Interior of the Earth – Class 11 Geography notes, important questions and MCQs

Chapter overview

The interior of the Earth cannot be observed directly, so scientists use indirect sources such as seismic waves, volcanic eruptions, gravity and magnetic surveys and meteorites, along with direct sources such as mines and deep drilling. Earthquake waves show that the Earth has three main layers: crust, mantle and core. Volcanoes and intrusive and extrusive igneous landforms are linked to the movement of molten material from the interior.

Sources of information and earthquakes

• Direct sources include rocks from mines and deep drilling projects, and lava from volcanic eruptions.
• Indirect sources include changes in temperature, pressure and density with depth, meteorites, gravity anomalies, magnetic surveys and, most importantly, seismic waves.
• Earthquakes release energy at the focus (hypocentre); the point on the surface above it is the epicentre. Body waves are P-waves (travel through solids, liquids and gases) and S-waves (travel only through solids); surface waves cause the most damage. The absence of S-waves beyond about 105° creates a shadow zone, indicating a liquid outer core.

Structure of the Earth

• The crust is the outermost layer, about 5 km thick under oceans and about 30 km (up to 70 km under mountains) on continents.
• The mantle extends to a depth of about 2,900 km; its upper part, the asthenosphere, is partly molten and is the main source of magma.
• The core is made mainly of nickel and iron (nife); the outer core is liquid and the inner core is solid.

Volcanoes and volcanic landforms

• Types of volcanoes include shield volcanoes (such as those of Hawaii), composite volcanoes, calderas, flood basalt provinces (such as the Deccan Traps) and mid-ocean ridge volcanoes.
• Extrusive landforms form from lava on the surface; intrusive landforms form when magma cools within the crust.
• Intrusive forms include batholiths, laccoliths, lapoliths, phacoliths, sills and dykes.

Key terms

Focus

the point inside the Earth where an earthquake originates

Epicentre

the point on the surface directly above the focus

Shadow zone

an area where seismic waves are not recorded

Asthenosphere

the partly molten upper part of the mantle

Batholith

a large dome-shaped intrusive body of granite

Important questions with answers

Quick revision

• Direct and indirect sources of information.
• Focus, epicentre; P, S and surface waves; shadow zone.
• Crust, mantle (asthenosphere), core (nife).
• Volcano types; extrusive and intrusive landforms.

Exam tips

• Draw a diagram of the Earth's layers with depths.
• Compare P-waves and S-waves in a table.

Practice MCQs

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

   1. hypocentre
   2. shadow zone
   3. epicentre
   4. core
   Show answer

   Answer: (c) epicentre. Damage is often greatest here.

2. The outermost layer of the Earth is the

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

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

3. The core of the Earth is made mainly of

   1. silica and aluminium
   2. nickel and iron
   3. carbon and oxygen
   4. silica and magnesium
   Show answer

   Answer: (b) nickel and iron. It is called nife.

4. Seismic waves that cannot travel through liquids are

   1. P-waves
   2. surface waves
   3. S-waves
   4. Love waves only
   Show answer

   Answer: (c) S-waves. This shows the outer core is liquid.

5. The partly molten layer in the upper mantle is the

   1. asthenosphere
   2. lithosphere
   3. inner core
   4. crust
   Show answer

   Answer: (a) asthenosphere. It is the source of magma.

6. The Deccan Traps are an example of

   1. shield volcano
   2. flood basalt province
   3. caldera
   4. mid-ocean ridge
   Show answer

   Answer: (b) flood basalt province. They were formed by fissure eruptions of lava.

7. The shadow zone of S-waves indicates that

   1. the inner core is liquid
   2. the mantle is gaseous
   3. the crust is liquid
   4. the outer core is liquid
   Show answer

   Answer: (d) the outer core is liquid. S-waves cannot pass through liquids.

8. A vertical wall-like intrusive body that cuts across rock layers is a

   1. sill
   2. laccolith
   3. dyke
   4. batholith
   Show answer

   Answer: (c) dyke. Sills are horizontal.