Geography and Environment

By the end of this unit, you must be able to describe the geographical structure of the earth.

Unit objectives
By the end of this unit, you must be able to:
(a) explain the external components of the earth
(b) describe the internal structure of the earth.

The components of the earth

Do this in pairs. Study the picture below and answer the questions that follow

1. Mention the natural features that you can see in the picture.
2. Discuss the importance of the things that you have listed.

3. What will happen to this neighborhood if the trees, grass and vegetation were all removed?
4. Suggest ways in which the people who live in this neighborhood can protect and conserve the environment.

Figure 4.1 shows us that the earth is composed of solid materials. They include soil, rocks, vegetation and the air that surrounds us.
It is important to note that both living and non-living things constitute the
environment. They form part of what makes up the external structure of the earth. They are the things that we can see and touch.
The structure of the earth is made up of two components, namely the external and internal structures.

The external structure is made up of:
• the atmosphere
• the hydrosphere
• the lithosphere
• the biosphere.
The internal structure is made of:
• the crust
• the mantle
• the core.

External structure of the earth

(a) The atmosphere

The atmosphere is a thin layer of gases that surrounds the earth. It is held onto it by the force of gravity. It is a mixture of gases like nitrogen, oxygen, water vapour and carbon (IV) oxide. The earth’s atmosphere extends out to 10,000 kilometres. The atmosphere is important since it determines the climate.
The atmosphere has to be protected since it influences human activities and wellbeing

(b) Hydrosphere
The hydrosphere is made up of water. It is all the water on the surface of the earth. 71% of the earth’s surface is covered by water. The oceans hold 96.5% of the earth’s water. The rest of the hydrosphere includes the fresh water found in the streams, rivers, lakes, glaciers, ice sheets, underground water and water vapour. Water moves from the oceans and seas to the atmosphere, then to the land and into the sea in a continuous process called the hydrological cycle. Water
bodies on earth play a role in maintaining our environment. Human activities such as agriculture and fishing depend on water found on the earth’s surface.

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         Fig 4.2 Fishing on Lake Kivu in Rwanda

(c) Biosphere
This is the biological component of earth’s systems. It includes all living organisms on earth, together with the dead organic matter that they produce. Living things are divided into plants and animals. The plants are referred to as flora and the animals as fauna.

(d) Lithosphere
This is the solid, rocky crust that covers the entire planet. This crust is composed of minerals. It is inorganic. It is this part that supports plant life. It includes land or soils and rocks.

   Fig 4.3 A summary of the external structure of the Earth

Do this in groups of three. Your teacher will take you to two different locations in the neighborhood of your school. One location has fertile soil, trees and vegetation. The second location has rocks and bare soil.
1. Observe the external components of the earth that can be observed in both locations.

2. Record your observations of the two locations separately.
3. Present your findings on the description of the two locations to the class and discuss.
4. In a question and answer session, answer the following.
• State the factors that make the two locations different in their appearance.
• Explain what can be done to conserve the environment in the second location.
• Explain the importance of conserving the environment.
• Explain how plants and animals benefit from the first location.

Task 4.1
1. Explain why it is important to conserve our environment.
2. Describe the composition of the external structure of the earth.
3. Explain how the earth’s external structure is important to humans.

Do this in pairs.
1. Discuss the importance of each of the components of the earth’s external structure.
2. Explain the importance of each component as observed in your home area.
3. Suggest ways in which each of the external components of the earth can be protected

4. Show how plants and animals benefit from the hydrosphere, atmosphere and lithosphere.
5. Present your findings in class for discussion.

Internal structure of the earth

Do this in groups of three. You will need an avocado fruit and a knife.
1. Cut the avocado into two using a knife.
2. Carefully study the parts of the cut avocado.
3. (a) How many layers can you see?
  (b) Name them
4. Write down your findings.
5. Discuss them in a class presentation.

Earth’s interior is made up of three parts:
 the crust
 the mantle
 the core

     Fig 4.4 The internal structure of the earth.

(a) The crust
This is the outermost layer of the earth’s structure. It measures about 30 kilometres at the continents and 5 kilometres under the oceans. 1% of the earth’s mass is made up of its crust. The crust rests on the mantle. It is divided into two sections, the sial and sima. These are the continental and oceanic crusts.

The sial (continental crust)

This zone makes the outermost layer of the crust. It forms the continents and is composed of granitic rocks. It is dry and has a thickness of between 35 and 40 kilometres. This layer has huge
quantities of silica and aluminium minerals hence the name sial. The density of this layer averages about 2700 – 2800 kg/m3. The rocks found here are bright in colour. The sial is solid and firm.
It holds the biosphere. 

The sima (oceanic crust) This is the inner layer of the crust which forms the oceanic beds. It has an average thickness of between 7 to 10 kilometres. Sima derives its name from the component minerals silica and magnesium. Sima occurs beneath the sial. It is more dense than the sial with a density of about 2800 to 3300 kg/m3. The rocks are dark in colour.

The upper part of the sima consists of sediments deposited by rivers and sea waves. It also has volcanic lava derived from the eruption of volcanoes on the ocean beds. The lower layer is made up of basalt rocks. The rocks result from the spreading of lava on ocean beds. This layer is thicker
than the upper layer. The crust is separated from the mantle by a transitional zone called the Mohorovicic discontinuity. It is also referred to as the Moho discontinuity.This is a zone of sharpchange in rock density between the crust and the mantle. It is between 5 and 10 kilometres with an average of 8 kilometres beneath the ocean floor. It is about 20 to 90 kilometres with an average of 32 kilometres beneath the continents. 

(b) The mantle

This is the layer that is found below the crust. It is the largest part of the earth’s interior. It makes up about 84% of the earth’s volume.
The mantle is a silicate rocky shell with an average thickness of 2,886 kilometres. It extends inwards to a depth of about 2,900 kilometres. In the mantle, temperatures range between 500 and 900 °C at the upper boundary with the crust to over 4,000°C at its boundary with the core.
It is divided into two parts, namely the upper mantle and the lower mantle. The mantle is separated from the core by a zone of discontinuity. This zone is referred to as the Gutenberg discontinuity. It occurs at a depth of 2900 kilometres beneath the surface.

(c) The core
This the innermost part of the earth’s interior. It is composed of iron and nickel. It extends from below the mantle to the earth’s centre, a depth of about 6,400 kilometres.
It is about 7,000 kilometres in diameter and 3,500 kilometres in radius. Temperatures in the core are about 6,000°C. The core is divided into two parts. They are the inner and the outer core.

         Fig 4.5 An illustration of the earth’s structure

Do this in groups of three. Using images from the Internet:
1. Identify the parts that make up the internal structure of the earth.
2. Describe their characteristics.
3. Draw well-labelled diagrams to aid your explanation.

Did you know?
• The average density of the earth as whole is about 5.51gms/cc.
• The total mass of the earth is about 5.9736 x 1024 kgs.
• The earth is mostly iron, oxygen and silicon.
• The earth’s molten iron core creates a magnetic field.
• The outer crust of the earth is broken up into regions known as tectonic plates.

1. (a) Name the components of the external structure of the earth.
(b) Describe the components of the external structure of the earth.
2. With the aid of a well-labelled diagram, name different parts of the earth’s structure.
3. Explain why the inner core of the earth is in a solid state.
4. Name the main minerals that form the following.
(a) Sial
(b) Sima.
5. Distinguish between the Mohorovicic and the Gutenberg discontinuities.