NCERT Solutions for Class 9th Social Science Chapter 2 The chapter's recap points, expanded — Before we move on …

Book page 37 Updated on2026-09-08

Q1.

The Earth is made up of layers, namely, the crust, mantle, and core.

Answer

What this recap means, expanded:

Crust — the outermost layer, the one we live on. Fig. 2.1 gives its thickness as 30–40 km under the continents but only 5–7 km under the ocean. That difference is not a detail: thin, heavy oceanic crust is exactly what sinks at a convergent boundary, while thick, light continental crust does not.
Mantle — the mostly solid layer between the crust and the outer core, about 2900 km thick in Fig. 2.1. It is very thick and very hot, and it is where the convection currents run.
Core — the innermost part, in two pieces: an outer core (about 2200 km), a fluid layer mainly of iron and nickel, and an inner core (about 1250 km), a solid, hot, spinning metal ball that is the densest part of the Earth.

Two layers cut across this three-fold division, and they are the ones plate tectonics actually needs:

Lithosphere — the rigid outer shell, about 100 km thick: the crust plus the upper part of the mantle. This is the layer that is broken into plates.
Asthenosphere — about 200 km thick, a hot, mobile layer of partially molten rock lying beneath the lithosphere. Because it can flow, the plates above can move.

Check the figures against the radius in Fig. 2.1:
crust + mantle (2900) + outer core (2200) + inner core (1250)
= 2900 + 2200 + 1250 = 6350 km
Fig. 2.1 marks the distance from surface to centre as 6375 km — the two agree, as they should, since the caption says the values are approximate.

Why the layers differ: they are sorted by density. The heaviest material — iron and nickel — sank to the centre, and the lightest rock floated to the top as the crust. That is why the inner core is the densest part of the Earth and the crust the lightest, and why continental crust rides higher than oceanic crust.

Q2.

Interior forces of the Earth (earthquakes, volcanoes, folding, and faulting) are responsible for the movement of the crust.

Answer

What this recap means, expanded. These four are the visible signs of one hidden cause — the slow movement of the plates driven by convection currents in the mantle.

Interior force	What it does to the crust	Where it happens
Folding	Rock layers are squeezed sideways and buckle into waves instead of breaking	Convergent boundaries where two continental plates collide — the Himalaya
Faulting	Rock cracks and one block slips past or over another; blocks are raised or dropped	Wherever the crust is stretched or sheared — rift valleys, transform boundaries
Earthquakes	Strain stored in locked rock is released in a sudden slip, and the ground shakes	All three kinds of boundary; the San Andreas Fault is the transform example
Volcanoes	Magma reaches the surface and builds new land out of lava and ash	Where an oceanic plate sinks under a continental one, and where plates pull apart

Why these forces are called constructive: all four add relief — they raise land, thicken crust, build cones and open basins. They work against the external agents, which do the opposite. Every landscape you see is a snapshot of that contest.

Note the direction of the logic: the recap line says these forces are responsible for the movement of the crust. It is more exact to say that the movement of the plates and these four phenomena share a single cause — the heat escaping from the Earth's interior, shown in Fig. 2.2 — and that folding, faulting, earthquakes and eruptions are how that movement shows itself at the surface.

Q3.

External forces like weathering and erosion carve smaller landforms over the Earth's surface which affect human life in multiple ways.

Answer

What this recap means, expanded. The internal forces make the big shapes; the external forces do the carving that decides what the land is actually like to live on.

The two are not the same process, and the chapter is careful about the difference:

	Weathering	Erosion
What happens	Rock breaks into smaller pieces	Material is worn away and carried off
Movement?	No — the pieces stay where they are	Yes — movement is the whole point
Types	Physical (heating and cooling, water, wind, ice expansion), chemical (substances in the air and in rain), biological (plants and animals) — Fig. 2.8	Water, wind, glacial, coastal — Fig. 2.9
Result	Broken rock, and the beginning of soil	Valleys, cliffs, caves, deltas — the shaped landscape

The order matters: weathering has to come first. Erosion can only carry away material that has already been loosened. That is why the two are always described together as the pair that wears down mountains, carves valleys, forms plains and creates caves, cliffs and river deltas.

And the effect on human life is direct. Weathering makes the soil that farming depends on. Erosion, unchecked, takes that same soil away: page 21 lists the consequences — lower yields for farmers, land, houses and roads washed away near rivers and coasts, unstable ground for construction and mining, and losses to tourism and fishing when beaches, rivers and fertile lands are destroyed.

Why the same process can help and harm: erosion in one place is deposition in another. The silt stripped from a hillside is the silt that builds the fertile floodplain and delta downstream. Whether it is a loss or a gain depends on where you are standing.

Q4.

The surface of the Earth is carved by agents of gradation like running water, waves and tides, glaciers, wind, and underground water.

Answer

What this recap means, expanded. Agents of gradation are the natural forces that wear down, transport and deposit material, and so level or smooth the surface over time. Gradation means bringing to a common grade — lowering the high ground and filling the hollows.

Agent	What it does (page 22)	Its signature landforms
Running water	Erodes rocks and soils to form valleys and plains	V-shaped valleys, waterfalls, rapids, meanders, oxbow lakes, floodplains, deltas, levees, alluvial fans
Glaciers	Scrape and carry huge amounts of material, carving U-shaped valleys	U-shaped valleys, cirques, aretes, hanging valleys, fjords, moraines
Wind	Shapes deserts by eroding and depositing sand	Yardangs, ventifacts, deflation hollows, desert pavements, dunes
Waves and tides	Erode coastlines to form cliffs, beaches and bays	Sea cliffs, shore platforms, sea caves, arches, stacks, beaches, sand bars
Underground water	Dissolves rocks such as limestone, creating caves and sinkholes	Caves, stalactites, stalagmites, pillars, sinkholes, underground rivers — Karst topography

Why each agent leaves a different shape: each one carries material differently. Ice is rigid and holds everything it picks up, so it grinds a broad U-shaped trough. Water is fluid and cuts hardest along the line of fastest flow, so it saws a narrow V. Wind can lift only fine grains, so it sorts the desert — carrying dust away, piling sand into dunes, and leaving the coarse stones as a desert pavement. Groundwater does not carve at all; it dissolves, so its landforms are hollows and dripstone. Recognise the shape and you can name the agent.

Q5.

Disasters like landslides, avalanches, glacial lake outflows, and sandstorms are associated with specific landforms.

Answer

What this recap means, expanded. A disaster does not strike just anywhere. Each one needs a particular landform to be possible at all — so knowing the landform tells you the hazard in advance.

Disaster	Landform it needs	Main causes given in the chapter
Landslide	Steep slopes with loose or weathered rock	Heavy continuous rainfall (water seeps in, adds weight, reduces friction); earthquakes and eruptions; deforestation, mining, road building and unplanned hillside construction; poor drainage and improper land use
Avalanche	Steep snow-covered mountain slopes	Heavy snowfall in a short time on weakly bonded layers; a sudden rise in temperature that melts snow and reduces friction; strong winds piling snow unevenly; earthquakes and vibrations; skiing, trekking and construction
GLOF (glacial lake outflow)	Glacial valleys holding lakes behind ice or moraine dams	Rapid glacier melting from rising temperatures raising the lake level; heavy rain or snow adding water; earthquakes, avalanches or landslides striking the lake or weakening the dam, which then collapses
Dust storm	Deserts and semi-arid plains with loose, dry soil	Strong winds lifting dry soil; prolonged drought and low rainfall; sparse vegetation from deforestation, overgrazing or poor farming; climate change and extreme weather

The common thread: in every one of the four, something is held in place only by friction — soil on a slope, snow on a slope, water behind a moraine dam, dust on dry ground. Add water, heat, shaking or wind and the friction gives way all at once. That is why these disasters are sudden, and why mitigation almost always means increasing the friction (roots, bunds, barriers) or removing the load (drainage, controlled release) before it fails.

Note the human share: three of the four lists above contain human causes — deforestation, mining, unplanned construction, overgrazing, poor farming. These are hazards we make worse ourselves, which also means they are hazards we can reduce.

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