The Earth’s Structure – SS1 Geography Lesson Note (First Term)

The structure of the earth is grouped into two zones: the outer structure and the internal structure. The outer structure has four layers: the lithosphere (solid), hydrosphere (liquid), atmosphere (gaseous) and biosphere (where living things are found). The internal structure has three concentric layers: the crust, the mantle and the core.

Behavioural objectives

By the end of the lesson, students should be able to:

• name the four layers of the outer structure of the earth and describe each one;
• state the importance of the lithosphere, hydrosphere, atmosphere and biosphere;
• list the gases that make up the atmosphere and their percentages;
• name the three layers of the internal structure of the earth;
• describe the crust, mantle and core, including their main minerals.

What is the outer structure of the earth?

The outer structure of the earth is made up of four zones or layers:

1. Lithosphere
2. Hydrosphere
3. Atmosphere
4. Biosphere

Lithosphere

This is the solid portion of the earth. It is the outermost zone of the earth, made up of rocks and mineral materials. It represents 30% of the earth’s surface.

Importance of the lithosphere

• It forms the basis for all human settlement.
• All mineral resources are derived here.
• It aids transportation through the construction of roads, railways, airports, etc.
• It aids farming activities.
• Most human activities, like mining and trading, are done here.

Hydrosphere

This refers to the liquid portion of the earth. It includes the oceans, seas, rivers, streams, springs, lakes, etc. It covers 70% of the earth’s surface.

Importance of the hydrosphere

• It provides water for domestic uses.
• It provides a medium for transportation.
• It provides water for industrial uses.
• It provides food (fish, prawns, etc.).
• It provides employment.
• It serves as a tourist centre.
• It is used for the generation of hydro-electric power (HEP).

Atmosphere

This is the gaseous portion of the earth. It contains 78% nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.97% rare gases.

Importance of the atmosphere

• It is the habitat of some living organisms.
• It provides oxygen for respiration.
• It provides carbon dioxide for photosynthesis.
• It provides oxygen for combustion.
• It provides a medium of transportation.
• It provides nitrogen for protein synthesis in plants.

Biosphere

This is the zone of the earth where living things are found.

Importance of the biosphere

• Plants provide food for man.
• It provides employment.
• It provides raw materials.
• It balances and purifies the atmosphere.

What is the internal structure of the earth?

The internal structure of the earth is made up of three concentric layers:

1. The crust (lithosphere)
2. The mantle (mesosphere)
3. The core (barysphere)

The crust

The crust is subdivided into upper and lower crust. The upper crust consists of granite rocks and forms the continents. The main minerals here are silica and aluminium, collectively referred to as SIAL. It has an average density of 2.7.

The lower part of the earth’s crust consists of basalt rocks and forms the ocean floor. It contains minerals like silica, iron and magnesium, collectively referred to as SIMA. It has an average density of 3.0.

The mantle

This zone is found just beneath the crust. It is about 2900 km thick with a density of 3.3. Its main mineral is olivine. It is plastic and in semi-liquid form.

The core

This is the innermost part of the earth. It is about 3500 km in radius. It contains two minerals, iron and nickel, collectively referred to as NIFE. The temperature of the core is estimated to be as high as 2000°C. It is in a molten state.

Evaluation

1. Name the four layers of the outer structure of the earth.
2. State four importance of the hydrosphere.
3. List the gases in the atmosphere and give the percentage of each.
4. What are SIAL, SIMA and NIFE, and in which layers of the earth are they found?

Assignment

1. Draw the inner structure of the earth.