GEOGRAPHY FORM FIVE TOPIC 4: THEORY OF PLATE TECTONIC IV

PLATE TECTONIC THEORY (Unifying theory)

It takes the combination of theory of theory of isostasy, continental drift and ocean floor spreading.

According to this theory, the earth has an outer shell(lithosphere) made up of several rigid piece called tectonic plates.

I) Geometrical part   – The crust is made up of segment called plates, which are of various sizes, Large and small plates. pacific,  N. American, Nazca, S. American, Africa, Indian,  Antarctica plates

II) Moments– Movement of the plates. The plates are in motion. Either they are diverging or converging cause of movement – Convectional currents from the interior of the earth’s especially aesthnosphere.

The movements can cause plates to collide or slide past one another. The rate of motion is very slow of the plates relative to one another 1cm to 2cm per year.

TYPES OF PLATE BOUNDARIES:

1) Divergent boundary – Is a boundary which is created when the plates are moving away from each other. This normally occurs in the ocean, where there are mid-oceanic ridges.

Example

–          Mid Atlantic ridges

–            Island Arcs ( volcanoes)
–            Rift. valley Graben

2)  Convergent boundary– Is boundary where by the plates are moving towards one another

3) Neutral / trans current boundary- This occurs when plates are sliding past one another. Normally occur on the Transform faults. (No uplift or submerge of the land (crust)

MECHANISMS OF THE PLATE MOVEMENTS AND THE PLATE BOUNDARIES:

• Plates are either continental or oceanic.
• Oceanic plates are simatic (denser).
• Continental Plates are Sialic (light).

CONVERGENT PLATE BOUNDARY

i)   Continental and continental   –  Both will go uplifted and result to folds and faults

ii)  Oceanic and continental -The denser will down lift and lighter will uplift and may result to volcanic eruption and subduction result to trenches for the denser plate.

iii)  Oceanic and oceanic   – Convergent boundary ( subduction result to trenches both will move down)
Plate Tectonic Theory states that “The lithosphere is made up of rigid segments called the plates and the plates are in constant state of motion relative to one another”

CAUSES OF MOVEMENTS:

I) Convectional current – During mantle convection some materials rise due to the influence of radioactive heat generation and later moves laterally below the lithosphere. The lateral movements drag the lithosphere leading to the 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

II ) Upwelling of magma in the lines of weakness for example in the mid-oceanic ridges where by the magma is pushed out of the surface through the weak lines of the crust, in order to form a  new crust. This may result to the cause of movements

III) Isostatic adjustment – May cause slight movement when trying to create balance.

IV) Cooling and heating of the crustal rocks -Expansion and contraction of rocks. The heat in the interior of the earth (mantle) causes rocks inside to expand and when the heat reduces, the rocks cool. This process causes the rocks to crack hence leads to the disturbance of the crust and causes movements.

EFFECTS OF PLATE MOVEMENT

Changes on the plate boundaries which are lines of weakness and on these boundaries, major landforms of the earth’s surface are going to be formed.

a)      Diverging plates

I) Oceanic

– Mid oceanic ridges

– Oceanic Islands

– Rifts e.g. Red sea

II)Continental

-Volcanic mountain

– Block mountains

-Rift valley

b)Convergent plate boundary– Collision may lead to subduction and uplift.

i)  Oceanic  – oceanic trends (Marianna trenches, Japan trenches )

ii) Oceanic and continental-Volcanic mountains on the coastal boundaries and also result into trenches.

iii) ( Continental )- Formation of Fold Mountains. Himalayas ( Indian and Russian plate formed)

c)      Neutral / Trans current boundary – There is neither uplift nor seduction. There is lateral displacement of the plates. N. America, San Francisco – San Andrea’s faults displacement of features of about 1000km.

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

The theory of plate tectonic can help to explain almost all of the landforms on the surface of the earth.

I. Deep sea Trenches: A sea trench is a long deep valley along an ocean floor. they form along a convergent destructive term of depth. the Mariana in the western Pacific with a depth of more than 36000ft.

II. Mid Oceanic Ridge: It refer to a giant undersea 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.

III. Island Arcs: 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.

IV. 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.

V. 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. 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.

VI. 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.

VII. 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.

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