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
