Give A Concise Account Of Plate Tectonics

As They Operate On The Earth.s Surface Today Essay, Research Paper

........... The earth

can be divided into concentric layers. These can be distinguished from one

another through varying chemical composition, temperature, density and phase.

The inner most central body is called the core. This consists of two parts: the

inner and outer core. The inner is understood to be solid due to the immense

pressures, and the outer core is liquid. Phase can be determined through

analysis of seismic waves passing through the earth. Two waves types; p

(compressional) and s (shear) waves are emitted during earthquakes. S waves

cannot however pass through liquids, and therefore if data is collected from

stations over the globe of an earthquake, a picture can be built up of the

internal structure of the earth. The next major layer of the earth is the

mantle, and this too can be divided into parts: the asthenosphere and layered

upon this, the lithosphere. The mantle is less dense than the core and accounts

for 82% of the volume of the earth. The asthenosphere is plastic like due to

partial melting, unlike the lithosphere which is solid but semi-rigid and can

flex. The lithosphere is capped by the crust, the final layer of the earth.

This is a less dense layer and very thin in comparison to the core and mantle.

There are two types of crust, being oceanic and continental. Continental crust

is generally thicker and less dense than oceanic. ........... Plate

tectonics are concerned with the movement of lithospheric plates, which are

chunks of lithosphere with their crustal caps. The earth.s surface consists of

seven major and thirteen minor plates. The boundaries between these plates can

be described as divergent, convergent or transform. At divergent boundaries

plates are moving apart, at convergent they are moving together and are moving

laterally past each other. Lithospheric plate is destroyed at convergent plate

boundaries when one plate is subducted under the other and melted after been

forced deep into the mantle. For the theory of plate tectonics to function the

following assumptions have to be made: i)

The earth.s surface has a constant area. This means that the

earth cannot be expanding and that the overall rate plate is destroyed (at

convergent plate boundary) is the same as the rate that it is created (at

divergent plate boundary) globally. ii)

Secondly, individual plates can alter in area. This will

depend of the type of plate boundary it has with its surrounding plates. For

example the African plate is expanding as it is bounded on three sides by

divergent plate boundary. iii)

Thirdly the length of constructive plate boundary is greater

than the length of convergent plate boundary. To retain the validity of the

first assumption the rate that plate is being produced per unit length of divergent

plate margin must be less than it is being destroyed per unit convergent. The mechanism for lithospheric plate movement is

less well understood. The most recognised theory is that of convection currents

in the plastic asthenosphere. Hot and therefore less dense material from deep

in the mantle rises up to the lithosphere where it then spreads laterally. This

results in drag on the plates and therefore movement. These currents may be

circulating just below the lithosphere, or the cells may stretch 700 km into

the mantle. Others theories suggest that circulating currents transgress the

whole mantle and are heated by hot spots on the core itself. Lava injection

theory states that at constructive plate margins up welling magma exerts

lateral pressure on the two plates, forcing them apart. The final mechanism to

consider is the subduction of the plate due to gravity as a result of a

thickening of the plate as it moves away from a divergent plate boundary or an

increase in density due to cooling and compression. The oceanic plate here

would be described as gravitationally unstable as it is denser than the mantle

below it. The density increase across the plate will result in a sinking and

horizontal slide into the mantle, therefore promoting plate movement. Diagrams Showing Possible Mechanisms Driving Plate Movement........... It is in my opinion likely that all

the mechanisms come into play to an extent and it is unlikely one mechanism is

solely responsible for the movement. For example I doubt that pressure from up

welling magma in a divergent boundary alone could force whole continents to

move at the rates of several centimetres a year. ........... Plate boundaries are indicated on

the globe by linear concentration of earthquake occurrences and volcanoes.

Boundaries can be differentiated between through differences in earthquake

magnitude and depth, and volcanoes through volcano form. Diverging plate

boundaries generally experience shallow earthquakes of low magnitude, whereas

converging

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