GEOGRAPHY O LEVEL(FORM THREE) NOTES – FORCES THAT AFFECT THE STRUCTURE OF THE EARTH

FORCES THAT AFFECT THE STRUCTURE OF THE EARTH

What are forces ?

Forces are the processes that operate (work) within or on the earth’s crust
There are different forces that affects the earth’s but can be grouped into two major types :-

1.Internal forces (endogenetic/endogenic processes)
2.External forces(exogenetic /exogenic forces )

INTERNAL FORCES (ENDOGENETIC/ENDOGENIC)

These are forces that operate within (inside) the earth’s crust
OR
These are forces which operates beneath (under)the earth’s surface. These forces are generally referred to as TECTONIC FORCES.

A word tectonic is derived from a Greek word tecton which means builder

♦ Tectonic means building

– internal forces(tectonic forces) which are divided into

(1)Earth movement (Diastrophism)

(2)Vulcanism/ Vulcanicity/Volcanic erruptions

EARTH MOVEMENTS
These are also known as Diastrophism
Definition: is the movement of the solid parts of the earth towards each other or away from one another or side way.

Types of Earth movement.
Earth movements are classified into two (2) main groups:-
(i) Vertical or radial movements
(ii) Lateral or horizontal movements or tangential.

I ) VERTICAL OR RADIAL MOVEMENTS.
⇒These are the upward and downwards movements or forces .These forces cause the uplift (epeirogenic) and the downward movement (cymatogenic).

⇒These forces which causes the vertical earth movements operate from the interior upward toward the surface or downward from the surface to the interior.

These forces cause

(a) The crustal rock to fault. When faults develop produce feature like plateaus, basin, Block Mountain (host) and escapements.
(b) Sea level changes due to the upward lift of the land or sinking of the land.

NB: This changes in the sea level is not eustatic change (not eustatic movement)but is due to vertical forces.

⇒The eustatic change is the changes of the sea level due to ice melt during ice ages

II LATERAL/HORIZONTAL MOVEMENTS
These are also referred as organic forces (movement) because they are responsible for the build of the mountains (Orogenesis means the process of mountain building)
lateral forces are of two(2) types:-

1) Compressional forces
2) Tensional forces.

Compressional forces: – Are forces which move towards each other ie move against each other.
– They tend to shorten the crust (the land) i.e. they squeeze the land.

They normally cause :-
(i) Folding of land hence fold mountains
(ii) Break the land to form faulting which may produce features like block mountains, rift valley and faults.

Tensional forces: – Are forces that tend to stretch the land i.e. the force move away from each other, they pull the land away.

–The forces cause faulting of the crust and produce features like faults, Block Mountains, rift valley.

FEATURES PRODUCED BY COMPRESSIONAL FORCES

1.Fold Mountains.
Fold Mountains are formed when a piece of the crust is subjected to compressional forces. These forces tend to shorten the crust so the land end up in folding(wrinkling or crumpling) hence develop fold.

-The earth’s crustal rocks produced upward fold known as anticlines (up fold) and downward folds known as synclines (down fold)

(a)Before folding (b)After folding

The anticline the crest part while the syncline is the trough part.

TYPES OF FOLDS

i)simple folds or symmetric fold:- the fold has equal limbs.

ii)Asymmetric fold :-is the fold with one limb steeper than the other.

iii) Over fold:-one limb is pushed over the other.

iv) Recumbent fold:-the limb is pushed over the other completely.

v)Overthrust fold(nappe):- when temperature is very great a fracture occurs in the fold (such that one limb breaks off)and the one limb is pushed over the other limb along the line of thrust plane

Example of fold mountains are Atlas (N. Africa), Aplas(in cape ranges in s. Africa),Himalayas(in Asia),Urals(in Rocky of N.America), Andes(s. America), Appalachian(U.S.A) Great divide range in Australia.

2) Rift valley (grabens).

This can be formed by both tensional and compressional forces.

Rift valley formed by compressional forces

How it formed:-

(a)layers of rocks are subjected to compressional force

b)Fault develop

c)The side blocks(outer blocks) move (thrust)upward over the center block

(d)later the over-hanging sides of the rift valley are removed (worn & back)by erosion

3) Block mountains (Horst) by compressional

Are formed when compressional force cause the formation of series of fault, such that the central block forced up (to form a block).Examples usambara, Ruwenzori, Vosges and Black forest.

How is block mountains formed by compressional force:-

a)A piece of crustal rock is subjected to compressional force

b) A series of fault develops.

c) The side blocks (outer blocks)and central block move (thrust) upward.

NB: Where series of faults occur both block mountain and rift valleys develop.

FEATURES PRODUCED BY TENSIONAL FORCES

Tensional forces can frm due to fault there are several features which may be produced they include Rift valley, block mountains(horsts) and fault.

1)Rift valley (Grabens)

⇒ A rift valley is an elongated trough with steep sides.

⇒A rift valley can either be formed due to tensional or compressional forces .

Formation of a rift valley by tensional forces

a).The line of weakness develop to form normal fault (Normal faults are formed )

b). The central block subsides(sinks)to form a rift valley

3. Block mountains (horsts) by tensional

These are tables like mountains formed due to fault

They are note extensive like fold mountains ,they have almost flat surface

Example Vosges and Black forest mountains(Europe) -Usambara, Uluguru in -Ruwenzori and Uganda boarder-Mount Sinai in asia.

Formation of blocks(horsts)by tensional forces.

a)A piece of crust is subjected to tensional force.

b)A series of fault develo

c)some parts subside leaving some standing high as block

where series of fault develop both block mountains and rift valleys are formed.

Fault scarp.

This is an escapement which is a steep slope when the land falls from a higher to a lower level

-it is formed soon after displacement

Note

First -Tensional forces cause fault only

-Tensional forces cause the normal fault

Second-compressional forces cause faulting and folding
-compressional force causes the reverse fault

Faulting: is the process which involves the fracturing of the crust rock due to tectonic forces to form faults.

This process is associated with the movement. The rock block (i.e. rock displacement

A fault –is a fracture in the crust due to stress or strain associated with rock displacement.

Fault is cause by both tensional and compressional forces

Joint: is a crack in the rock due to stress but is not associated with rock displacement

-The paths of a fault on the surface of the earth s called the fault line (line of fault)

-some of the fault line may cover hundreds of kilometers.

FOLDING :is the wrinkling ,bending or crumpling of the earth crut producing upward folds(anticlines) and downward folds (synclines)

-folding is caused by compressional forces.

Significance of
features produced by earth movements(effect of feature produced by earths movement).

-Both vertical and lateral force cause either folding and or faulting which in turn from fold and block mountains ,plateaus , rift valley basin and other feature s

These different features have the following positive (good)effects and negative( bad)effects.

Positive (good) effects

1.climatic influence mountain both fold and block receive orographic rainfall due to relief influence the windward side receive heavy rain at ll temperature are modified into cool(cold)

2.source of river : d to heavy rains and snow on mountains many rivers originate from them. Water used for irrigation.HEP generation, industrial and domestic uses.

3.source of minerals:-different minerals are obtained from fold mountain such as coal in Appalachian mountain gold in Colombia, silver in Peru.

-other mineral from block mountains example dolomite

4.attract tourism

Mountain skimming attract a lot of tourists for sporting activities (skidding) e.g. the cross Alps

-also rift valley forms a very attractive area due to presence of different lakes like Tanganyika , Natron, Manyra, Naivasha, etc ,presence of different volcanic cones and crater within it

5.slope of mountains have fertile soils for agriculture

Negative (bad) effects

1.The leeward side receive little or no rainfall hence hinder agricultural and a lot

2.sometimes folding causes some minerals to be buried deeper hence not easy to exploits

3.fault may cause some rivers to disappear I the ground

4.both mountains and rift valley hinder communication network

some mount sides are too steep or ruge, or have thick fog hidering both land and air transport

-earth movement also cause earthquakes and volcanic

EARTHQUAKE

These are sudden earth movements or vibration in the earth’s crust; they are caused by.

i. When one tectonic plate sliding over/or past another plate along the line of a faulty

ii. Volcanic eruption- The movement of molten rock below or onto the earth’s crust which in turn is caused by the movement of plates.

Nature of Earthquakes

•The point of which on earthquake is originate is called focus. And sometimes it is several kilometres below the surface.

•The point on the earth’s surface immediately above the focus is called the Epicentre, This is where the shock waves first hit the surface. It is the shock waves which gives rise to an earthquake.

Types of shock waves

There are two types of shock waves:-

1.Body waves.

Are waves which travel through the erust and are of two types

i. Primary waves- which cause the crustal rock to move back and forth in the direction of wave movement

ii. Secondary waves- Are waves which cause the crustal rock to move side to side ie right angles to the direction of wave movement.

2.Surface wave.
These travel through the surface and are of two types
i.Love wave;- Which cause the surface rock to move side to side of right angles to the direction of wave movement.
ii. Rayleigh wave;- Wave which cause the surface waves to have a circular movement very similar to that of water wave movement.

I/
II/

DETECTING MAGNITUDE AND INTENSITY
The intensity of an earthquake is measured by an instrument called seismograph, This is the instrument which record the vibration produced by an earthquake.
The magnitude of an earthquake -refer to the total amount of energy released and the scale which gives the magnitude is called the Richter scale, The scale range from 0 to 8.9
Effects of Earthquake

•Rise and fall of the sea floor Eg. Agadir Earthquake in Morocco in 1960

Displacement of the earth’s crust, it can happen vertically or laterally.
Land slides and open up deep cracks in the surface rocks eg. The El Asnam earthquake in Algeria 1954 destroyed an area of radius 40km and open surface cracks up to 3m deep.
Destruction of infrastructure and properties
Loss of life, can lead to death’s
They can raise or lower erosion rocks eg. In Alaskan the earthquake of 189 lead to raise of some rock for about 16km.

2. VOLCANIC ERRUPTIONS/VULCANICITY/VULCANISM

The map above is a part of a recently released world map that shows, in blue, the presence of the underground water

The map above is a part of a world map that shows, in red, the presence of Active volcano centers

VULCANICITY

Vulcanicity is the range of processes by which molten materials and gases are either intruded (injected) or extruded (ejected) into the earth’s crust or into the earth’s crust respective

Vulcanicity is the formation of various feature due to the intrusion or extrusion of molten materials, and gases.
The molten materials are called magma when found within the earth’s crust and magma when poured on the earth’s crust.

Vulcanism/Vulcanicity – Is a broader term which includes both extensive and intrusive igneous activities while volcanicity – Refers to the extensive volucancity in which the materials are forced at onto the surface.

Origin of magma

Origin of magma is within the earth’s crust where it kept as a molten rock due the influence of temperature.

Magma can be ejected out through fissures or vent.

Magma when passed through vent result the formation of volcanic features like volcanic cones.
Magma when passed through fissures leads to the formation of lava plateau.

Types of vulcanicity

There are two types of vulcanicity which are classified as follows

1. INTRUSIVE VOLCANIC FEATURES

This is when magma intruded within the earth’s interior. The features resulted due to the intrusive volcanic erruption is called intrusive features.

The intrusive volcanic features are the features which are found within the earth’s interior. The following are intrusive volcanic features.

Sill – is a rock sheet formed when the magma solidifies horizontally along the bedding plane. Eg:- Tyolo scarp in Malawi

Dyke- is a rock sheet formed when the magima solifies vertically across the bedding planes. Eg:- Kinkon

Laccolith is the cone dome shaped mass of rock with flat formed of viscus lava. It look like a mushroom. Eg:- Laccolith found in Morafonobe in Madagases.

Lapolith –is a scar shaped mass of rock formed in glosyncline. It forms a saucer – like shape may be due to the increased weight of the deposits

Phacolith – is a leans shaped strip of igneens rock formed when the magima solidies along the anticline or syncline. Eg: Cordon wills in U.K Phacolith

Batholith is the large mass of solidified rock formed when magma cools plutonically at the great dept Eg: at the heart of the mountain ranges. Eg:- Chilu Batholic in Gabon

2.Extrusive volcanic features

Are the features formed when the magma cools and solidifies on the earth’s surface.

The extrusive volcanic features can be classified into the following

I. Central features

II. Fissure features

The central features due to the violent eruption include

Ash and cinder cone. Lava is blown to great heights when it is violently ejected and it breaks into small fragments which fall back to earth and build up a cone, example Busoka and Bitale in South west Uganda.

• Composite cone

This type of cone is formed of alternate layers of lava and ash. The volcano begins each eruptions with great violence forming a layer of ash. As the Eruption proceeds the violence ceases and lava pours out forming a layer on top of the ash, Lava often escapes from the sides of the cone where it builds up small conelets.Example mount Kilimanjaro in Tanzania and mount Cameroon

• Volcanic plug/plug dome volcano. Is a rigid cylindrical plug formed when very viscous lava is forced out of the volcano and form a plug dome that may completely block the vent. Good examples of a plug dome occurs in the Atakor volcanic area of Haggar mountains in Algeria. These are almost 300 plug domes in these region

• Crater is a depression formed on the summit of the cone after the plug dome has been blown off on the top of the cone.

• Caldera is a large crater formed when the upper part of the volcano is either bombarded away by a violent eruptions or subsides into the crust. Eg:- Alaska, Ngorongoro in Tanzania, Eboga in Cameroon

Central features due to peacefull eruptions
The lava coming onto through the vent can be acidic and hence viscous or basic and hence less viscous.
The following are the central features due to peacefull eruptions
• Acid lava cone/comulo dome volcano- is the dome shaped volcano with convex slopes formed when acidic lava solidifies around the vent Eg:- Ntumbi dome (Mbeya)

•Shield volcano:- is an extensive cone with gentle sloping sides formed when basic (basalt) is poured on the surface and spread to occupy large area.
Sometime times these features can be formed when lava comes out through a single fissure or many fissures.

• Fissure eruption

The fissure eruption involve lava coming out through a crack or fault. The feature formed is called lava plateau.

Minor volcanic features

A variety of minor volcanic forms can be distinguished, usually though not necessarily associated with volcanoes approading extinction. They include the following
•Mofettle – is the volcano which emit carbondioxide gas.
•Hot spring – is the volcano which emits hot water.
•With dissolved minerals suspension.Salfatara – is the volcano which emits sulphur gas.
•Fumerole – is the volcan which emits steam, mud and other gas than sulphur.

OR:-

Is the outflow of superleated water from the ground which contains some mineral substances in solution or in suspension

Hot springs mostly occur in island or in some of the African countries such as Kenya and Ethiopia.

1.Geysers – are the forceful omission of hot water from the ground to high level in the atmosphere when emitted contains fine material forms mud volcano which later can lead to the formation of very fertile soild

Types of volcanoes according to the activity

There are three types of volcanoes according to the activities. These include:-

I. Active volcano: is the volcano which erupts frequently. Eg:- Oldonyo Lengai in Tanzania and Mount Cameroon.

II. Dormant volcano is the one which has stopped erupting but not extinct and it is expecte to erupt.

The dormant volcano is also known as sleep volcano.

III. Extinct volcano. Is the volcano which has stopped erupting for a very lon time in history and is not expected to erupt.

It is also known as dead volcano.

INFLUENCES OF VOLCANIC ERUPTION TO MAN AND ENVIRONMENT

The following include the economic importance of volcanic eruptions to man.

i. Lava on weathering head to the formation of very fertile soil which support agriculture.

ii. Volcanicity eruption lead to the formation of mineral deposits like copper deposits of butte in USA, diamond of kimberley in South Africa.

iii. Volcanic eruption provide geothermal power for electric generation

iv. Some hot hot springs utilized for heating homes in glaciated region

v. Volcanic features attract tourist

vi. Volcanic cones are the source of rivers.

DISTRIBUTION OF VOLCANIC ACTIVITIES IN THE WORLD

Distribution of volcanic activities corresponds to the earthquake belts. They are common along the zones of convergence like in the circum pacific ring and Alpine – Himalayan belt, zones of divergence like Atlantic ridge belt and along the fealty belts like the African rift valley.

EARTHQUAKE

These are sudden movements or vibration in the earth’s crust caused by

i) One tectonic plate sliding over another along a line of a features

ii) Volcanic eruption – The movement of molten rocks below or on to the earth’s crust.

iii) Mass movements like landslides and rock falls. Can causes earthquake but in local scale.

iv) Falling of large objects from the atmosphere such as meteorites lead to the shaking of the earth’s crust.

v) Man’s activities or influence, such as quarrying rocks using explosive like dynamite, transept vellsel’s e.g trains and heavy trucks

THE NATURE OF EARTHQUAKE

FOCUS: Is the point at which an earthquake originates and sometimes it is several kilometer below the surface

EPICENTRE: Is the point as the earth’s surface immediately above the focus. This is where the shock waves first lit the surface. It is the shock waves which give rise to an earthquake.

There are two types of shock waves.

i) Body waves

ii) Surface waves

i) Body waves – this travel through the crust and is of two types.

a) Primary waves – Which cause the crustal rock to more back and forth in the direction of wave movement.

b) Secondary waves – Which cause the crustal rock to more from side to side that is at right angles to the direction of waves movement.

ii) Surface waves – These travel through the surface rocks and of two types

a) Love (L) waves which cause the surface rocks to more from side to side , at right angles to the direction of wave movements

b) Rayleigh (R) waves which cause the surface rocks to have a vertical circular movement very similar to that of water in a sea were. The vibrations caused by the surface wave produce most of the damage that occurs in an earthquake. The violent shaking of the surface rocks often cause great damage to buildings and sometimes considerable loss of life.

HOW TO DETECT AN EARTHQUAKE

The intensity of an earthquake is measured by an instrument called a seismograph. This instrument records the vibrations produced by an earthquake. The magnitude of an earthquake refers to the total amount of energy released and the scale which gives the magnitude s called the Richter scale. This scale ranges from O to 8.9. A magnitude of 2.0 is ten times greater than that of 1.0 and one of 5.0 is 10,000 times greater than one of magnitude 1.0.

The intensity of an earthquake – refers to the effect produced by the earthquake of course these varies from place to place so while the intensity of a specific earthquake varies its magnitude does not vary. It is important not to confuse magnitude with intensity.

EFFECTS OF EARTHQUAKES

1) Development of rift valley or basins

2) Landslides can occur and these in turn can lead to land degradation

3) Collapse of houses and other structures as well as loss of life. E.g in 1906 the earthquake killed a number of 700 people in San – Transits

4) Fire outbreak in cities, San – Transits damage due to fire outbreak.

5) Tsunamis (waves in the ocean due to earthquake)

POSSIBLE AREAS WHERE EARTHQUAKE IS LIKELY TO OCCUR ON WORLD MAP.