At divergent plate margins where plates move apart, 2 main global structures are formed. The type and nature of these structures are determined by the location of the divergent plates. Where plates move apart in oceanic areas, an ‘oceanic ridge’ is likely to be formed. This can be seen where the South American plate and African plates are moving apart. Magma has shot up through the crack created and solidified, to create the ‘mid-Atlantic ridge’. Oceanic ridges are the longest continuous global uplifted structure on the planet stretching up to 60,000 km, and in some parts rising up to 3000 m above the ocean floor. Volcanic activity is also seen along the ridge, forming underwater or submarine volcanoes. Occasionally these volcanoes rise above sea level, forming volcanic islands. Iceland is a prime example of this, however Iceland formed due to an irregular plume, rather than a large volcano. These volcanoes are fairly gentle due to the volatile nature of the lava. As the ridge widens (10-90+mm/year) new basaltic oceanic crust is formed and spreads outward, however faults form and the variety of movement along the oceanic ridge has inevitably led to earthquakes.
Divergent plate margins in continental areas such as Africa, have a different feature associated with them. As the crust move away from each other, the brittle and thin parts of the crust on the sides of the ‘scarps’ fall away, landing in between the parallel faults. Over time, this debris builds up and leads to the formation of a rift valley as seen in Africa. The largest rift valley is the African rift valley, which stretches 4000 km from Mozambique all the way to the red sea. From the red sea it carries on further north into Jordon, measuring a total distance of 5,500 km. Identified by the characteristic series of step faults, scarps on the sides of the valley are steep and inward facing and can measure up to 600 m above the valley floor. Tension created by the much thinner crust, has led to a large amount of volcanic activity clustered around rift valleys, eg Kilimanjaro based along the great African rift valley is an inactive destructive volcano, and has been formed as a result of the tension in the thin crust. There is also evidence to suggest that the eastern part, and the horn of Africa, will split away from Africa eventually separated by sea, as a result of the constantly moving rift valley.
Conversely we see the opposite occurring at convergent plate margins come together. Unlike divergent plate margins, the nature and type of structure formed at convergent plate boundaries is dependant on the nature of the two plates in question. For example the Nazca plate (oceanic) converges on the less dense S. American plate (continental), and therefore the Nazca plate sub-ducts leading to the ‘down-warping’ of the ocean floor leading to the formation of an Ocean trench. In this case it is the Peru-Chile trench reaching a maximum depth of 8000m. Sediment trapped and accumulated on the continental plate has been deformed by folding and faulting due to the collision with oceanic plate. This sediment has been twisted and transformed upwards leading to the creation of Fold Mountains. The Andes follow the Nazca/S. American boundary, and are a direct result of the deformation of sediment on the continental shelf. Both earthquake and volcanic activity are present at varying levels of intensity. The tremors are caused by the continual pressure built up by the friction between oceanic and continental crust, with increasing pressure leading to more violent tremors. Volcanic activity is also rife in these areas, due to the increasing amount of magma being produced in the crust, leading to a build up of pressure in the lithosphere, which then breaks through onto the surface leading to violent eruptions.
This process is similar when two oceanic plates collide, forcing one to subduct, and the other to remain on the surface. This also leads to the creation of trenches and mountain ranges, as well as tectonic activity. When two continental plates collide, sediments and fluvial deposits between the two plates are compressed and deformed as the plates come together, and are forced upwards into Fold Mountains. This is evident throughout the world and has led to the formation of the world’s most famous structures such as the Himalayas, and the Alps.