After the theory of plate tectonics was proven, scientists did further research into the characteristics of the two different types of plates, oceanic plates and continental plates. Both have completely different characteristics, and produce different structures across our world. I am going to briefly describe their characteristics and later on in this essay I will draw out the global structures that result at different plate boundaries.
Oceanic plates are the first of two types of tectonic plates; oceanic crust is the younger of the two and, as a result, oceanic crust is much more active with its movement. Oceanic plates are considerably thinner plates at only 5km thick, but, despite this, is the denser of the two plates, and as a result, oceanic crust tends to sink. There are two main types of features that are associated with oceanic crust; abyssal plains which are very deep parts of the ocean, and can be between 4000 and 6000 meters deep. The other feature that is associated with oceanic crust is seamounts, which are extinct volcanoes formed by stationary hot spots in the mantle or less commonly by volcanism at mid-Atlantic ridges.
Continental plates are the second type of plates that I will be looking at in this essay. They are made from shale, sandstone and limestone. The oldest continental plates dates old as 3.98 billion years, and are considerably older then oceanic plates. They are the thicker of the two plates, and can be as thick as 50km thick. Continental plates are less dense when compared to oceanic crust, and, as a result, do not sink at plate margins. There are two type’s features that are associated with continental plates; Mobile belts, and shields. Mobile belts are Fold Mountains, where erosion exposes roots of former mountains into which batholiths of granite are seen to have been intruded. Shields occur when prolonged erosion occurs to crations; they are occasionally flooded due to eustatic rise or tectonics down flexing of cration and results in the accumulation of sediments.
There are three main types of plate margins, Constructive Plate Margins, Destructive Plate Margins and Conservative Plate Margins. Each different margin has a different process that it is associated with and also results in different global structures forming.
The most common type of Destructive Plate Margin is Oceanic-Continental plate boundaries. At Destructive margins, the plates converge, resulting in one being forced downwards beneath the overriding plate. As the oceanic crust moves down below the continental, rising temperatures and frictional effects cause the oceanic rocks to melt. As the two plates slide past and over one another, substantial friction stores energy within the rocks. This energy is released in earthquakes with foci along the destructive plate margin or the subduction zone. As the oceanic crust is pushed downwards and forms deep ocean trenches along the plate margin. Deep Oceanic Trenches the deepest parts of the ocean and are a major global structure that are formed as a result of Destructive Plate Margins. The deepest Oceanic trench in the whole of the world is the Marinas Trench in the Pacific Ocean. The Marinas Trench is 11,040 deep and occurs where the fast moving pacific-plate converges with the slower moving Philippine plate, Marinas Trench plunges deeper into the Earth's interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 m).
Over two thirds of the surface of the Earth is covered by ocean and underlain by oceanic crust. Some of the continental regions are slowly moving apart - at rates of some 5 - 10 cm per year. Constructive Plate Margins are found where two Continental Plates are gradually moving apart. The movement of the continental plates is due to the up welling limbs of convection currents deep within the mantle. Heat energy rises from the hot core and spreads sideways below the crust, moving the continental plates sideways. New oceanic crust must be created to fill the gap left by the retreating continents. This New Crust can be known as an ocean ridge, or can be described as large mountains underneath the ocean. The main example of a Oceanic Ridge is the Mid-Atlantic Ridge, and it runs down throughout the whole of the Atlantic Ocean, and starts as far north as Iceland, and even rises above sea-level. Iceland is one of the few countries that claim that the surface area is increasing each year, which indicates that the two continental plates are still moving away from each other.
Conservative margins occur where two plates slide horizontally past each other. There is no subduction zone with one plate being destroyed beneath another nor is there a constructive zone as at the Mid-Atlantic ridge. As the two plates slide past each other, the high levels of friction create large strains along the slippage zone faults. Eventually the strain energy is released in an earthquake. When this happens there is likely to be large scale movement along the transform fault with substantial damage to nearby buildings. There are no structures that are associated with conservative plate margins as no new crust is formed, nor is any crust destroyed. The plates simply slip past each other along transform faults. The best known example of a conservative plate margin can be found along the west coast of the USA where the Nazca plate is moving in a North Westerly direction and sliding past the North American Plate. The fracture zone between the two plates is the San Andreas Fault - a huge fault running for hundreds of km along the Californian coastal region from San Francisco to Los Angeles. The region has a number of other substantial faults running parallel to the major San Andreas fault - the Hayward Fault runs almost parallel and to the east of the San Andreas fault with the modern city built across the two active fault planes. Earth tremors and earthquakes regularly occur along these fault planes - in 1906 the city of San Francisco was destroyed in a magnitude 8.2 quake on the San Andreas fault; in 1989 substantial damage and some 69 fatalities resulted from the magnitude 7.1 Loma Prieta earthquake.
To conclude, the theory behind plate tectonics was first discovered in 1910, and allowed us to understand how different global structures were formed and what different processes initiated them. The evidence that supported this was both geographical and biological and included the fitting together of different continents across the world, as well as specific species fossils that were found on opposite sides of the world on separate continents. Due to this new evidence, the original idea of “land bridges” was destroyed and the new scientific knowledge allowed scientists to find out more about different plates and margins themselves. The two main plates that were discovered were Continental and Oceanic plates, and both have completely different characteristics and resulted in different landforms being formed. Destructive plate margins are the site of subduction zones and result of the destruction of the oceanic plate, and can result in the formation of fold mountains and deep sea trenches. Constructive plate margins can occur at continental-continental plate margins, where the two plates are moving apart, and this results in the formation of new oceanic crust in the form of a oceanic ridge. Conservative is the last type of plate margins and doesn’t involve either the formation of new crust or the destruction of plates. The improved scientific technology that allowed scientists to find out more information about the different plates and the processes and landforms that form as a result, and allowed us, today to understand more about the formation of the landforms on our earth.
