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