Topic 1: Structure Of The Earth - Geography Form 3

 Topic 1: Structure Of The Earth – Geography Form 3

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Structure Of The Earth

The earth is a system which composed of two zones. These are

1)      Outer zone

2)      Inner zone

Thus, the structure of the earth consists of external structure and internal structure.


The internal structure of the earth consists of three zones. These are

i)        Lithosphere/crust

ii)      Mesosphere/Mantle

iii)    Barysphere / Core.


Is the outermost and thinnest zone of the earth which found between 8 – 50km or 5 – 30 miles.

– It is largely composed of igneous rocks.

– Other types of rocks also exist as a result of changes on the earth’s surface. When subjected to forces or any stress.

– Igneous rocks are hard and brittle.
– The crust also consist of two layers are sial and sima layers.


Is the outer layer of the crust which rich in silica and Aluminium minerals.

– The sial layer consist of granites rocks with density of about 2.65.

– The sial for the basis of the continent.

– The presence of silica and aluminium minerals collectively form SIAL layer.


– Is the layer which found beneath the sial.

– Is the inner layer of the crust which separated from sial layer by the zone called Conrad discontinuity line.

– The sima layer is composed by silica and magnesium.

– It has the density of about

– It forms the basis of ocean floor.

Note: – Sial and sima layer together forms the crust.


Mesosphere or mantle which found between the crust and core.

– It lies beneath the crust

– It separated from the crust by the zone of separation called Mohorovic discontinuity line has temperature which may reaches to.

– It consist of denser rock to about 3.03 –

– It consist of pale green minerals called Olivine (Ferromagnesium silicate) in form of ultra basic rock

– It consists of lower and upper mantle.

– The upper mantle is rigid and crust to form a large layer called lithosphere.

– The lower mantle is less rigid and forms the moltern layer within the earth’s interior called asthenosphere.

– Asthenosphere is the molten layer layer which responsible for the balancing movement of the earth’s material called isostatic readjustment.

– Asthenosphere has been investigated is found between 100 to 200km below the upper surface.


The core is the innermost zone of the internal structure of the earth.

– It has diameter of about 69000km (4300 miles) density of about.

– The core is also classified into two parts i.e. the outer and inner core.

– It separated from the mantle by zone of separation called Gutenberg discontinuity

– The outer core is liquid in nature because of temperature of up to

– The outer core is consist of nickel and Iron (NIFE)

– It estimated to be 2100km

– Its density is about.

– The inner core is solid in nature because of high pressure exerted from different parts toward the center

– It composed mainly by iron

– Its density is about 16 – 17

– It has diameter of about 2600 – 2700km. (1600 – 1700 miles)

– The temperature at the center is about


i) The average density of the whole earth is about 5.17gm/cm3

ii ) The total mass of the earth is about 5.976 x 1021 tons.


External structure of the earth consists of four main layers’. These are

  • Atmosphere
  • Hydrosphere
  • Lithosphere / Land mass
  • Biosphere


– Is the thin layer of gases held on the earth by gravitation’ attraction.

– It composed by abiotic (non living matter) and biotic living organism.

– Non living matter found in the atmosphere includes mixture of gases, water vapor and dust particles.

– The living organism include the smallest or microscopic organisms like bacteria


Atmosphere is the outer zone or external structure of the earth composed by Abiotic and Biotic components.


The abiotic components of the atmosphere include the following.

  • Mixture of various gases.

These include Nitrogen (78%), oxygen (21%), argon (0.009%) and carbon dioxide (0.03%).

– Other gases include neon, helium, Krypton, xenon and other which are present in minite (small proportion) percentage.

  • Water vapor.

Is the colorless and odorless (smell less) gas in the form of water which makes up a perfect mixture with other gases.

– The degree to which water vapor is present in the atmosphere is called humidity.

– Humidity is very important to weather as condensed to form clouds and fog.

– Excess water vapor brings about precipitation in form of rain, hail, snow and sleet.

– Water vapor is capable of absorbing heat which penetrates into the atmosphere in the form of radiant energy from the sun to the earth.

– It is also act as a blanket which prevents the rapid escape of heat from the earth’s surface and therefore maintain heat budget.

  • Dust particles.

The dust particles may exposed to the atmosphere naturally or artificially

– Natural dust particles are those caused by natural phenomena like winds and volcanic eruptions

– Artificial dust particles are those derived from industrial pollutions such as soot and ashes. It includes the particles caused by other man’s activities like construction, mining and farming activities

– The function of dust particles serve as a nuclear or center around which water vapor condenses to produce clouds.


According to the temperature changes, atmosphere divided into two zones. These are

  • Homosphere
  • Heterosphere.


Homosphere is the layer which found between 0 – 80km above the sea level.

– This is the lowest part of the atmosphere which composed of uniform composition gas of uniform composition of gases and temperature

– Homosphere consist of three layers. These are

i) Troposphere

– This layer extends by 0 – 15km above the sea level.

– Troposphere is the first layer of homosphere located nearest to the earth

– It contains water vapor, gases and dust particles

– It is the layer of atmosphere which support life on the earth due to the presence of plenty oxygen gas.

– All processes of rainfall formation take place in this layer and the temperature decreases as the altitude increases at the rate of per every 100 meters or per every 1000 meters.

Note: – This situation where by temperature decreases as altitude increases is called lapse rate and because it occur near to the ground is called environmental Lapse rate.

⇒The upper limit of Troposphere which separates it to the next later is called Tropopause.

Tropopause makes the upper limit of troposphere to the next layer called stratosphere.

ii) Stratosphere

Stratosphere exists between 15 – 48 km above the sea level.

– This is the second layer of homosphere which lies above the tropopause.

– It is also composed of water vapor, dust particles and various gases

– It is the layer of atmosphere which characterized by high concentration of Ozonic gases. This gases form Ozone layer which found particularly at 20 – 35 km in the stratosphere

– The Ozonosphere or ozone layer is the layer which form a shield or cover that prevent the earth’s surface from destroying by the sun rays.

– It prevents the direct incoming of harmful rays from the sun to fall direct on the earth’s surface.

– The temperature remains unchanged about between 20 – 35 km from the earth’s surface. Then temperature increases with height to about at the upper limit of stratosphere called stratopause.

– The increase in temperature with height is referred to as temperature invasion.

iii) Mesosphere

– This layer extends between 48- 80 kilometers above the sea level.

– Mesosphere is the third part of the homosphere where temperature decreases as the altitude increases.

– It separated from the stratosphere by the zone of separation called stratopause.

– The upper limit of mesosphere is called mesopause.

– Mesopause record minimum temperature of this zone that may fall to making this zone to be coldest.

– It is at this zone where strong upper air streams of wind like jet streams are experienced.


– Is the second layer of atmosphere which extends from 80km towards the interplanetary space

– Heterosphere divided into two layers which include.

  • Thermosphere
  • Exosphere

– Is the lower part of heterosphere where temperature increases as the altitude increases from – i.e. temperature invasion. This is because; there is no water vapor or dust particle in this zone.


– Is the part of heterosphere which found above the thermosphere.

– It has high temperature through it has little significance as it has not been greatly researched.

Note: – Within the heterosphere, there is also a scientific significant layer called ionosphere.

– Ionosphere consists of some ions which influence radio waves. This is because, ionosphere is electrically charged with free electrons that allow the passage of radio waves, television waves and telephone or mobile phone waves.


– Is the layer of water bodies of the earth including all oceans, rivers, precipitation and underground water.

– It is estimated that 75% of the Earth’s surface is covered by water bodies.


– Is the whole solid body of the earth with various landforms such as mountains, valleys and plateaus.

– The lithosphere is also known as the crust.

– It includes all land masses. The major land mass is called continent and the minor land mass is called islands.


– Biosphere is the complex zone which comprises all living things.

– It includes a lower level of atmosphere and the upper level of lithosphere and hydrosphere.

– Biosphere receives substantial supply of energy from the sun which gives it condition necessary for life and does not occur in any part of the solar system.

– The living organisms that inhibit biosphere interact with each other and their environment.

– The sum of all these interaction components is called the ecological system or ecosystem.

– Biosphere comprises all living organism both macro and micro organisms living in water bodies, soils and on air.


1)  Insulation

Atmosphere is an insulator it acts as a shield or blanket and therefore regulates temperature during the night and during the winter.

2) Filtration. The atmosphere is the filter. It filters solar insulation and percent ultra violet rays of certain length due to the presence of ozone layer in the stratosphere.

3) Scientific function. Atmosphere is the scientific field

– It is the field through which the scientific experiments and observation carried out. Example ionosphere layer of atmosphere reflects some electromagnetic waves and ration signals back to the earth.

4) It supports much on hydrological cycle.

The surface water, evaporation, condensation and precipitation formation take place in the atmosphere.

5) It support life some gases particularly oxygen is important for living organisms

– Air has weight which contributes to the occurrence of atmospheric pressure variations without which breathing would be impossible.

–  Wind movement and direction that balances temperature, humidity and precipitation also result from pressure variations.


The earth’s crust is composed of different materials ranging from elements, minerals and rocks. These materials differ in their physical and chemical composition.


They refer to the smallest particles of matter which can not be split into different forms by any means. Examples of elements are magnesium, potassium, sodium, iron, aluminum and silicon.


They are naturally occurring substances which have definite shape, colour and resistance formed due to combination of different elements. They are formed as a result of the combination of two or more elements. Some single elements like gold, silver and diamond may occur as minerals.

Mineral Element
Quartz Silicon and oxygen
Feldspar Potassium, sodium, calcium and aluminum


A rock is an aggregate of minerals in a solid state. On the other hand the term rock can include substances like clays, shells, sandstones and corals. Rocks which contain metallic compounds are called ores.

Rocks Minerals
Lime stone Mica, feldspar, calcite, iron ore
Granite Mica, iron ore, quarts and feldspar
Basalt Calcite, dolomite
Sand stone Quarts, calcite, feldspar and iron ore


Rocks are aggregates of minerals in a solid state. Examples of rocks are such as lime stone, granite, basalt, sand stone and shale. Rocks can be classified depending on various criteria such as mode of formation, texture, structure, colour, permeability, age and the degree of resistance.

Rocks can be classified as follows:


Rocks can be classified into:-

        Igneous rocks

        Sedimentary rocks

        Metamorphic rock


These are rocks which are formed by the cooling and solidifying of the molten material from the interior of the earth. The molten materials can solidify either intrusively or extrusive. When molten materials are still in the earth’s crust are formed as magma but when they reach on the earth’s surface are called lava.


Igneous rocks can be classified into two criteria;

1)     According to the place of occurrence

2)     According to chemical composition.

1. According to the place of occurrence, igneous rocks can be classified as;

a) Intrusive rocks

These are igneous rocks formed when the molten materials cool and solidify within the earth. The cooling and solidification of the molten materials can be near the surface or very deep in the crust. Igneous rocks formed when molten materials cool and solidify near the surface are called hypabyssal igneous rocks. Examples include granite, porphyries, and dolerite. These rocks can be exposed to the surface if there is severe erosion. Some rock masses like lopolith, laccolith, phacolith, sill and dykes are also hypabyssal igneous rocks.

When molten materials cool and solidify deep in the crust they form plutonic igneous rocks. These rocks consist of large grains and they are hard since they cool and solidify slowly. Examples include Granite, diorite, gabbro, pumice, and peridotite.

b) Extrusive rocks

These are igneous rocks formed when the molten material solidifies on the earth’s surface and form lava. These rocks have small grains because they cool and solidify fast due to low temperature on the earth’s surface.

        Hypabyssal igneous rocks

These are rocks formed when the magma cools and solidifies closely or nearly to the earth’s surface. They have medium size particles, example dolerite.

        Plutonic igneous rocks

These are formed when the molten material (magma) solidifies deep down in the crust. They are consisted of large grains and they are hard because of the slow cooling process for example granite, diorite, gabbro etc

2.According to Chemical composition, igneous rocks can be classified into;

i)       Felsic (acidic) igneous rocks: These are igneous rocks which consist of great amount/content of silica and feldspar with a very little or no iron or any other metallic metal for example gramorphyte, granite etc

ii)      Mafic (basic) igneous rocks: These are igneous rocks which consist of small amount of silica with large amount of magnesium, iron and other minerals like aluminum.

iii)     Ultra mafic (ultra basic): These are rocks which consist of a very large amount of metallic minerals like iron, magnesium and little amount of silica less than 45% for example peridotite.

iv)      Intermediate igneous rocks: These are igneous rocks with silica content between basic and acidic degree. That is both acidic and basic oxides are in equal proportions.


        They are hard and non stratified.
        They are formed by cooling and solidification of the molten materials.
        They can be acidic ,basic, or intermediate depending on the amount of silica.
        They are crystalline ( having a definite shape).
        They can undergo metamorphism to form metamorphic rocks.
        They can undergo weathering process and after sedimentation can form sedimentary rocks.
        They contain minerals like iron, magnesium etc.
        They don’t consist fossils.


These are rocks formed by the process of sedimentation that is deposition or accumulation and lithification of sediments and some weathered particles and other minerals. These sediments may be deposited by water, wind or moving ice.

The particles are either deposited by running water or by moving ice.


a)     Originally formed sedimentary rocks.
b)     Mechanically formed sedimentary rocks.
c)     Chemically formed sedimentary rocks.

a)Organically formed sedimentary rocks.

These are formed from deposition of the remains of living organisms for example they include coral reefs, carbonaceous rocks and siliceous rocks formed as a result of remains of organisms. The remains of once living organisms may accumulate in layers to form sedimentary rocks. They can be classified into the following types;

1)     Carbonaceous rocks

These are formed from deposition of plants remains only. They are formed from accumulation of plants which were rich in carbonates and being buried for many years ago. Example Coal

2)     Calcareous rocks

These are formed from accumulation of remains of animals only. They are formed from lithification of skeletons and shells of animals. Examples are coral, chalk, limestone etc

3)     Siliceous rocks

These are rocks formed from the remains of organisms like diatoms and radiolarians whose skeletons and shells are rich in silica/silicon. Example diatomic rocks

b)Mechanically formed sedimentary rocks.

These are rocks which have been formed by compaction and cementation of sediments which have laid down on sand or on the sea floor. They are formed through accumulation/deposition and lithification of weathered materials e.g.Igneous rock- weathering- sediments- deposition- lithification- sedimentary rocks.
A particular rock is disintegrated and the weathered materials formed are deposited in layers to form sedimentary rocks. These rocks can be classified into the following;

1)     Arenaceous

These are formed through deposition of particles with medium size of sand e.g. Sand stone

2)     Argillaceous

These are formed from deposition and lithification of weathered materials of very fine or small particles e.g. clay and silt. Argillaceous particles may include clay stone, salt stone, mud stone etc.

c) Chemically formed sedimentary rocks.

These are rocks formed through chemical process or decomposition. The basic chemical process includes carbonation which is the reaction between weak carbonic acid with rocks containing calcium carbonate.

Calcium carbonate + Water + Carbon dioxide → Calcium bicarbonate

CaCO+ H2O + CO→ Ca (HCO3)2

Other processes include those of sulfates, chlorides, silicates, iron stone etc

Mechanically rocks
Organically rocks
Chemically rocks
 Shale Chalk Gypsum
Mud stone Limestone Rock salt
Silt stone Coal Flint
 Grit Coral reef Ironstone


        They are stratified and young rock layers overlay the old rock layers.

        They are non crystalline.

        They contain fossils as a result of accumulation of skeletons and shells of once living organisms.

        They can undergo changes to form metamorphic rocks when they are influenced by pressure and temperature.

        They are consisted of some fragments which were deposited and then cemented to form a rock.


These are rocks which are formed when one type of rock change into another type of rock after being subjected under intense heat or pressure or both. Any rock can change to form metamorphic rock. For example :-

– Sedimentary rocks to metamorphic rock.

a)     Sand stone to Quartzite.

b)     Lime stone to Marble.

c)     Coal to Graphite.

d)     Clay/Shale to Slate.

e)     Mud stone to Slate.

– Metamorphic rock to metamorphic rock,example slate to schist.

– Igneous rock to metamorphic rock.

a)     Augite to Hornblende.

b)     Granite to Gneiss.
The process which involves the change of one rock type to another rock type is called Metamorphism.


There are three kinds of metamorphism;

(i) Dynamic metamorphism.

This is influenced by pressure because of the earth’s movement and brings about mountain formation. Examples;

        Shale to Schist

        Clay to Slate

        Granite to Gneiss

(ii) Thermal or contact metamorphism.

This is caused by intense heat. This can take place when the rock comes into contact with hot molten material like magma or lava. Examples

⋅       Lime stone to Marble

⋅       Sand stone to Quartzite

(iii) Thermal dynamic metamorphism

This is the process that takes place as a result of a combination of heat and pressure. It is when the existing rocks are subjected to both pressure and heat to change their shape and appearance. Example Coal to Graphite


i)       They are more hard and more resistant than the original rock.

ii)      They are formed when any type of rock even the other metamorphic  rock changes into other types of rocks

iii)      It can undergo weathering to form sedimentary rock or after melting and cooling it can form igneous rock.

iv)      Most of them are foliated (they are composed of thin layers)


It is the relationship into which rocks tend to change from one type to another type of rock . For example the rock can change from igneous rock to sedimentary to metamorphic and go back to igneous. In the rock cycle, any type of rock can form another type of rock through different processes

Processes in the rock cycle

i)  At first igneous rock may be formed due to cooling and solidification of magma or lava.

ii) Then igneous rock can be attacked by weathering agents to form sediments which can be deposited and compacted or cemented together to form a sedimentary rock.

iii) It is the igneous rock or sedimentary rock can undergo metamorphism due to the influence of pressure and heat to form metamorphic rocks.

iv)  Likewise metamorphic rocks can undergo further metamorphism to form either another or more metamorphic rock like Slate which is changed to form slate.

v)  Lastly only rock that is either sedimentary or metamorphic when subjected to heat under high temperature can melt and cool to form an igneous rock.


Rocks can be classified according to age. The age of each rock can be indicated on the geological time scale. Geological time scale is a chat used for dating the history of the earth including rocks. This chart attempts to describe different periods and eras which rocks can be formed. The geological time scale divides geological time extending to about 600 million years.

According to the ages rocks can be classified into;

i)      Pre Cambrian era

ii)    Paleozoic era – ancient life

iii)  Mesozoic era – middle life

iv)  Cenozoic era – recent life

The last three eras are described as primary, secondary and tertiary eras respectively. The quaternary era represents rocks aged 1-2 million years ago. Each era is divided into period/system which in turn may be divided into series of formation.


Years in Million
Major Geological event in Africa











 Main period of volcanic activity in E.A began

Main period of faulting in E.A,

Alpine earth movement formed Atlas mountains and lava flow in Ethiopia.








Deposition of Marine sediments in Sahara and S. Nigeria.

Formation of Enugu Coal Fields.





Break up Pangaea into Gondwanaland and Laurasia; Marine invasion in  E.A and separation of Madagascar from mainland.




 Drankesberg lava  formation of upper Karroo and volcanic activity in W. Africa












Formation of lower Karroo beds;

Formation of Coal deposits in TZ, Zimbabwe, and South Africa;

Ice ages in C. Africa and S. Africa

Carboniferous 280 Formation of Cape fields




Marine invasion in Libya, Sahara and West Sudan, Continental basins formed by warping.  



Ages of Fishes





Continental sedimentation in Zaire, Tanzania, and S.Africa followed by folding.




Deposition of sediments, formation of sandstones in Guinea, Volta basin and North-West Ethiopia.


Marine invasion of West Sahara and Kalahari basin. Age of introverted brutes



Upper Middle Lower Azoic



Ancient mountains building and ancient glaciation in south of equator

Oldest recorded rocks of 3500millions years from S. Africa.


Age of Algae

Advantages of the geological time scale

i)     It shows the ages of the rocks when they were formed i.e. other rocks were formed by deposition of igneous rocks etc

ii)     It helps to understand when and how various land forms were formed for example the mountains of different types like the volcanic and the Fold Mountains have been accounted for.

iii)     It can help one to know and predict the occurrence of crustal development likely to take place.

iv)     It helps in the recording of plant and animals by so doing this helps to understand the relationship between living things and the geological process.

Disadvantages of the geological scale

i)   The method used in determining the age of rocks were largely based on estimation

ii)    Crustal deformation like over folding and gaps caused by denudation.

iii)    It is not certain.


i) They help in soil formation which can be good for agricultural activities.

ii) Storing underground water. Water is stored in the water holding stratum and can come out as a spring.

iii) Some rocks are used for fuels like coal and mineral oil.

iv) Rocks are used for building. A wide range of rocks like limestone are used for building houses and for manufacturing of cement.

v) Salt extraction, various salts are obtained from rocks occurring in some places.

vi) Manufacturing of some chemicals. Some rocks have salt such as nitrate or phosphate.

vii) Mineral deposits. Mineral ores occur in veins of some rocks such as igneous rocks.

viii) Some rocks are so impressive such that they attract some tourists to come and view them.