NCERT Solutions for Class 9th Social Science Chapter 2 Chapter opening — The Big Questions

Two sets of forces shape it, and they work against each other all the time.

1. Internal (endogenic) forces — these come from the heat inside the Earth. That heat sets up convection currents in the mantle, which move the tectonic plates. Plate movement produces earthquakes, volcanic eruptions, folding and faulting, and these build relief: mountains, valleys, ocean basins and plateaus.

2. External (exogenic) forces — these work on the surface: weathering, erosion and deposition, carried out by running water, glaciers, wind, waves and underground water. These wear down what the internal forces have raised, and fill up the hollows. The chapter calls them the agents of gradation, because they grade — level — the surface.

The surface you see at any moment is the balance of the two. The Himalaya is still being pushed up by the collision of plates while rivers, glaciers and frost are cutting it down at the same time. If the internal forces stopped, the external agents would eventually grind the continents flat.

Plate tectonics is the theory in earth science — given by W.J. Morgan — which explains the movement of the Earth's crust. It says that the outermost layer of the Earth is not one single piece but is broken into several large and small slabs called tectonic plates, which move slowly over the semi-molten layer beneath them.

What the theory rests on:

• The crust plus the upper mantle form the rigid lithosphere (about 100 km thick, Fig. 2.1). Beneath it is the asthenosphere (about 200 km), a hot, mobile layer of partially molten rock. A rigid layer floating on a layer that can flow is exactly what makes movement possible.
• Plates are of three kinds — continental plates (carrying continents), oceanic plates (carrying ocean floors) and mixed plates (carrying both). The major ones named on page 15 are the Pacific, Eurasian, African, North American, South American, Indo-Australian and Antarctic plates.
• They move at only a few centimetres per year — about the speed at which your fingernails grow — but over millions of years that adds up to thousands of kilometres.

What makes them move: convection currents in the mantle. Heat from within causes molten material to rise; the cooler material near the top sinks; this endless circulation pushes and pulls the plates in different directions.

The effects of plate movement depend on which way two plates move at the boundary between them:

Beyond these, plate movement also explains the distribution of continents and oceans, and the fact that most earthquakes and volcanoes are concentrated along plate boundaries — especially around the Pacific Ocean, the Ring of Fire.

A landform is a natural feature on the Earth's surface formed by weathering, erosion, deposition and the movement of the Earth's crust (the definition in the margin on page 13).

How they are formed — in two stages.

1. Internal forces raise the raw material. Plate movement folds, faults, uplifts and erupts. This gives the big shapes: mountain ranges, plateaus, ocean basins, volcanic cones.
2. External agents carve the detail. Weathering loosens rock; erosion carries the loose material away; deposition drops it somewhere else. Over long periods this cuts valleys, opens caves, builds deltas and beaches.

How they are classified. The chapter classifies them in two useful ways.

(a) By the agent of gradation that made them — this is the order the chapter itself follows:

(b) By the force behind them — landforms of internal origin (fold mountains, rift valleys, volcanic cones, ocean basins) and landforms of external origin (all the ones in the table above). The first group is built up; the second group is carved out or laid down.

Running water shows the whole story in one river, because the same river does different work along its course:

Closely — landforms decide where people can farm, settle, travel and earn a living, and they decide which plants and animals can survive there.

Landform by landform, as the chapter sets it out:

The connection also runs through history. Page 22 makes the point: the fertile plains of the Ganga, Nile, Brahmaputra and Indus gave rise to agricultural societies and early cities. Mountains acted both as barriers and as protectors — the Himalayas shielded India from invasions but still allowed cultural exchange through passes such as the Khyber Pass. Deserts such as the Thar limited large settlements but encouraged trade routes such as the Silk Route. Coasts and harbours supported trade and travel, helping kingdoms in south India flourish.

And it runs the other way too. When a landform is damaged, livelihoods go with it. Page 21 lists it plainly: erosion strips the fertile topsoil farmers need, so yields fall; it washes away land, houses and roads near rivers and coasts; it destabilises land for construction and mining; and it hurts tourism and fishing when beaches, rivers and fertile lands are destroyed.

Every landform carries its own hazard, and the damage follows the shape of the land.

The chapter's own case. The flood that struck Chamoli district in Uttarakhand in February 2021 shows the full pattern: many people and livestock lost their lives; there was severe damage to buildings, roads, bridges and hydel projects; and connectivity to villages was badly affected. Notice that the losses are of three kinds — lives, property and connectivity — and the third one is what makes rescue in mountains so difficult.