (Geography) · CBSE Class 9 · NCERT ‘Understanding Society: India and Beyond’ · Notes, Solutions & 25 Quiz
Chapter 2 — Shaping of the Earth’s Surface (Notes · Geography)
The Earth’s surface is constantly transformed by forces acting from within (internal/endogenic) and on the surface (external/exogenic). The theory of plate tectonics explains how large pieces of crust move over the molten mantle, giving rise to mountains, volcanoes, plains and valleys.
The Big Questions
What shapes the Earth’s surface? What is plate tectonics and its effects? How are landforms formed and classified? How are living beings connected to landforms? How do landform-related disasters affect human life?
Landform: a natural feature on the Earth’s surface formed by processes such as weathering, erosion, deposition and crustal movement — e.g., mountains, valleys, plateaus, plains, deserts and coastal features.
The Earth’s Interior
The Earth has three main layers: crust (outermost, where we live), mantle (thick and hot), and core (innermost, extremely hot and dense). The crust + upper mantle form the rigid lithosphere, which is broken into tectonic plates. Beneath it lies the asthenosphere — semi-molten rock that allows plates to move. The core has a fluid outer core (iron & nickel) and a solid, hot inner core.
Plate Tectonics
The theory of plate tectonics (given by W.J. Morgan) states that the Earth’s outermost layer is broken into several large and small tectonic plates that move slowly (a few cm/year). Types: continental plates (carry continents), oceanic plates (carry ocean floors), and mixed plates (carry both). Major plates: Pacific, Eurasian, African, North American, South American, Indo-Australian and Antarctic. Plate movement is caused by convection currents in the mantle — heat from the core makes molten material rise while cooler material sinks, pushing and pulling the plates.
Plate Boundaries
• Convergent boundary — plates move towards each other. Continent–continent collision forms fold mountains (e.g., the Himalaya); ocean–continent collision makes the oceanic plate sink, causing volcanoes and earthquakes.
• Divergent boundary — plates move apart; magma rises to form new crust and mid-ocean ridges (e.g., the Mid-Atlantic Ridge).
• Transform boundary — plates slide past each other (no crust created/destroyed), mainly causing earthquakes (e.g., the San Andreas Fault, USA).
Most earthquakes and volcanoes occur along plate boundaries, especially around the Pacific Ocean — the Ring of Fire. (In ancient India, Varāhamihira’s Bṛihatsaṁhitā discussed earthquakes — ‘bhūkampa’ — linking them to elemental forces Vāyu, Agni, Indra and Varuṇa.)
Weathering vs Erosion
Weathering = breaking down of rocks in place (NO movement). Types: physical (temperature, frost, wind), chemical (reactions with water/air/acids), biological (plants, animals, microbes — e.g., roots splitting rock).
Erosion = wearing away AND movement of material by water, wind, ice or waves. Types: water, wind, glacial, coastal. Erosion removes fertile topsoil (hurting farmers), washes away land/houses near rivers & coasts, and destabilises land for construction — directly affecting human livelihoods.
Agents of Gradation & Their Landforms
Agents of gradation (running water, glaciers, wind, waves, groundwater) wear down, transport and deposit material, levelling the surface over time.
• Running water (rivers): Upper course — V-shaped valleys, waterfalls, rapids. Middle course — meanders, oxbow lakes, floodplains. Lower course — deltas, levees, alluvial fans. Deltas are highly fertile (rice, jute) but flood-prone; the Sundarbans is a famous delta.
• Waves & currents (coasts): deposition forms beaches and sand bars; erosion forms cliffs, wave-cut platforms, sea caves, arches and stacks.
• Glaciers: erosion forms U-shaped valleys, cirques, aretes, hanging valleys, fjords; deposition forms moraines (lateral, terminal, medial). Glaciers are crucial freshwater sources.
• Wind (deserts): erosion forms yardangs, ventifacts, deflation hollows, desert pavements; deposition forms dunes (barchan, longitudinal, star, parabolic).
• Underground water (in limestone/soluble rock) forms Karst topography: caves, stalactites (hang from ceiling), stalagmites (rise from floor), sinkholes/dolines, and underground rivers.
Landforms and Disasters
• Landslides — slope failure from heavy rain, earthquakes, steep slopes, loose rock, deforestation, mining and unplanned hillside construction.
• Avalanches — sudden slide of unstable snow on steep slopes (heavy snowfall, warming, wind-piled snow, vibrations, skiing).
• GLOFs (Glacial Lake Outburst Floods) — sudden release of water from glacial lakes when ice/moraine dams fail (rapid glacier melt, heavy rain, earthquakes).
• Dust storms — strong winds lift loose dry soil in desert/semi-arid regions (drought, sparse vegetation, deforestation, overgrazing, climate change).
Before we move on: The Earth has crust, mantle and core. Internal forces (earthquakes, volcanoes, folding, faulting) move the crust; external forces (weathering, erosion, deposition) carve smaller landforms. Agents of gradation — running water, waves/tides, glaciers, wind, underground water — continuously reshape the surface. Disasters like landslides, avalanches, GLOFs and dust storms are tied to specific landforms.
Key Terms
Lithosphere = rigid crust + upper mantle; Asthenosphere = semi-molten layer below; Convection currents = circulating mantle material that moves plates; Ring of Fire = earthquake/volcano belt around the Pacific; Contouring/Bunding/Terracing = soil-water conservation techniques; Till = rock/soil debris carried by glaciers; Karst = limestone topography of caves and sinkholes.
Textbook Solutions — Questions & Activities
Full model answers to the chapter’s ‘Questions and activities’, with activity tasks explained.
1What are the sources of energy required to cause movements associated with the internal forces of the Earth?
The main source is heat energy from the Earth’s interior — radioactivity and residual heat in the core and mantle. This heat drives convection currents in the mantle: hot molten material rises, cooler material sinks, and this continuous circulation pushes and pulls the tectonic plates, producing earthquakes, volcanoes, folding and faulting.
2Relate the physiographic divisions you studied earlier with the endogenic (internal) forces responsible for their origin.
Answer links earlier learning: the Himalaya (fold mountains) formed by the convergent collision of the Indo-Australian and Eurasian plates; the Northern Plains formed by deposition in the trough created by that collision; the Peninsular Plateau is an ancient, stable block of old crust; and volcanic activity (e.g., the Deccan Traps) came from magma reaching the surface. Thus endogenic forces (plate collision, folding, faulting, volcanism) created the broad physiographic divisions.
3Why and where do earthquakes occur frequently? Is it possible to predict earthquakes?
Earthquakes occur mainly along plate boundaries, where plates collide, move apart, or slide past each other, releasing built-up stress — especially around the Ring of Fire. In India the Himalayan belt and Kutch (Gujarat) are vulnerable. Earthquakes cannot be precisely predicted (exact time/place/magnitude); scientists can only identify high-risk zones and issue probabilities, so preparedness and earthquake-resistant building are the best defences.
4“Plate movements are responsible for the distribution of earthquakes and volcanoes.” Explain.
Where plates meet, enormous stress builds and is released as earthquakes; at convergent and divergent boundaries magma reaches the surface as volcanoes. This is why earthquakes and volcanoes are concentrated along plate boundaries (e.g., the Ring of Fire around the Pacific) rather than scattered randomly — comparing a plate-boundary map with an earthquake/volcano map shows they match closely.
5Draw and label a diagram of a meander and a delta.
Meander — show a winding river bend with: outer bank (erosion/steep cut-bank), inner bank (deposition/point bar), and an oxbow lake cut off from the loop. Delta — show a river splitting into distributaries at its mouth, depositing sediment to form a triangular/fan-shaped area of islands/bars before it meets the sea. (Label: river, distributary, islands/bars, sea.)
6How are deforestation and erosion associated with each other? Explain.
Tree roots bind soil and leaves break the force of rain. When forests are cut (deforestation), the soil is left bare and loose, so rain and wind wash or blow it away far more easily — accelerating erosion. This removes fertile topsoil, causes landslides on slopes, and silts up rivers. Thus deforestation is a major human cause of increased erosion.
7Develop a plan to protect the land in your local area from erosion.
Answer will vary. A good plan includes: afforestation/tree planting and keeping vegetation cover; contour ploughing, bunding and terracing on slopes; building check dams across small streams to slow water; maintaining proper drainage; avoiding overgrazing and unplanned construction on hillsides; and using cover crops to protect topsoil.
8Which disasters might you experience in your region? Discuss a mitigation plan.
Answer will vary by region — e.g., floods, landslides, earthquakes, cyclones or dust storms. A mitigation plan: identify risk zones, build strong/earthquake-resistant structures, create early-warning systems and evacuation routes, plant protective vegetation, educate the community through drills, and maintain emergency supplies.
9Prepare a model of landforms created by underground water.
Activity — build a Karst/cave model showing: a limestone cave with stalactites hanging from the ceiling, stalagmites rising from the floor, a pillar where the two join, a sinkhole where ground has collapsed, and an underground river. Explain that slightly acidic water slowly dissolves limestone to form these features.
10What precautionary measures will you take if you are staying in an earthquake-prone region?
Live/build in earthquake-resistant structures; fix heavy furniture to walls; keep an emergency kit (water, torch, first-aid); know safe spots (under a sturdy table) and the ‘Drop–Cover–Hold’ drill; identify open evacuation areas; keep exits clear; and participate in community awareness and mock drills.
11-15Activities — map, poster, folk stories, case study, group projects
These are project/activity tasks (answers will vary): (11) map landform-related disasters from the current year; (12) poster of sacred/important landforms in your region with folk stories; (13) document a past disaster and its effects on human activities; (14) translate the landslide poster into your native language; (15) in three groups, present projects on water, wind and glacier landforms covering causes, impact on human life/environment, and mitigation.