Topic 2: Forces That Affect The Structure Of The Earth - Geography Form 3 - Dukarahisi

Topic 2: Forces That Affect The Structure Of The Earth – Geography Form 3

Geography Notes Form Three (3) - All Topics, Map Reading And Interpretation 2 Topic 5: Map Reading And Interpretation - Geography Form 3, Map Reading And Interpretation, Topic 2: Forces That Affect The Structure Of The Earth - Geography Form 3, Topic 7: Application Of Statistics - Geography Form 3, Topic 6: Photograph Reading And Interpretation - Geography Form 3, Topic 4: Elementary Surveying - Geography Form 3, Topic 3: Soil - Geography Form 3, Topic 1: Structure Of The Earth - Geography Form 3

Topic 2: Forces That Affect The Structure Of The Earth – Geography Form 3

What are forces ?

Forces are the processes that operate (work) within or on the earth’s crust

here 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 )


These are forces that operate within (inside) the earth’s crust
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


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.


⇒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


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.


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.






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.