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 margins exhibit deeper earthquake foci of higher magnitude.

Diverging plate boundaries are lined with basic shield volcanoes, whereas

converging boundaries show concentrations of composite con volcanoes. ??????????? Diverging plate boundaries result in

the formation of mid oceanic ridges. Mid oceanic ridges take the form of a

series of roughly parallel ridges around a central rift. The boundary here is

under tension and the two oceanic plates are moving apart. As this occurs magma

upwells from below and solidifies to form new crust and lithosphere. This magma

is basic, of low silica content and forms basaltic rock. Oceanic plate is as a

general rule basalt based whereas continental plate is granite based. Examples

of this today are the Mid-Atlantic Ridge and the Carlsberg Ridge (Indian

ocean). Divergent boundaries often form volcanic islands where the ridge

protrudes above sea level, an example of this being Iceland. ??????????? However if a convection plume in the

mantle occurs under a continent, the continent becomes heavily stressed. This

results in primarily the formation of domes on the surface as the rocks bulge

upwards. If several hot spots are close enough then a rift system will be

formed. This has occurred in the East African Rift system, where 11 hot spots

have formed 29 separate rift valleys, with the Red Sea being the most developed

of these.??????????? Collision of two oceanic plates

results in the creation of a volcanic island arc and the formation of an ocean

trench. The Philippine Island Arc zone and the Tongan Island Arc zone are

examples of these. The Tongan ocean trench represents the subduction of the

Pacific plate under the Fiji plate. The subducted plate will be slightly denser

and so will more readily sink into the mantle. This plate will be forced under

at, in the Fiji ? Pacific case around 20 degrees. When it reaches a depth of

around 600 kilometres where it is completely absorbed into the mantle. Rising

plumes of andesitic magma erupt at the surface on the ocean floor, forming

volcanoes and eventually volcanic island arcs. These island arcs are common

around the west side of the Pacific Ocean. The non-subducted plate may become

buckled to produce an outer arc ridge, of which Barbados is an example today. ??????????? ??????????? ??????????? Continental margin subduction zone

occurs when oceanic plate and continental plate collide. The best example of

this is the collision of the Nazca and the South American plate. During a

collision of this type the oceanic plate always is subducted. This is due to

the fact that continental crust is thicker and less dense, which means that

through isostacy it stands considerably higher than the oceanic plate. Again

the oceanic plate is forced below to a depth where it can be completely melted

and absorbed into the mantle. Andesitic magma from wet ocean sediment rises up

through the continental plate from the oceanic plate to form andesitic

volcanoes. The continental plate becomes folded and fractured due to the

collision and fold mountains are formed. The Andes represent this in the Nazca

? South American collision example. However this mountain range here is an

accumulation of folded continental rock, folded sediments scraped from the

subducted plate and a volcanic island arc that collided with the South American

plate as it migrated with the Nazca plate. These three bodies form the Eastern

Cordillera, the Altiplano and the Western Cordillera. If plate movement continues

in this fashion then an area of oceanic plate can be completely destroyed, and

the two continental plates on either side will then collide. Here no subduction

occurs, but sheets of oceanic sediments are folded upward to form fold

mountains. The continental plate will become folded and buckled but volcanic

activity is rare, even though large batholiths will be expected to form

underneath the fold mountains. An example of this type of plate margin is

Indian ? Eurasian plate boundary. Here the continental crust of India has

collided with Asia and formed the Himalayas fold mountain range. This shows how

tectonics control the major relief features on the earth today, with the

Himalayas being the tallest mountain range, while uplift is still occurring

today. Plate is neither created or

destroyed at transform boundaries. Bar the fault line the only landforms

created will be local ones due to localised compressions and tensions. An

example of a transform fault is the San Andreas fault line.??????????? Plate tectonics can be seen overall

as a global conveyor system, destroying and creating oceanic crust, with

sediments being scraped off the ocean beds and returned to the continents. Many

of the processes are cyclic, but consideration must still be given to the super

continent cycle, involving the contraction of the continents into one body,

only to be broken up again.Bibliography????? The Physical Geography of Landscape ? Roy Collard????? Global Geomorphology ? Michael A. Summerfield????? A ? level notes