Compare and contrast the topographical features at divergent and convergent plate margins.
Throughout our landscape there are volcanoes, rift valleys, cinder cones, basalt dykes and flows which are formed due to the movement of the semi-molten rock of the asthenosphere that causes the tectonic plate boundaries to meet and move away from each other. There are three types of plate boundaries (transform, convergent and divergent) however, there are only two types that erect these topographical features mentioned above.
Divergent plate boundaries occur when two plates move apart from each other. This type of plate boundary deals with the movement of the continental and oceanic crusts due to the rising of conventional currents from the asthenosphere. (Strahler, 2011). The oceanic plate boundary occurs beneath it, causing the rising of the convection currents below it which lifts the lithosphere producing mid-oceanic ridge. Extensional forces stretch the lithosphere producing a deep fissure in which pressure is reduced on the super-heated mantle material below it. The magma then solidifies and the process repeats itself. Continental plates occur beneath a thick continental plate. As the two plates pull apart, normal faults develop on both sides of the rift in which and the central block slides downwards. Earthquakes occur as a result. The continental plate is much thicker than the oceanic plate. Due to these processes a rift valley is formed. From this process a fault scrap is also formed in which a block mountain can develop as well. The central block that slides downward is referred to a graben. An example is the East African Rift valley. Since the two plates drift apart, magma sometimes doesn’t rise up forming ridges to develop. Example Mid-Atlantic Ridge which is located between Africa and South America. Volcanoes are also formed due to the plates moving apart from each other. An example is in Iceland where the North-Atlantic mid-oceanic ridge is exposed on land due to the basaltic lava type ejected from the volcano.
Convergent plate boundaries are where plates move towards each other forming subduction zones, volcanoes and other topographical features. The two plates are the oceanic and the continental plate. In order for a subduction zone to occur, one of the plates must be denser than the other to allow the plates to slide under one another hence the oceanic plate being the thinner and denser plate it slides below the continental plate producing a subduction zone. At the subduction zone volcanoes are formed. The type of lava emitted from the volcano is of andesitic type. Cinder cones are also developed from convergent plate margins. An example of this is the Washington-Oregon coastline USA and the Andes mountains. An example of a plate producing a subduction zone is the Nazca plate subducting under the South America plate. Convergent plates are based on its different densities and due to this when oceanic-oceanic convergence occur there are no subduction zones being formed however, there are magma chambers produced due to the melting of the plates since there is no plate subducting under each other. Once the magma is erupted it forms cones producing island chains due to continue development. An example is the Eastern Islands of Martinique, St. Lucia, St. Vincent and the Grenadines. Another type is the continental-continental convergences. These plates are thick and have a density much lower than the mantle. They are welded together at a suture zone. A collision zone is formed between the two continental-continental convergence in which fragments of crust is found between the two plates forming a highly deformed mélange of rock. From these mountains can be formed. An example is the Himalayan mountain range where the Indian and Eurasian plates are constantly colliding.
Diagram 1: Showing a Rift Valley at a Divergent Plate Boundary
Diagram 2: Showing a Divergent Boundary
Diagram 3: Showing the plates moving apart at the divergent plate boundary
Diagram 4: Showing a Convergent Plate Boundary
Diagram 5: Showing the different types of convergent plates
Both of these plate boundaries produce extensive topographical features from mountains to volcanoes to rift valleys etc. Convergent plate boundaries are all based on the density of the plate in order formation of volcanoes and mountains while the divergent plate boundaries just move apart from each other to produce the volcanoes and mountains. They are all based on the theory of the plate tectonics and why we are able to see these landforms today.