THE PLATE TECTONIC THEORY

December 17, 2025 kindaya

THE PLATE TECTONIC THEORY

Tectonic plates

The earth’s crust and their major features are cracked in to huge pieces called tectonic

plates. These plates float on top of the semi-molten rock called asthenosphere. They

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move at very slow speed. The movements of the tectonic plates mean that some

continents are moving apart and some are moving towards each other. This process is

referred to as continental drift and has been going on for hundreds of millions of years.

Tectonic plate movements have been split up to form the continents as we know them

today.

The plate tectonic theory

Plate tectonic theory, was formulated by some scientists like Russian scientist known

as Dukcher in the mid of 1960 (20) th Century. However the plate tectonic theory is the

new version of continental drift theory which in reality come to correct some weakness

of continental drift theory.

The theory explain that, the earth crust and the upper part of rigid mantle are divided in

to several separately parts called plates. The plates are varying in size and movement

since the minor plates are moving fast while the major plates are moving slowly.

However those plates are categorized in to oceanic plates and continental plates.

According to the theory there about seven (7) major plates which are, Pacific plate,

North American plate, South American plate, Eurasian plate, African plate, Antarctic

plate and the Indian plate. The minor plates include, The Nazca plate, the Cocos plate,

The Juan de Fuca plate, the Philippine plate, the Arabian plate, the Caribbean plate, the

Indian plate and the Scotia plate. The theory assume that the plates floats on the

underlying semi molten mantle called Asthenosphere.

THE PLATES OF THE PLANET EARTH

Normallly the platent earth is made up by numbers of plates which are categorized in to

major and minor plates.

TYPES OF PLATES

1. CONTINENTAL PLATES

Continental plate: These are plates which are made of sialic materials (Silva and

Aluminium elements) of granite rock types. These plates form continents of the world.

They usually less denser. They have old rocks formed over 1500 million years ago.

Example of continental plates with old rocks are such, African continent, North

American continent, South American continent.

2. OCEANIC PLATES

Oceanic plates: Are the plates made up of simatic young and denser rocks of basalt

composition. ( Silca and magnesium elements). They usually form ocean floor. They

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usually much denser than continental crust. Oceanic plates consisted of young rocks

formed about 200 million years ago. Normally the boundary between lower crust

(oceanic crust and the lower mantle (Asthenosphere) is called Moho or Mohorovicic

Discontinuity.

THE MAJOR PLATES OF THE PLANET EARTH

Qn: The plate tectonic theory has identified several plates, critically discusss seven (7)

major plates of the planet earth.

The earth’s crust is broken up in to seven (7) major plates and several minor plates,

which are:-

1. The pacific plates: Is the largest oceanic crust plate in the planet earth. The plate is

presently shrinking as the Atlantic Ocean increases in size pushing the North American

Plate slowly westward. This movement has created subduction zones that cause

volcanoes to erupt, large subduction zone earthquakes, and tsunamis. Normally the

plate occupy the whole pacific region.

2. The North American Plate: This is a tectonic plate covering most of North America,

Greenland, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the

Azores. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky

Range in eastern Siberia. The plate includes both continental and oceanic crust. The

interior of the main continental landmass includes an extensive granitic core called a

craton. Along most of the edges of this craton are fragments of crustal material called

terranes, accreted to the craton by tectonic actions over a long span of time.

3. South America plate: This is a tectonic plate which includes the continent of South

America and also cover the region of the Atlantic Ocean seabed extending eastward to

the African Plate creating the Mid-Atlantic Ridge. The easterly side is a divergent

boundary with the African Plate forming the southern part of the Mid-Atlantic Ridge. The

southerly side is a complex boundary with the Antarctic Plate and the Scotia Plate. The

westerly side is a convergent boundary with the subducting Nazca Plate. The northerly

side is a boundary with the Caribbean Plate and the oceanic crust of the North American

Plate.

4. The Eurasian Plate: This is a continental crust plate which includes most of the

continent of Eurasia. It covered the whole part of European and Asian continent, the

Arabian subcontinent and the area east of the Chersky Range in East Siberia. It also

includes oceanic crust extending westward to the Mid-Atlantic Ridge and northward to

the Gakkel Ridge. The eastern side is a boundary with the North American Plate to the

north and a boundary with the Philippine Sea Plate to the south and possibly with the

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Okhotsk Plate and the Amurian Plate. The southerly side is a boundary with the African

Plate to the west, the Arabian Plate in the middle and the Indo-Australian Plate to the

east. The westerly side is a divergent boundary with the North American Plate forming

the northernmost part of the Mid-Atlantic Ridge, which is straddled by Iceland. All of the

volcanic eruptions in Iceland, such as the 1973 eruption of Eldfell, the 1783 eruption of

Laki, and the 2010 eruption of Eyjafjallajökull, are caused by the North American and the

Eurasian plates moving apart, which is a result of divergent plate boundary forces.

5. The India Plate: This is a continental plate in the eastern and largely southern

hemispheres. This plate occupy the whole part of Indian continent and Australia as well

as the oceanic crust of India and the parts of Pacific ocean. Plate tectonic theory

suggest that Gondwanaland, Australia remained connected to India and Antarctica until

approximately 100 million years ago when India broke away and began moving

Northward. Australia and Antarctica began rifting 85 million years ago and completely

separated roughly 45 million years ago. The Australian plate later fused with the

adjacent Indian Plate beneath the Indian Ocean to form a single Indo-Australian Plate.

However, recent studies suggest that the two plates have once again split apart and

have been separate plates for at least 3 million years and likely longer. The Australian

plate includes the continent of Australia, including Tasmania, as well portions of New

Guinea, New Zealand, and the Indian Ocean basin.

6. The African Plate/The Nubian plate: This is a continenal plate occupy the equator as

well as the prime meridian. It includes much of the continent of Africa as well as

oceanic crust which lies between the continent and various surrounding ocean ridges of

Atlantic ocean. Plate tectonic theory suggest that between 60 and 100 million years ago

the Somali Plate began rifting from the African Plate along the East African Rift valley.

Since the continent of Africa consists of crust from both the African and the Somali

plates, some literature refers to the African Plate as the Nubian Plate to distinguish it

from the continent as a whole.

7. The Antarctic plate: This is a tectonic plate containing the continent of Antarctica

and extending outward under the surrounding oceans. After breakup from Gondwana

(the southern part of the supercontinent (Pangea), the Antarctic plate began moving the

continent of Antarctica south to its present isolated location causing the continent to

develop a much colder climate. The Antarctic Plate is bounded almost entirely by

extensional mid-ocean ridge systems. The adjoining plates are the Nazca Plate, the

South American Plate, the African Plate, the Somali Plate, the Indo-Australian Plate, the

Pacific Plate and across a transform boundary, the Scotia Plate.

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MINOR PLATES

There about thirteen (13) minor plates where some of theme are such as:-

i. The Nazca Plate.

ii. The Cocos Plate

iii. The Juan de Fuca Plate

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iv. The Phillippine Plate

v. The Arabian Plate

vi. The Caribbean Plate

vii. The Indian plate

viii. The Scotia plate.

CAUSES OF PLATE MOVEMENTS

Normally the plate tectonic movement is caused by the following factors:-

1. Convectional current of mantle: During mantle convection some materials rise due to

the influence of radioactive heat generated within the interiors of the mantle and later

moves laterally below the lithosphere. Therefore lateral movements tend to drag the

lithosphere which resulted in to plate tectonic movements. On cooling the materials

sink down to the lower level of the mantle where they melt again due to the constant

motion of the plates. Normally to a large extent mantle convectional currents is caused

by movement of magma. The magma tend to move in different direction both vertical,

horizontal and circular form within the interiors of the mantle.

2. Formation of new oceanic crust: Normally the new oceanic crust is formed at the

mid oceanic ridge as the result of moving magma when it push the old rocks on the side

of the oceanic floor hence cool and solidifies. Hence this process cause plate

movement.

3. Cooling and sinking of oceanic crust: TNormally the oceanic crust is less denser

than asthenosphere, the high heat within the interiors cause the expansion of the

oceanic crust rocks and when the heat reduced cause the contraction (cooling) and and

when the oceanic crust cool become denser than the asthenosphere which sink to the

mantle and resulted in to the plate movement.

4. Gravitational different between oceanic ridge and oceanic trench: These are the

forces between oceanic ridge and ocean trench. Normally the risen part of the ocean

ridge consists of heavy materials which cause sinking of ocean trench. Hence this

process cause gravitational attraction and resulted in to plate movement.

5. Oceanic topography: This is also known as the elevation of the mid ocean ridge. It

believed that the elevated point of the mid oceanic ridge consisted of heavy weight

which cause sinking of materials which resulting cause the movement of plates.

However the plate may move away from each other, towards (collide) each other or

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slide over each other.

6. Isostatic adjustment: When the earth try to find its equilibrium due to the different

forces and disturbances acting upon this cause the movement of plates.

CONDITIONS/ASPECTS OF PLATE TECTONIC MOVEMENT

QN: By giving eight (8) points explain the principles governing plate tectonic

movements.

1. The continental crust does not sink down: This is due to the low density, hence

remain floating permanently on denser oceanic plate. Therefore the oceanic crust is

being formed and destroyed continuously.

2. Some continental plates consists of both oceanic crust and continental crust: This

include Eurasian plate.

3. Continental crust may extend far beyond the margin of the land masses.

4. Plates cannot overlap: Normally plates they may be pushed upward to form

mountains can be pushed downward in to mantle and destroyed but cannot overlap.

5. No gap may occur on the earth’s surface: Normally when two plates are moving

away from each other no gap can emerge because there is replacement within the

plates. Example when the continental plates are moving away from each other the

oceanic plate come to form the ocean floor.

6. The earth size is constant: This means that the earth’s size neither expanding nor

shrinking in size. Therefore when the new oceanic crust is formed in a one place the

older oceanic crust must be destroyed in another part.

7. Plate movement is slow: This means that plates are moving very slow and is usually

continuously process.

8. Most important landforms are found at plate boundaries: These include landforms

like fold mountains, volcanoes, islands, arc, deep sea trenches, and batholith intrusion.

Very little chance occur in plate centres.

THE PLATE BOUNDARIES/MARGINS

Plate boundaries: Are zones where two plates collide or separates and normally

resulted in to either creation of destruction of the crust. Normally the major causes of

plate tectonic movement is mantle convectional currents below the crust, however this

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is called endogenic forces which operate within the interior of the mantle.The line where

two tectonic plates meet is called a boundary.

TYPES OF PLATE BOUNDARIES

There are three main types of boundaries, These are:-

i. Divergent plate boundaries (Constructive boundaries)

ii. Convergent plate boundaries (destructive boundaries).

iii. Transform boundary (conservative boundaries)

A. DIVERGENT PLATE BOUNDARIES (CONSTRUCTIVE BOUNDARIES)

Divergent plate boundaries: Is the boundary or zone where two plates move away from

one anotherher. However the divergent plate boundary may be oceanic to oceanic or

continental to oceanic divergent. Example of divergent plate boundary is found in the

middle of Antlantic ocean (mid oceanic ridge) which cause the eruption of convectional

currents of magma which resulting push and cause the north America and south

America to move a way from Eurasian and African plate. Sometimes divergent boundary

is called constructive boundary because when two plates move away from each other

cause the eruption of materials (magma) which cause the formation of some features

like mid oceanic ridges.

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Types of divergent movement

i. Oceanic-oceanic divergent movement: It occurs when two oceanic plates moved a

way from one another. However this movement can led to the formwtion of mid oceanic

ridge. Normally when two oceanic plates move apart no gap will be formed because of

eruption of new materials from the mantle to fill the gap. Example: Mid Atlantic Ridge

with about 1000km to 2500m high. However also oceanic to oceanic divergent can

cause the formation of rift valley at the mid of oceanic ridge when the oceanic ridge

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break at the center. The Mid Atlantic Ridge is an example sea floor spreading. It formed

due to the divergent of South America and African plates.

ii. Continental-continental divergent movement: This movement occurs when two

continental plates separated from each other. Normally when two continental plates

move away from each other lead to the formation of rift valley. Example the Great east

African Rift Valley formed when the African plate moved North eastward from south

American plate which resulted in the formation of rift valley and volcanic eruption along

the fault lines.

2. CONVERGENT BOUNDARIES (DESTRUCTIVE BOUNDARIES)

Convergent plate boundaries: is the type of plate boundary which occur when two

plates collide or moves towards each other. Therefore, the convergent plate boundary

are can be oceanic to oceanic plate boundary, continental to continental plate boundary

or oceanic to continental plate boundary. Example The formation of ocean trenches like

Mariana Trench, fold mountain like Himalayas and volcanic islands like Japan. The

convergent movement cause sinking processes of plates. Therefore this sinking

process is called subduction. Normally the sinking process of oceanic plate will be

destroyed by melting in the mantle hence which resulted in to eruption of magma.

Types of convergent plate boundaries

i. Oceanic-oceanic convergent movement: It occurs when two oceanic plates move

toward each other and collide. Therefore the denser oceanic plate tend to sink down

beneath the convergence zone. However this movement can led to the ocean floor

spreading and the formation of new crust, ocean trench and formation of volcanic

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islands.

ii. Oceanic-continental convergent movement: It occurs when continental plate and

oceanic plate move towards each other and collide. However this cause the oceanic

plate to sink down because it is heavier than the continental plate. Therefore the

oceanic crust will be burned by mantle which resulted in to melting and led to the

formation of volcanic eruption. Example: South American plate collide to Nazc plate.

The south America plate to collide with east of Nazca plate. Therefore due to the fact

that Nazca plate is oceanic plate and it is so denser, will sink down beneath the south

American plate which is less denser at the point of collision.

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iii. Continental-continental convergent movement: This movement occurs when two

continental plates move towards each other. However in this movement the heavier

continental plate will sink down beneath the convergent zones. Therefore this

movement cause the formation of fold mountains. Example Himalayas, Atlas and Alps

mountain.

C. TRANSBOUNDARY/CONSERVATIVE MOVEMENT

Transboundary movement: It occurs when two plates slide over each other. Normally

this movement sometimes is called neutral or Trans current boundary. It usually

responsible by occurrence the earth quakes and the formation of faults along the line of

weakness where the plates slide over each other due to either uplift or subduction of

the crustal rocks (lithosphere). Example: North America, San Francisco – San Andrea’s

faults displacement of features of about 1000 km.

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LANDFORMS /FEATURESPRODUCED BY PLATE TECTONIC MOVEMENT

Qn: Elaborate seven landforms which resulted from plate tectonic movement.

Normally the plate tectonic movement is responsible by the formation of different

labdforms which largely determined by the nature of that movement which ocvured.

A. LANDFORMS WHICH RESULTED FROM DIVIGENT MOVEMENT

Normally the the divergent movement resulted in to the formation of the following

landforms:-

1. Formation of mid oceanic ridges: It occurs when two oceanic plates separate or

move away from each other.

2. Formation of oceanic islands: The oceanic island when two oceanic plates move

away from each other it can cause eruption of magma which resulted in the formation

of volcanic islands.

3. Formation of volcanic mountains: It occurs when two continental plates moved a

way from each other the gap between moved plates give room for eruption of magma

and formation of volcanic mountains. Example mount Kilimanjaro was formed when the

African (Nubia) plate moved eastward.

4. Forrmation of riftvalleys: The divergent movement is responsible for the formation of

riftvalley which occurs when two continental plates moved apart, the gap between two

two moved plates give out the formation of rift valleys. Example the Great East Africa

Rift Valley which run from middle east to Zambia.

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B. LANDFORMS PRODUCED BY CONVERGENT BOUNDARY

Converging plate boundary causes a number of associated Features as follow

1. Formation of ocean trenches: These happens whe two oceanic plates collide each

other hence the hevier one sinkdown beneath the less denser plate. Example Marianna

trench in pacific ocean and Japan trenches.

2. Formation of volcanic islands: They are formed when oceanic and continental plates

collide hence, the oceanic plate sink down beneath the continental plate. Therefore this

cause the formation of volcanic mountains along the coastal boundaries and also

result into trenches.

3. Formation of fold mountains: It formed when two continental plates collide. Example:

Mount Himalayas was formed when Indian and Russian plate collide.

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C. LANDFORMS FORMED BY TRANSBOUNDARY

1. Occurence of earthquake and faulting: Normally the neutral or Trans current

boundary is responsible by occurrence the earth quakes and the formation of faults

along the line of weakness where the plates slide over each other. However there is

neither uplift or subduction of the crustal rocks (lithosphere). Example: North America,

San Francisco – San Andrea’s faults displacement of features of about 1000 km.

EFFECTS OF PLATE TECTONIC MOVEMENT

QN: Assess six effects which resulted from plate tectonic movement.

The plate tectonic areas are areas of instabilities which results to earth quake areas are

volcanoes. The theory of plate tectonic can help to explain almost all of the landforms

on the surface of the earth.

1. It contribute to the formation of deep sea trenches: The plate tectonic movement

cause the formation of deep sea trench. The sea trench is a long deep valley along an

ocean floor formed when two oceanic plates collid and the heavier one sink down.

Example the Mariana trench found in the western Pacific with a depth of more than

36000 below the sea level.

2. It led to the formation of mid oceanic ridge: This a result of divergent movement

when plates moved a way from each other. Oceanic ridgeis the part of ocean floor rised

above as a mountain range made up mostly basalt. It may be more than 80000km long

and 1500 to 2500km wide and it may rise to 2.3km above the ocean floor. The feature is

associated with a divergent plate boundary. As plate diverge, magma rises repeated and

eventually cools to form the mid oceanic ridge. Example: East Pacific as Nazea and

Pacific diverges North Atlantic as North America diverges the Eurasian.

3. It contributes to the formation of Islands: Are caused by both convergent and

divergent movement. Sometimes basalt eruption along the ridge or near may build up

volcanoes that protrude above sea level to become Oceanic Island.They may vary in

size. Example: Iceland, Japan, Hawaiian is lands, Mauna Loa, Easter Islands near the

East Pacific ridge, west Indies.

4. It cause the formation of magmatic Arc: It refers to island arcs at sea and belts of

igneous activity on the edges of continents such as batholiths in mountain belts.

Example: Aleutian Island. cascade volcanoes of the pacific North West,along Andes.

5. It contributes to the formation of mountain Belts: At a convergent collision

boundary,the sea floor is denser and will be subducted making the ocean thinner and

narrower hence collision of the continents. Eventually the oceanic lithosphere breaks off

leaving the continental crumple to form mountain ranges.The thick sequences of

sedimentary rocks that had built upon both continental margins are intensively.

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Example The Himalayas as India collided lided, Alps as African and Europe, Atlas in

Northern Africa. In addition to that at a convergent destructive boundary, young

mountain like the Andes form due to the folding of the young sediments.

6. It contributes to the formation of rifting and associated features: At a passive

divergent plate boundary ,the continental crust is stretched and thinned producing faulty

landforms like the great East Africa Rift Valley.The faults may be path associated

volcanic landforms.This may explain the volcanic landforms in the rifted areas of East

Africa. Due to thermal expansion emanating from rising mantle plume, it causes uplift

of landscape.

7. It cause the formation of new Oceanic Crust: This is associated with a divergent

plate margin where the would be gap is filled with the up welled magma to form a new

oceanic crust.

IMPORTANCE OF PLATE TECTONIC THEORY

Qn: Discuss six usefulness of plate tectonic theory to Tanzanian society.

Qn: How the plate tectonic theory become relevant to the today’s geographers? (Seven

points)

Normally the plate tectonictheory is important and it has relevant to all people who are

interested on geological studies due to the following aspects:-

1. The theory help to explain the formation of rift valleys on different parts of the world.

2. It help geographers to understand causes of plate tectonic movement and their

associated features.

3. The theory helps in explaining the causes and distribution of earthquake.

4. The theory helps in understanding the distribution of volcanic activities worldwide.

5. The theory helps in explaining the reasons behind the sea floor spreading.

6. The the theory provides base for studying indogenic forces affecting the planet earth.

7. The theory has provided framework for further geological studies to the learners.

WEAKNESS/SHORTCOMINGS OF PLATE TECTONIC THEORY

1. There is weak evidences on the thermal convection of the mantle which is

considered among the driving forces of plate motion.

2. There is assumption that, the solid lithosphere is pushed down to the mantle to the

depth of about 700km.

3. The theory has faced to provide clear evidences on the formation of many faults in

different parts of the earth which are common to both continents.

4. The theory did not explained well to why the old rock cannot be formed near the crest

of Mid oceanic ridge.

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THE PLATE TECTONIC THEORY

THE PLATE TECTONIC THEORY

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