PLATE TECTONIC THEORY – ADVANCED PHYSICAL GEOGRAPHY

PLATE TECTONIC THEORY – ADVANCED PHYSICAL GEOGRAPHY

Plate tectonic theory is the theory that describe the earth’s outer shell (lithosphere) is divided into several plates that glide (move) over earth’s mantle. OR

Plate tectonic theory is a scientific theory describing the large-scale motion of the plates making up earth’s lithosphere. OR

Plate tectonic theory is a scientific theory that explain how major landforms are created as a result of earth’s subterranean movements.

The word “tectonic” comes from Greek word “tekton” which means “builder”. This theory was developed by several scientists after comprehensive research such as Dan McKenzie, L. Pichlate, R. Parker, Frederick Vine, John Tuzo-Wilson and Drummond Matthews from U.S.A in 1960’s.

The Plate Tectonic Theory state that; the earth’s lithosphere is comprised by a number of separates segments called Tectonic plates (lithospheric plates), these plates are mobile floating on partial molten materials of the upper mantle in asthenosphere. The movement in these plates is horizontal but extremely slow motion driven by convectional currents of magma in the asthenosphere.

There are seven major plates and several minor (small) plates. These are: –

1. The major plates are as follows;

1. Antarctica plate.
2. North American plate.
3. South American plate.
4. Pacific plate.
5. Indio-Australia-New Zealand plate.
6. Africa with the eastern Atlantic floor plate.
7. Eurasia and the adjacent oceanic plate.

2. The minor plates are;

1. Cocos plate: Between Central America and Pacific plate.
2. Nazca plate: Between South America and Pacific plate.
3. Arabian plate: Mostly the Saudi Arabian landmass.
4. Philippine plate: Between the Asiatic and Pacific plate.
5. Caribbean plate.
6. Philippine plate.
7. Scotia plate.
8. Juan De Fuca plate.
9. Goda plate.

TYPES/FORMS OF LITHOSPHERIC PLATES

There are two (2) forms or types of lithospheric plates. These are: –

1. Oceanic plates:

These are the plates comprised by oceanic crust and solid upper mantle, mostly without presence of land masses. They are totally covered by oceans or seas.

Example of oceanic plates are Nazca plate in the Pacific Ocean, etc.

2. Continental plates:

These are plates comprised mostly by continental crust and solid upper mantle. They are covered by continents.

Examples of continental plates are South America plate, Africa plate, etc.

MOTION AND DIRECTION OF THE PLATES (PLATES MOVEMENT)

The tectonic plates move extremely slowly. Each of the plates has it own direction as it moves. There are three (3) types of movements exhibited by plates. These are: –

1. Divergent plate movement.

This is the movement of plates where plates spread a part or move away from each other. In the oceanic basin it occurs in the mid oceanic ridges and on the land masses it occurs the rift valley. Example between Africa plate and South America plate.

2. Convergent plate movement.

This is the movement of the plates where by two plates move towards each other in horizontal direction. Example Nazca plate and South America plate.

3. Transform plate movement.

This is the movement of plate where by plates slide and past one another. This plate moves parallel in horizontal direction. Example Juan De Fuca plate and North America plate.

CAUSES OF PLATE MOTION/MOVEMENTS

The plate tectonic movement might have been caused by the following;

1. Mantle convectional current:

This is the circulation movement of molten materials in the asthenosphere. They caused by influence of radio active generations of rocks in the asthenosphere. The current rises up and move laterally below the lithosphere plate, therefore causing motion of plate.

2. Gravitational force:

These forces pull the lithosphere toward the center of the earth. In this case it causes disturbance on lithosphere, therefore triggering plates motion.

3. Ocean topography:

The surface of oceanic floor is not uniform as there are ridges and basins. At the mid oceanic ridges, the weight of lithosphere rock is too heavy enough to cause sinking movement, therefore trigger plates motion.

4. Slab pull and push process:

The injection of new slab (piece of rock) process of rock in the mid oceanic ridges causes divergent motion of plates away from each other. Likewise, as the plates moved away for convergent zone it is subducted and pulls down the rock of the plate behind. It is also called formation of new oceanic crust or features.

5. Cooling and sinking of the continents:

Geologists believe that the continents continue to cool and therefore they become denser. As they continue to cool become much denser they cause sinking movement and ultimately the plates move.

THE PLATES BOUNDARIES/PLATES MARGINS

Are the zones where plates interact each other. In these zones the earth’s dynamic processes occurs (never cases) i.e seismic process, vulcanism process, folding process, faulting process, and warping process.

TYPES OF PLATES BOUNDARIES

There are three types of plates tectonic boundaries according to direction of the plate’s movements.

These are; –

i. Transform plate boundaries.

These are plate margins also referred as “conservative plate zone” where by neither plate is destroyed nor constructed during interaction. In this type of plate boundaries, the plates sliding past each other moving into different direction.

Example of transform plate margins is found when North America plate interact with Juan De Fuca plate as a result of San Andrea’s fault.

EFFECTS/CHARACTERISTICS OF TRANSFORM PLATE BOUNDARIES

• Formation of San Andreas fault in N/America.
• Occurrence of earth quakes.
• Formation of crustal displacement.
• It triggers volcanic process.

ii. Divergent plate boundaries.

These are the plate margins where by the lithosphere is constructed. It is also known as “Constructive plate zone”. These are the plate margins, in which the plates move apart in divergent fashion i.e away from each other, leading the up welling of molten materials from asthenosphere in the mid oceanic ridges.

The up welling of molten materials cools and solidify along the margin and forcefully push the two plates laterally from the mid oceanic ridges i.e sea floor spreading. On the continents these plate margins are marked by rift valley.

Example of divergent plate margins is the mid-Atlantic oceanic ridges where Africa plate and South America plate diverges.

(a) Continent divergent plate boundary

LANDFORMS/CHARACTERISTICS FEATURES OF DIVERGENT PLATE MARGIN

1. Formation of new lithosphere along the mid oceanic ridges due to solidification of molten materials from asthenosphere.

2. Formation of rift valley in the continents eg. the Great East Africa rift valley.

3. Formation of submarine rift mountain ranges called mid oceanic ridges eg. Mid Atlantic Oceanic Ridges.

4. Formation of offset transform fault.

5. Formation of volcanic Islands eg. the Iceland in the N/Atlantic Ocean.

6. Occurrence of shallow focus earth quakes.

(b). Convergent plate boundaries.

These are the plate margins in which the plates move toward each other. These plate margins are zones in which plates are destroyed. It is also known as “Destructive plate zones”.

TYPES OF CONVERGENT PLATE BOUNDARIES

There are two types of convergent plate boundaries. These are: –

1. Collision plate zones (continental – continental converging plates).

These are convergent plate zones which occur when two continental plates zone collide but neither plate forced to go beneath the other plate because both plates have similar density.

Example of collision plate zone is the collision between India plate and Eurasia plate as a result to the formation of Himalaya mountain.

2. Subduction plate zones.

These are the convergent plate zones where by two plates meets and the denser plate should be sinking and get subducted beneath the other plate.

Depending on the type of plates involved on subduction, there are two subtypes of subduction plate zones. These are;

3. Oceanic – continental plate subduction zone.

These are plate zones in which two plates converge one of which being oceanic plate and other is continental plate. The denser oceanic plate is forced to go beneath the less dense continental plate, therefore subducted.

Example oceanic – continental subduction plate zones is the subduction of oceanic Nazca plate beneath the continental of S/America plate.

4. Oceanic – oceanic plate subduction zone.

These are plate subduction zone involving convergence of two oceanic plates having the same density. One of it must be subducted beneath the other plate.

Example is the subduction between Philippine plate and Pacific plate.

Oceanic – oceanic convergence

CHARACTERISTICS FEATURES OF CONVERGENT PLATE BOUNDARIES

1. Formation of volcanic island eg. the Japanese Island and Australia Island in the Pacific Ocean.

2. Formation of volcanic mountain when faults reach to asthenosphere and magma penetrate upwards through these faults and cooling on the earth surface.

3. Formation of Fold Mountain when plates converge eg. Andes, Himalaya mountain

4. Formation of deep oceanic trench eg. the Mariano and Tonga trench.

5. Formation of faulting features like rift valley, faults scarps, etc.

6. Formation of earth quakes and earth tremors.

EVIDENCES OF PLATE TECTONIC THEORY

1. Occurrence of Fold Mountains like the Himalaya ranges. Fold mountain are the results of convergence of two continental plates forming anticlines and syclines.

2. Formation of oceanic ridges. This shows that two oceanic plates are spreading apart allowing molten materials to be emitted out forming a new crust for example the mid Atlantic ridge, Indian ocean ridge and Pacific oceanic ridge.

3. Presence of oceanic trench; they develop when oceanic and continental plates converge. Example Mariana Trench in Western Pacific Ocean which is 11,034metres deep.

4. Widening of the Antarctic Ocean basin; this is caused by divergent movement of oceanic plates. Studies shows that Antarctic Ocean basin is gradually increasing in area which tells that the South American plate is moving western ward while the African plate is spreading eastward.

5. Existence of rift valleys; like the Great East African Rift Valley; this indicate that continental plates are diverging from each other.

6. Occurrence of volcanicity in different parts of the world; this is a results of convergence of oceanic and continental plates at the zone of subduction where the rigid oceanic plate sinks to the mantle and melts. The molten materials then poured out to form volcanoes.

7. Narrowing of the sea floors; this shows that plates are coverging; example; the Meditteranean sea floor is experiencing narrowing effects due to the northern ward movement of the African plate. This, perhaps may result to complete disappearance of the Meditteranean Sea.

8. The Jig-saw fitting of continents; this shows divergent movements of the continents.

REASONS FOR WHY PLATE TECTONIC IS THE NEW VERSION OF CONTINENTAL DRIFT THEORY

Qn: Plate tectonic theory is a new version of continental drift theory. Discuss

It is true to say that, without continental drift there were no plate tectonics, therefore the plate tectonic theory is a new version of continental drift theory due to the following reasons;

1. The plate tectonic theory was provided by modern scientists in late 1960’s after comprehensive researches followed discovery of Amazing evidence to support the continental drift theory which left among unaware question. The continental drift theory was provided in 1915 by a meteorologist with narrow knowledge on geology.

2. The plate tectonic theory describes process and forces for movement of plate tectonic that is convectional heat currents in the asthenosphere. The continental drift theory not only failed to state the driving mechanism of continents but also it did not explain the manner and how continents had moved after breaking up of Pangea (how and why drifting). Therefore, the theory of plate tectonics is more complete.

3. The continental drift theory only proposed movement of continents. The plate tectonic theory a detailed describe the fact that, the movement did not involve continents alone but rather both continent and oceanic floor. Basically, some plates are comprised entirely of oceanic crust but majority are consisted by both continental crust and oceanic crust.

4. The plate tectonic theory is a framework from which to view most other geological processes such as rock cycle, igneous activities, etc. The continental drift did not show the linkage between the movement of continent and such processes.

5. The continental drift theory never indicated the exact rate of continental drifting process. On the other hand, details show that the plate vary in their rates of motion. For instance, it is 2cm apart in the Atlantic Ocean while the movement is little but higher in the Pacific Ocean ranging 3cm to 8cm and maximum of 10cm per year.

6. The plate tectonic theory reveals the effects of plate interaction along the plate margins in which is formation of various geomorphological processes with different features like folding, faulting, vulcanism and warping.

7. The plate tectonic was able to show linkage and relationship in the distribution of earth quakes, volcanism and plate boundaries which were not described in continental drifting theory.

8. Scientific community now understood the fact that in the mid oceanic ridges are points of new creation of lithosphere and the rock are younger than elsewhere due to plate tectonic. This was not understood during era of continental drift theory.

9. Plate tectonic theory clarify the weakness of continental drift theory which state continents are floating on the oceanic crust also theory state that lithosphere plate supported by asthenosphere.

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