Shaping of the Earth's Surface – Class 9 Social Science notes, important questions and MCQs

Chapter overview

The Earth's surface is constantly changing. Internal (endogenic) forces such as plate movements, earthquakes and volcanoes build landforms, while external (exogenic) forces such as rivers, wind, glaciers and waves wear them down and deposit material elsewhere. Together these processes create mountains, plateaus, plains, valleys and coasts.

Structure and plates

• The Earth has three main layers: crust, mantle and core.
• The crust and upper mantle form the lithosphere, which is broken into large plates that move slowly.
• Plates may move apart, collide or slide past each other; the Himalayas formed when the Indian plate collided with the Eurasian plate.

Internal forces

• Earthquakes are sudden shaking of the ground caused by movement along faults; they are measured on the Richter or moment magnitude scale.
• Volcanoes form where molten rock (magma) reaches the surface as lava.
• Folding and faulting of rocks create fold mountains, rift valleys and block mountains.

External forces

• Weathering breaks rocks in place; erosion carries the material away; deposition lays it down.
• Rivers make V-shaped valleys, waterfalls, meanders, ox-bow lakes, flood plains and deltas.
• Glaciers make U-shaped valleys and moraines; wind makes mushroom rocks and sand dunes; sea waves make cliffs, sea caves, arches and beaches.

Key terms

Lithosphere

the rigid outer layer of the Earth made of the crust and upper mantle

Tectonic plate

a large slab of the lithosphere that moves slowly over the mantle

Erosion

the wearing away and removal of rock and soil by water, wind, ice or waves

Delta

a triangular landform made by river deposits at its mouth

Epicentre

the point on the Earth's surface directly above the focus of an earthquake

Important questions with answers

Quick revision

• Layers: crust, mantle, core.
• Plates move: apart, together, sideways.
• Earthquakes, volcanoes, folding = internal forces.
• Rivers, glaciers, wind, waves = external forces.

Exam tips

• Make two columns: internal forces build, external forces level.
• For each agent (river, glacier, wind, waves) list one erosional and one depositional landform.

Practice MCQs

1. The outermost layer of the Earth is the

   1. core
   2. mantle
   3. outer core
   4. crust
   Show answer

   Answer: (d) crust. It is thinnest under the oceans.

2. Molten rock that comes out on the Earth's surface is called

   1. magma
   2. sediment
   3. granite
   4. lava
   Show answer

   Answer: (d) lava. Inside the Earth it is called magma.

3. A V-shaped valley is mainly made by

   1. a river
   2. a glacier
   3. wind
   4. sea waves
   Show answer

   Answer: (a) a river. Rivers cut down in their upper course.

4. The Himalayas were formed by the collision of the Indian plate with the

   1. Eurasian plate
   2. African plate
   3. Pacific plate
   4. Antarctic plate
   Show answer

   Answer: (a) Eurasian plate. They are young fold mountains.

5. The point on the surface directly above the focus of an earthquake is the

   1. fault
   2. crater
   3. delta
   4. epicentre
   Show answer

   Answer: (d) epicentre. Damage is usually greatest there.

6. A U-shaped valley is formed by

   1. wind
   2. a river in its lower course
   3. a glacier
   4. sea waves
   Show answer

   Answer: (c) a glacier. Glaciers erode the sides and floor.

7. A lake formed when a river meander is cut off is called

   1. a crater lake
   2. a glacial lake
   3. an ox-bow lake
   4. a lagoon
   Show answer

   Answer: (c) an ox-bow lake. It forms in the river's middle and lower course.

8. Why are the Himalayas still rising slowly

   1. the Indian plate is still pushing against the Eurasian plate
   2. rivers deposit sand on them
   3. wind adds dust
   4. glaciers push them up
   Show answer

   Answer: (a) the Indian plate is still pushing against the Eurasian plate. Plate collision continues today.