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The earth has Internal and External structures

The earth is composed of three internal, concentric layers of increasing densities. These layers are the crust, mantle and core. They are made up of different layers of rocks, with their densities increasing towards the centre of the earth. That is, densities of rocks that make up the earth increase as you move from the surface towards the interior
The Crust (Lithosphere)
This is the outermost part of the earth. It consists of silica and aluminium (sial). It forms the upper layer of the continent and is mostly composed of granite rocks. The layer below SIAL is called SIMA. This layer is made of silica and manganese. It is a layer of basaltic rocks which are denser and underlies the continental block to form the ocean floor.
The Mantle (Mesosphere)
This is the layer below the crust. It is composed of iron and manganese. It lies between the crust and the core. The layer which separates crust and mantle is called Mohorovic discontinuity. The mantle is made up of very dense and hot igneous rocks, found in semi liquid states. It extends downwards 2900 km and the temperature ranges between 5000°C and 7000°C. The density of the mantle is 3 – 3.3 g/cm3. It is divided into two parts namely, the upper and lower mantle. The upper mantle is rigid and combines with the crust to form a layer called lithosphere. Below the upper mantle there is a layer called asthenosphere
The Core (Barysphere)
This is the innermost layer of the earth. It is composed of nickel and iron. Its diameter is approximately 2500 – 2700 km and its temperature is around 5500°C. The average density of the barysphere is about 5.2 g/cm3. Most geographers believe that the core is divided into solid and liquid core. The total mass of the earth is about 5.976 x 1021 tonnes.

External structure of the earth consists of four main layers’. These are Atmosphere, Hydrosphere, Lithosphere / Land mass Biosphere, 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.

• Abiotic components of Atmosphere 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.
• Biota components of atmosphere includes bacteria et.

According to the temperature changes, atmosphere divided into two zones. These are Homosphere and 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 consists 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 and 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 – 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 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.


 1. Igneous rocks Are rocks that formed when molten rock cools and solidifies within or outside the earth’s crust. The origin of igneous rocks is inside the earth where they are under great pressure. Igneous rocks do not occur in layers and they don‘t contain fossils. Igneous rocks solidify either within the earth‘s crust and form intrusive features or outside the earth‘s surface and form extrusive features. Igneous rocks are formed when the molten magma is forced out from the upper mantle to the earth‘s surface, where it cools and solidifies due to low temperature. Crystals form on cooling and the rocks are called crystalline rocks. There are two main types of igneous rocks:
1. Plutonic: these have solidified deep in the crust and they are seen on the surface only after being exposed by prolonged erosion.
2. Volcanic: these have been poured on the earth‘s surface where they are called lavas.

Characteristics of igneous rocks
• Igneous rocks reflect light.
• They are not found in layers.
• They do not contain fossils.
• They are crystalline rocks.
• They are formed through cooling and solidification of magma.
• They can undergo metamorphic and weathering processes.
• They contain different minerals like iron, magnesium etc. In Tanzania igneous rocks are found in Dodoma, Iringa and in the shores of Lake Victoria (Mwanza). The main examples are granite, gabbro, basalt and diorite. Some are found in Kilimanjaro and Rungwe (Mbeya) such as basalt, pumice, diorite, gabbro, syenite and peridotite rocks.

2. Sedimentary rocks
Sedimentary rocks are rocks formed through weathering processes when sediments are accumulated, compacted and cemented together. The sediments are compacted by compression to form sedimentary rocks. Sedimentary rocks are found in layers; they contain fossils and are very soft. These are weathered particles formed through deposition and lithification processes

Characteristics of sedimentary rocks
• They are formed when particles or sediments are accumulated, compacted and cemented together.
• They contain fossils.
• They are found in layers (strata).
• They do not reflect light.
• They are non-crystalline rocks.
• They can undergo metamorphic process.

Types of sedimentary rocks

a) Mechanically-formed sedimentary rocks These are formed through weathering process. When weathering agents erode and deposit rock particles, they are accumulated, compacted and cemented together to form sedimentary rocks. Examples of mechanically formed sedimentary rocks are clays, gravels and alluviums (all deposited by water), moraines, boulder clay and gravels (deposited by ice) and loess (deposited by wind); sandstones and shale.

b) Chemically-formed sedimentary rocks These are formed through chemical precipitation process. They include carbonate (as it is in stalactite and stalagmite), sulphate, chloride, etc. The main examples are gypsum, rock salt, lignite, dolomite, flint, borax, limonite, haematite, etc.

c) Organically-formed sedimentary rocks These are formed through mineralization process of decaying and decomposition of dead organisms such as animals and plants. The remains of living organisms are accumulated, compacted and cemented together to form these sedimentary rocks. The main examples are chalk (limestone) and coral (formed from animals), and peat, coal and lignite (formed from plants).

3. Metamorphic rocks

These are rocks which have changed from one type of rock to another due to the contact of heat, pressure or both. This process is referred to as metamorphism. Any rock can be changed into a metamorphic rock. Examples of metamorphic rocks are slate, marble and granite.
There are three kinds of metamorphism
(i) Dynamic metamorphism. This is influenced by pressure of the earth’s crust. 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

Characteristics of metamorphic rocks
• They are very hard due to prolonged action of heat and pressure.
• These rocks can change to another to another type of rocks.
• They can undergo weathering process.

Rock cycle is a relationship in which rocks tend to change from one type of rock to another. This is the cycle in which rocks tend to change from one type to another. For instance igneous rocks may change to metamorphic rocks or sedimentary rocks; sedimentary rocks to metamorphic or igneous rocks, etc. Necessary conditions for rock cycle to take place or Process of rock cycle.

1. First, the molten rocks erupt from the interior of the earth and then cool and solidify to form igneous rocks.
2. Secondly, the igneous rocks are subjected to denudation process to form sedimentary rocks.
3. Third, either igneous or sedimentary rocks undergo metamorphism, due to prolonged heat and pressure, to form metamorphic rocks.
4. Fourth, metamorphic or igneous rocks can undergo weathering process through erosion and transportation of sediments which are further deposited in layers in the ocean or lake floors where they are cemented and consolidated to form sedimentary rocks and vice versa.

5. Fifth, metamorphic or sedimentary rocks can be subjected to heat and pressure where melting take place and later cooling, due to low temperature, to form igneous rocks.

Simplified geological time scale
The geological time scale is a chart for dating the history of the earth including rock span. It tries to explain the age of rocks as far back as 600 million years ago. Era Period Years in millions before present Major geological events in Africa Man and animals Cenozoic Quaternary 1 Glaciation of East Africa mountains. Formation of river terraces and raised beaches. Age of man Tertiary 163 Formation of the Atlas mountains. Lava flows in Ethiopia. Age of mammals. Mesozoic Cretaceous 135 Deposition of marine sediments in the Sahara and Southern Nigeria. Formation of Enugu coalfield. Age of reptiles Jurassic 180 Break-up of Gondwanaland and Marine invasion of East Africa coastlands and separation of Malagasy Island from mainland. Triassic 230 Drakensburg lavas and formation of upper Karro beds. Volcanic activity in West Africa. Paleozoic Permian 280 Formation of lower Karro beds. Formation of rich coal deposits in Tanzania and South Africa. Ice age in central and South Africa. Age of amphibians Carboniferous 345 Cape fold formed. Devonian 405 Marine invasion of Libya, the Sahara and Western Sudan. Continental basins formed by crustal warping Silurian 425 Continental sedimentation in Zaire basin, Tanzania and South Africa, followed by intensive folding. Ordovician 500 Extensive deposition of sediments. Formation of sandstones in Guinea, Mali, Volta basin and North West Ethiopia Age of marine invertebrates Cambrian 600 Marine invasion of Western Sahara and Kalahari basin. Proterozoic Pre Cambrian or Archarean Glaciations of Africa South of Equator. Extensive metamorphism of oldest known fossilized, unicellular algae formed in Swaziland and Mali. Algae

The importance of rocks
1. Rocks are very important in the formation of soils which can be used for agricultural production.
2. Rocks are used for building purposes: some rocks such as limestone, sandstone, gravels and sand are used for building houses, construction of roads, etc.

3. Some rocks are used as sources of energy or fuel such as coal and petroleum (mineral oil).
4. Limestone is widely used for cement manufacturing. In Tanzania, cement is produced at Tanga,
 Mbeya and Wazo Hill.
5. Salt extraction: salt usually originate from rock accruing strata, for instance, in Tunisia and Morocco there are large deposits of salt.
6. Manufacture of chemicals: some rocks contain nitrate or phosphate, while others have potash. This kind of rocks can be used for making dyes, fertilizers and medicines.
7. Mineral deposits: mineral ores occur in veins of some rocks such as igneous rocks. The minerals are formed when the magma coos down. Valuable minerals extracted from rocks include gold, lead, tin, silver, diamond, copper, zinc, aluminium, calcium and manganese.
 8. Some rocks are so impressive such that they attract tourist to come and view them. In so doing, the country earns a lot of foreign exchange.
9. Some rocks are used for decoration of houses as ornaments or they are grinded to produce powder which is used for decoration.

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