The Lithosphere

Internal structures of the earth

There are three layers that constitute earth’s interior namely crust (lithosphere), mantle (mesosphere) and core (barysphere).

a.      Lithosphere

This is part of the outermost part of the earth consisting of solid rocks. It is also known as crust and is divided into two distinct parts namely granitic layer and basaltic layer.

        i.            Granitic layer

It is the outermost layer which is made up of light rocks known as granite. It is discontinuous layer and contains a large proportion of silica and aluminium hence it is referred to as sial. It forms the continents.

      ii.            Basaltic layer

This is the continuous layer which is made up of heavy rocks known as basalt that contain a large proportion of silica and magnesium, hence it is referred to as sima. It forms the basis of continents (ie on which the continents sit) and the floor of oceans hence it is referred to as oceanic crust.

The lithosphere (crust) is divided into portions that vary in sizes. These divisions are referred to as plates. They are also called crustal plates or lithospheric slabs or tectonic plates. They are classified as oceanic and continental plates.

        i.            Continental plates include African, Iran, Arabian, Australian, Eurasian, North American, South American, Philippines, Caribbean and Antarctica plates.

      ii.            Oceanic plates include Nazca, Pacific plates.

These plates are constantly moving in various directions as described below in part of theory of plate tectonics.

The world map below shows the plates

b.      Mesosphere (mantle)

This is second layer after crust. While the outer mantle consists of molten rocks known as magma due to intense heat the inner part is solid due to intense pressure. Asthenosphere is the name given to outer layer of the mantle. Convectional currents in the asthenosphere adequately influence the movements of plates.

It is separated from the lithosphere by the boundary called Mohorovicic discontinuity shortened as Moho. This boundary is where earthquake waves change speed.

c.       Barysphere (core)

This is the innermost structure of the earth. It is divided into inner and outer core. The outer core is semi-liquid while the inner is solid due to high temperatures and intense temperature respectively. It is mainly composed of nickel (Ni) and iron (Fe).

It is separated from the mantle by the boundary called Gutenburg discontinuity.

The diagram below shows the structures of the earth’s interior.

Theory of plate tectonics

This theory suggests that the earth’s crust has divisions known as plates which are varied in size and are constantly moving.

The theory explains the formation of major landforms of the world such as plains, mountains, plateaus and valleys.

Movement of plates

The plates move in three different ways. Some plates move towards each other such as Nazca and South American plate, Caribbean and Pacific plates, Eurasian and Pacific plates. Others move away from each other such as African and South American plates. They are other plates that neither move towards each other nor move away from each other, they rather slide (shear) such as Iran and Eurasian plates.

The movement of plates is mainly influenced by the convectional currents in the asthenosphere on which the crust sits.

Plate boundaries (margins)

These are regions which separate the plates and they are characterized by instability. The changes that occur in these regions lead to formation of major landforms of the world.

a.      Convergent boundary

This is the boundary that separates that plates that move towards each other. The movement results into the plates bending into the mantle (subsidence), hence the boundary is also called zone of subduction. The subduction zones become features known as ocean deeps (trenches) whose examples are Marianas, Tonga, Java, Peru Aleutian trenches.

The plates bend into the mantle where oceanic plate melts (becomes molten). This magma may find its way into the crust or on the earth’s surface it solidifies to form rocks which form mountains called volcanic mountains. Good examples are Popocatepeti, Pelée, Cotopaxi, Krakatoa, Mauna Loa, and Fujiyama.

Apart from that, the continental plate is subject to folding which results into formation of Fold Mountains such as Andes, Rockies, Himalaya and Atlas.

b.      Divergent boundary

This is the boundary which separates the plates that move apart (away from each other). It is referred to as constructive margin (boundary) or zone of sea floor spreading because it is where magma outpours resulting into formation of new crust.

The features which are formed along this boundary include mid oceanic ridges such as Mid-Atlantic ridge, Indian Ocean ridge and Pacific ridge; volcanic islands such as Iceland.

c.       Transform boundary

This is the boundary which separates the plates that move past each other (sliding plates). Along these boundaries earthquakes and faulting are common phenomena.

NB earthquakes are common phenomena along all plate boundaries. A detailed account of earthquakes is described below.

Formation of mountains

A mountain is simply defined as a higher ground than its surrounding. Mountains are classified into four groups namely volcanic mountains, fold mountains, block mountains and residual mountains.

a.      Volcanic mountains

These are mountains that are formed from the accumulation of lava on the surface of the earth. They are also called mountains of accumulation. They are categorized into three namely ash and cinder cones, lava cones and composite cones.

i.        Ash and cinder cones

These are formed from the accumulation of lava on the surface of the earth during eruption. The lava is forced out of the crust with great pressure, thereby thrown to height and broken into small fragments (cinder). Good examples of such cones include Vulcano de Fuego in Guatemala and Paracutin in Mexico.

ii.      Lava cones

These are formed from either fluid lava or viscous lava. Fluid lava leads to formation of cones that have gently sloping sides such as Mount Mauna Loa in Hawaii whereas viscous lava leads to formation of cones that have steeply sloping sides such as Mont Pelée in Martinique. Such lava cools rapidly on the surface of the earth.

 

iii.    Composite cones

These cones are formed from lava and ash that are laid alternately in layers during eruption. The eruption begins with violent eruption of ash followed by eruption of lava. Lava often escapes from the sides of the cones to form small conelets. Good examples of such cones include Mount Kilimanjaro (Tanzania), Mount Cameroon (Cameroon), Vesuvius (Italy) and Stromboli (Italy), Etna (Italy).

b.      Fold mountains

These are also referred to as mountains of elevation since the earth’s crust is lifted up. They are formed when the crust bends (fold) in response to compressional forces under which it can be when the plates move towards each other.

Compressional forces lead to four forms of crustal folding.

i.        Simple fold: has equal limbs (sides of the fold)

ii.      Asymmetrical fold: one limb is steeper than the other

iii.    Overfold: one limb is pushed over the other

iv.    Overthrust (nappe): one limb is pushed forward over the other and has a fracture (fault).

The diagram below shows the folds.

Examples of Fold Mountains include the Andes of South America, the Rockies of North America, Himalaya, Atlas (Morocco), Drakensberg (South Africa).

The Fold Mountains are the highest and longest mountains in the worlds. They are usually associated with volcanic eruptions and earthquakes.

Influences of Fold Mountains

i.        The act as barriers to communication that is they hinder the construction of the roads and railways which are significant in the transportation of goods and people from one point to another

ii.      They give rise to important rivers such as Indus, Colorado, Colombia, Ganges which significant for HEP production and irrigation

iii.    They contain precious minerals such tin, copper, gold spur economic growth of countries through exportation

iv.    They have valuable timber resources such as coast ranges of Western America and foothills of Himalayas

v.      They act as climatic barriers as they divide the region into two of which one has climatic conditions different from the other. For example, western coast of North America experiences mild winters and warm summers and throughout the year while the Eastern region of the Rockies experiences cold winters and hot summers and maximum rainfalls in summer.

c.       Block mountains

These are mountains formed by the uplifting of crustal block as a result of the either compressional forces or tensional forces.

i.        Tensional forces

The crust may be under tensional forces when the plates move apart. Under such circumstances, it is divided into three parts (blocks) by faulting (formation of normal fault). While the middle block sinks (subsides) the side blocks rise to become block mountains as shown in the diagrams below. These mountains are characterized by steep sides and generally flat surface (plateaus). Such being the case they are called horsts such as Black Forest, Mount Ruwenzori, Korea

ii.      Compressional forces

The crust might be under compressional forces when the plates move towards each other. This folding yields faulting (formation of reverse fault) which divides the crust into two crustal blocks. One block subsides (sinks) while the other rises to become a block mountain as shown in the diagrams below. These mountains have one side steeper than the other (escarpment). As such they are called tilt blocks such as Deccan plateau, Arabian plateau and Brazilian plateau.

NB. The blocks that subside (are depressed) become rift valleys or great basins.

The lifting up is facilitated by the theory of isostacy which explains the vertical and horizontal movement of continental blocks which are floating on the semi-liquid layer of the asthenosphere.

d.      Residual mountains

These are mountains that are formed as a result of denudation processes (weathering, erosion and transport), hence they are referred to as mountains of denudation.

The earth’s crust consists of more resistant rock and less resistant rocks to erosion. Under denudation, the less resistant rocks are carried away leaving behind the more resistant rocks which become the mountains as shown in the diagrams below

NB. Denudation processes encompass weathering, erosion and transport. Weathering is the breakup of rocks by alternate heating and cooling; chemical actions and action of living organism. Erosion is the breakup rocks by action of rock particles being moved over the surface of the earth by running water, wind and moving ice. Transport is the movement of rock particles over the earth’s surface by water, wind and ice.

NB: Mountains of denudation are often formed from volcanic features which are formed from magma that solidifies within the crust such as batholith, laccolith

Batholith is the large mass of magma that accumulates in the crust. It forms the root of mountains. It is exposed by processes of denudation

Laccolith is the large mass of magma that solidifies near the surface of the earth.

Sill is the sheet of magma that solidifies along the bedding plane.

Dyke is the sheet of magma that solidifies across the bedding plane

The diagram below shows the volcanic features that formed within the crust

Earthquakes

These are vibrations of the earth’s crust as a result pressure resulting from development of faults as the plates collide or movement of the magma (melted rocks) along the fissure (fault) in the crust.

Earthquakes originate from the focus (a point in the earth’s crust) and travels through waves to different parts of the earth’s surface including the epicenter (a point on the surface of the earth which is directly above the focus).

Earthquake waves can be body waves through which earthquakes travel within the crust or surface waves through which earthquakes travel on the surface of the earth.

The diagram shows how earthquakes are formed

Effects of earthquakes

a.       The cause loss of lives and property

b.      They cause vertical and lateral displacement of parts of the crust

c.       They cause the rising and lowering of the parts of the sea floor which give rise to tsunami (tidal waves)

d.      They cause the rising and lowering of coastal regions

e.       They cause landslides

Theory of continental drift

Continental drift is simply defined as the horizontal movement of the crust in which the continents seemingly move apart. It is the basis of the continental drift theory which states that the continents originated from a single super continental called Pangaea which later broke into various continents. It explains that the continents attained the present positions as a result of moving apart of continents. The Pangaea was surrounded by the super sea known as panthalassa.

The Pangaea split into Laurasia and Gondwanaland with time. Laurasia also divided into North America, Greenland and Eurasia, so did Gondwanaland into Africa, South America, Australia and Antarctica. 

The diagram below show continental drift

Evidence of continental drift

a.       The shapes of South America and Africa seem that they can fit in a jig saw which implies that they were once joined.

b.      The structure of rock ranges of Argentina and Cape of South  Africa are similar

c.       Rocks of the West Africa and South America are similar

d.      Presence of glacial deposits in Congo basin

e.       Similar magnetism of ancient rocks in all continents

f.       Fossilized trees in the Antarctica

Weaknesses of continental drift

a.       There is no indication that continental drift is currently taking place

b.      There is no indication that that there are great forces that can move the continents to the positions at which the continents are today

c.       Some continents do not have shape that shows that they can fit in a jig-saw

Shields

These are parts of the earth’s crust which have never gone vehement changes but have remained quiescent (stable) over a very long period of time.

These include Laurentian, Brazilian, Baltic, African, Arabian, Siberian, Deccan, China and Australian shields 

The map below shows shields

Rocks

There are three major groups of rocks according to how they are formed namely igneous, sedimentary and metamorphic rocks.

a.      Igneous rocks

These are rocks formed by solidification of magma either on the surface of the earth or within the crust. They are divided into volcanic (extrusive) rocks and plutonic (intrusive) rocks

i.        Volcanic rocks

These are rocks which are formed from lava that solidifies on the surface of the earth. They contain small crystals due to rapid cooling of magma. Examples include basalt, pumice, andesite etc.

ii.      Plutonic rocks

These are rocks which are formed from magma that solidifies within the crust. They contain large crystals due to slow cooling of magma. Examples include granite, gabbro etc.

Characteristics of igneous rocks

i.        They are crystalline

ii.      They are not stratified

iii.    They do not contain fossils (remains of animals and plants)

b.      Sedimentary rocks

These are rocks which account for a large proportion in the earth’s crust. They are divided into three categories according to how they are formed.

i.        Mechanically formed rocks (clastic rocks)

These are rocks formed by compaction (compression) and cementation (lithification) of rock particles called sediments (clastics). Examples include loess (wind-deposited rocks), gravel and alluviums (river-deposited rocks), moraines, sandstone and boulder clay (glacier-deposited rocks), breccia, conglomerate, siltstone, limestone

The diagram below shows how clastics rocks are formed

ii.      Chemically formed rocks

These rocks are formed in precipitation or evaporation processes such as rock salt, gypsum, borax, and potash

iii.    Organically formed rocks

These rocks are formed by decomposition of fossils (plant and animal remains). From plants, examples are peat, lignite, coal while from animals, chalk and coral

Characteristics of sedimentary rocks

i.        They are fossilized

ii.      They are stratified

iii.    They are not crystalline

c.       Metamorphic rocks

These are rocks which are formed from pre-existing rocks by great heat and pressure which change the structure and appearance of rocks. The process under which these rocks are formed is metamorphism.

There are three types of metamorphism which include:

i.        Thermal metamorphism

This is a process influenced by heat that leads to re-mineralisation and the change of rock appearance. In re-mineralisation, new minerals are added to the rock. 

ii.      Dynamic metamorphism

This is a process in which the great pressure influences changes in the rock which lead to recrystallization and re-mineralisation 

iii.    Regional metamorphism

This is influenced by both heat and pressure

iv.    Metasomatism

This is a process in which chemical processes occurs leading to changes in the rock that is chemical reactions.

The table below shows examples of metamorphic rocks

Metamorphic rock	Formed from
Marble	Limestone
Slate	Clay
Gneiss	Granite
Quartzite	Sandstone
Graphite	Coal
Schist	Slate, granite

Rock cycle

This is the process by which rocks change from one form to another within or beneath the earth’s crust. Igneous rocks can be derived from either sedimentary or metamorphic rocks through igneous activities. Igneous activities encompass melting of rocks and solidification (cooling) of magma. Sedimentary rocks can be formed from either igneous or metamorphic rocks through compression and lithification processes while metamorphic rocks can be formed from igneous rocks or sedimentary rocks through metamorphism process.

The diagram below shows the rock cycle

Importance of rocks

a.       They contain precious minerals such as uranium, aluminium, iron which can be exported to earn foreign exchange

b.      They important for farming in form of soil

c.       They are used in construction of roads and buildings

d.      They are used as fuel such as coal

e.       They provide beautiful scenery in form of mountains which influences tourism thereby earning foreign exchange 

The Lithosphere

Internal structures of the earth

There are three layers that constitute earth’s interior namely crust (lithosphere), mantle (mesosphere) and core (barysphere).

a.      Lithosphere

This is part of the outermost part of the earth consisting of solid rocks. It is also known as crust and is divided into two distinct parts namely granitic layer and basaltic layer.

        i.            Granitic layer

It is the outermost layer which is made up of light rocks known as granite. It is discontinuous layer and contains a large proportion of silica and aluminium hence it is referred to as sial. It forms the continents.

      ii.            Basaltic layer

This is the continuous layer which is made up of heavy rocks known as basalt that contain a large proportion of silica and magnesium, hence it is referred to as sima. It forms the basis of continents (ie on which the continents sit) and the floor of oceans hence it is referred to as oceanic crust.

The lithosphere (crust) is divided into portions that vary in sizes. These divisions are referred to as plates. They are also called crustal plates or lithospheric slabs or tectonic plates. They are classified as oceanic and continental plates.

        i.            Continental plates include African, Iran, Arabian, Australian, Eurasian, North American, South American, Philippines, Caribbean and Antarctica plates.

      ii.            Oceanic plates include Nazca, Pacific plates.

These plates are constantly moving in various directions as described below in part of theory of plate tectonics.

The world map below shows the plates

b.      Mesosphere (mantle)

This is second layer after crust. While the outer mantle consists of molten rocks known as magma due to intense heat the inner part is solid due to intense pressure. Asthenosphere is the name given to outer layer of the mantle. Convectional currents in the asthenosphere adequately influence the movements of plates.

It is separated from the lithosphere by the boundary called Mohorovicic discontinuity shortened as Moho. This boundary is where earthquake waves change speed.

c.       Barysphere (core)

This is the innermost structure of the earth. It is divided into inner and outer core. The outer core is semi-liquid while the inner is solid due to high temperatures and intense temperature respectively. It is mainly composed of nickel (Ni) and iron (Fe).

It is separated from the mantle by the boundary called Gutenburg discontinuity.

The diagram below shows the structures of the earth’s interior.

Theory of plate tectonics

This theory suggests that the earth’s crust has divisions known as plates which are varied in size and are constantly moving.

The theory explains the formation of major landforms of the world such as plains, mountains, plateaus and valleys.

Movement of plates

The plates move in three different ways. Some plates move towards each other such as Nazca and South American plate, Caribbean and Pacific plates, Eurasian and Pacific plates. Others move away from each other such as African and South American plates. They are other plates that neither move towards each other nor move away from each other, they rather slide (shear) such as Iran and Eurasian plates.

The movement of plates is mainly influenced by the convectional currents in the asthenosphere on which the crust sits.

Plate boundaries (margins)

These are regions which separate the plates and they are characterized by instability. The changes that occur in these regions lead to formation of major landforms of the world.

a.      Convergent boundary

This is the boundary that separates that plates that move towards each other. The movement results into the plates bending into the mantle (subsidence), hence the boundary is also called zone of subduction. The subduction zones become features known as ocean deeps (trenches) whose examples are Marianas, Tonga, Java, Peru Aleutian trenches.

The plates bend into the mantle where oceanic plate melts (becomes molten). This magma may find its way into the crust or on the earth’s surface it solidifies to form rocks which form mountains called volcanic mountains. Good examples are Popocatepeti, Pelée, Cotopaxi, Krakatoa, Mauna Loa, and Fujiyama.

Apart from that, the continental plate is subject to folding which results into formation of Fold Mountains such as Andes, Rockies, Himalaya and Atlas.

b.      Divergent boundary

This is the boundary which separates the plates that move apart (away from each other). It is referred to as constructive margin (boundary) or zone of sea floor spreading because it is where magma outpours resulting into formation of new crust.

The features which are formed along this boundary include mid oceanic ridges such as Mid-Atlantic ridge, Indian Ocean ridge and Pacific ridge; volcanic islands such as Iceland.

c.       Transform boundary

This is the boundary which separates the plates that move past each other (sliding plates). Along these boundaries earthquakes and faulting are common phenomena.

NB earthquakes are common phenomena along all plate boundaries. A detailed account of earthquakes is described below.

Formation of mountains

A mountain is simply defined as a higher ground than its surrounding. Mountains are classified into four groups namely volcanic mountains, fold mountains, block mountains and residual mountains.

a.      Volcanic mountains

These are mountains that are formed from the accumulation of lava on the surface of the earth. They are also called mountains of accumulation. They are categorized into three namely ash and cinder cones, lava cones and composite cones.

i.        Ash and cinder cones

These are formed from the accumulation of lava on the surface of the earth during eruption. The lava is forced out of the crust with great pressure, thereby thrown to height and broken into small fragments (cinder). Good examples of such cones include Vulcano de Fuego in Guatemala and Paracutin in Mexico.

ii.      Lava cones

These are formed from either fluid lava or viscous lava. Fluid lava leads to formation of cones that have gently sloping sides such as Mount Mauna Loa in Hawaii whereas viscous lava leads to formation of cones that have steeply sloping sides such as Mont Pelée in Martinique. Such lava cools rapidly on the surface of the earth.

 

iii.    Composite cones

These cones are formed from lava and ash that are laid alternately in layers during eruption. The eruption begins with violent eruption of ash followed by eruption of lava. Lava often escapes from the sides of the cones to form small conelets. Good examples of such cones include Mount Kilimanjaro (Tanzania), Mount Cameroon (Cameroon), Vesuvius (Italy) and Stromboli (Italy), Etna (Italy).

b.      Fold mountains

These are also referred to as mountains of elevation since the earth’s crust is lifted up. They are formed when the crust bends (fold) in response to compressional forces under which it can be when the plates move towards each other.

Compressional forces lead to four forms of crustal folding.

i.        Simple fold: has equal limbs (sides of the fold)

ii.      Asymmetrical fold: one limb is steeper than the other

iii.    Overfold: one limb is pushed over the other

iv.    Overthrust (nappe): one limb is pushed forward over the other and has a fracture (fault).

The diagram below shows the folds.

Examples of Fold Mountains include the Andes of South America, the Rockies of North America, Himalaya, Atlas (Morocco), Drakensberg (South Africa).

The Fold Mountains are the highest and longest mountains in the worlds. They are usually associated with volcanic eruptions and earthquakes.

Influences of Fold Mountains

i.        The act as barriers to communication that is they hinder the construction of the roads and railways which are significant in the transportation of goods and people from one point to another

ii.      They give rise to important rivers such as Indus, Colorado, Colombia, Ganges which significant for HEP production and irrigation

iii.    They contain precious minerals such tin, copper, gold spur economic growth of countries through exportation

iv.    They have valuable timber resources such as coast ranges of Western America and foothills of Himalayas

v.      They act as climatic barriers as they divide the region into two of which one has climatic conditions different from the other. For example, western coast of North America experiences mild winters and warm summers and throughout the year while the Eastern region of the Rockies experiences cold winters and hot summers and maximum rainfalls in summer.

c.       Block mountains

These are mountains formed by the uplifting of crustal block as a result of the either compressional forces or tensional forces.

i.        Tensional forces

The crust may be under tensional forces when the plates move apart. Under such circumstances, it is divided into three parts (blocks) by faulting (formation of normal fault). While the middle block sinks (subsides) the side blocks rise to become block mountains as shown in the diagrams below. These mountains are characterized by steep sides and generally flat surface (plateaus). Such being the case they are called horsts such as Black Forest, Mount Ruwenzori, Korea

ii.      Compressional forces

The crust might be under compressional forces when the plates move towards each other. This folding yields faulting (formation of reverse fault) which divides the crust into two crustal blocks. One block subsides (sinks) while the other rises to become a block mountain as shown in the diagrams below. These mountains have one side steeper than the other (escarpment). As such they are called tilt blocks such as Deccan plateau, Arabian plateau and Brazilian plateau.

NB. The blocks that subside (are depressed) become rift valleys or great basins.

The lifting up is facilitated by the theory of isostacy which explains the vertical and horizontal movement of continental blocks which are floating on the semi-liquid layer of the asthenosphere.

d.      Residual mountains

These are mountains that are formed as a result of denudation processes (weathering, erosion and transport), hence they are referred to as mountains of denudation.

The earth’s crust consists of more resistant rock and less resistant rocks to erosion. Under denudation, the less resistant rocks are carried away leaving behind the more resistant rocks which become the mountains as shown in the diagrams below

NB. Denudation processes encompass weathering, erosion and transport. Weathering is the breakup of rocks by alternate heating and cooling; chemical actions and action of living organism. Erosion is the breakup rocks by action of rock particles being moved over the surface of the earth by running water, wind and moving ice. Transport is the movement of rock particles over the earth’s surface by water, wind and ice.

NB: Mountains of denudation are often formed from volcanic features which are formed from magma that solidifies within the crust such as batholith, laccolith

Batholith is the large mass of magma that accumulates in the crust. It forms the root of mountains. It is exposed by processes of denudation

Laccolith is the large mass of magma that solidifies near the surface of the earth.

Sill is the sheet of magma that solidifies along the bedding plane.

Dyke is the sheet of magma that solidifies across the bedding plane

The diagram below shows the volcanic features that formed within the crust

Earthquakes

These are vibrations of the earth’s crust as a result pressure resulting from development of faults as the plates collide or movement of the magma (melted rocks) along the fissure (fault) in the crust.

Earthquakes originate from the focus (a point in the earth’s crust) and travels through waves to different parts of the earth’s surface including the epicenter (a point on the surface of the earth which is directly above the focus).

Earthquake waves can be body waves through which earthquakes travel within the crust or surface waves through which earthquakes travel on the surface of the earth.

The diagram shows how earthquakes are formed

Effects of earthquakes

a.       The cause loss of lives and property

b.      They cause vertical and lateral displacement of parts of the crust

c.       They cause the rising and lowering of the parts of the sea floor which give rise to tsunami (tidal waves)

d.      They cause the rising and lowering of coastal regions

e.       They cause landslides

Theory of continental drift

Continental drift is simply defined as the horizontal movement of the crust in which the continents seemingly move apart. It is the basis of the continental drift theory which states that the continents originated from a single super continental called Pangaea which later broke into various continents. It explains that the continents attained the present positions as a result of moving apart of continents. The Pangaea was surrounded by the super sea known as panthalassa.

The Pangaea split into Laurasia and Gondwanaland with time. Laurasia also divided into North America, Greenland and Eurasia, so did Gondwanaland into Africa, South America, Australia and Antarctica. 

The diagram below show continental drift

Evidence of continental drift

a.       The shapes of South America and Africa seem that they can fit in a jig saw which implies that they were once joined.

b.      The structure of rock ranges of Argentina and Cape of South  Africa are similar

c.       Rocks of the West Africa and South America are similar

d.      Presence of glacial deposits in Congo basin

e.       Similar magnetism of ancient rocks in all continents

f.       Fossilized trees in the Antarctica

Weaknesses of continental drift

a.       There is no indication that continental drift is currently taking place

b.      There is no indication that that there are great forces that can move the continents to the positions at which the continents are today

c.       Some continents do not have shape that shows that they can fit in a jig-saw

Shields

These are parts of the earth’s crust which have never gone vehement changes but have remained quiescent (stable) over a very long period of time.

These include Laurentian, Brazilian, Baltic, African, Arabian, Siberian, Deccan, China and Australian shields 

The map below shows shields

Rocks

There are three major groups of rocks according to how they are formed namely igneous, sedimentary and metamorphic rocks.

a.      Igneous rocks

These are rocks formed by solidification of magma either on the surface of the earth or within the crust. They are divided into volcanic (extrusive) rocks and plutonic (intrusive) rocks

i.        Volcanic rocks

These are rocks which are formed from lava that solidifies on the surface of the earth. They contain small crystals due to rapid cooling of magma. Examples include basalt, pumice, andesite etc.

ii.      Plutonic rocks

These are rocks which are formed from magma that solidifies within the crust. They contain large crystals due to slow cooling of magma. Examples include granite, gabbro etc.

Characteristics of igneous rocks

i.        They are crystalline

ii.      They are not stratified

iii.    They do not contain fossils (remains of animals and plants)

b.      Sedimentary rocks

These are rocks which account for a large proportion in the earth’s crust. They are divided into three categories according to how they are formed.

i.        Mechanically formed rocks (clastic rocks)

These are rocks formed by compaction (compression) and cementation (lithification) of rock particles called sediments (clastics). Examples include loess (wind-deposited rocks), gravel and alluviums (river-deposited rocks), moraines, sandstone and boulder clay (glacier-deposited rocks), breccia, conglomerate, siltstone, limestone

The diagram below shows how clastics rocks are formed

ii.      Chemically formed rocks

These rocks are formed in precipitation or evaporation processes such as rock salt, gypsum, borax, and potash

iii.    Organically formed rocks

These rocks are formed by decomposition of fossils (plant and animal remains). From plants, examples are peat, lignite, coal while from animals, chalk and coral

Characteristics of sedimentary rocks

i.        They are fossilized

ii.      They are stratified

iii.    They are not crystalline

c.       Metamorphic rocks

These are rocks which are formed from pre-existing rocks by great heat and pressure which change the structure and appearance of rocks. The process under which these rocks are formed is metamorphism.

There are three types of metamorphism which include:

i.        Thermal metamorphism

This is a process influenced by heat that leads to re-mineralisation and the change of rock appearance. In re-mineralisation, new minerals are added to the rock. 

ii.      Dynamic metamorphism

This is a process in which the great pressure influences changes in the rock which lead to recrystallization and re-mineralisation 

iii.    Regional metamorphism

This is influenced by both heat and pressure

iv.    Metasomatism

This is a process in which chemical processes occurs leading to changes in the rock that is chemical reactions.

The table below shows examples of metamorphic rocks

Metamorphic rock	Formed from
Marble	Limestone
Slate	Clay
Gneiss	Granite
Quartzite	Sandstone
Graphite	Coal
Schist	Slate, granite

Rock cycle

This is the process by which rocks change from one form to another within or beneath the earth’s crust. Igneous rocks can be derived from either sedimentary or metamorphic rocks through igneous activities. Igneous activities encompass melting of rocks and solidification (cooling) of magma. Sedimentary rocks can be formed from either igneous or metamorphic rocks through compression and lithification processes while metamorphic rocks can be formed from igneous rocks or sedimentary rocks through metamorphism process.

The diagram below shows the rock cycle

Importance of rocks

a.       They contain precious minerals such as uranium, aluminium, iron which can be exported to earn foreign exchange

b.      They important for farming in form of soil

c.       They are used in construction of roads and buildings

d.      They are used as fuel such as coal

e.       They provide beautiful scenery in form of mountains which influences tourism thereby earning foreign exchange