Plate Tectonics Constructive Plate Margins Constructive plate margins, this is where there are two plates moving away from each other causing new oceanic crust to be formed and mid-ocean ridges are created by the build up of molten rock on the sea floor due to the mantle building up. This new crust pushes the old crust aside and forcing the plates apart at a very slow rate (10kmper 1million years) volcanoes or volcanic islands are usually associated with the Mid-Atlantic Ridges.

The Mid-Atlantic Ridge The Mid-Atlantic Ridge runs all the way up the Atlantic ocean and was created by the Eurasian and North American plate moving away from each other and the separation of the African and South American Plate. This ridge runs straight through the middle of Iceland and this has caused a lot of problems for Iceland and its inhabitants such as: Earthquakes and Volcanic eruptions.

Iceland Case Study On November 14th, 1963 it was reported that there was an explosion under the sea which was followed by smoke steam and emissions of pumice stone and left a 130m cone of ash from the sea bed and later formed the isle of Surtsey. Five months later on the 4th of April, 1964 lava flowed from the sea and covered the ash and this made sure that the Island was to survive.

Iceland Case Study (Continued) Nine years later on the 23rd January 1973, about 2`0’clock in the morning there was an earth tremor that stopped the clocked at Iceland’s main fishing port of Heimaey. The tremor was caused by the separation of the Eurasian and North American plate and fisherman saw this happen and say that “the earths crust was broke open and lava was just pouring out of this 2kms long fissure. Once the volcano had calmed down the inhabitants of Heimaey were evacuated and when the volcano had finally stopped erupting six months later the inhabitants were only to find their houses burnt or covered in a layer of ash up to five metres deep and the entrance to their harbour blocked.

Rift Valleys As the crust is being pulled apart fractures appear. The fractures break up the crust. As the crust pulls away the pieces of land fall and slot downwards. As the land has been pulled apart and has fallen down it has formed a rift valley. On the left is an air photo of the Koko rift section of O’ahu.

Destructive Plate Margins Destructive margins occur when Continental crust and Oceanic crust converge. At one point the Oceanic crust will dip down and form a subduction zone. In this zone high friction breaks up the plates causing earthquakes. As the plate descends the increase in pressure can trigger major earthquakes. Fold mountains and Island Arc are also formed. Examples are: The Nazca and American Plate, Also Peru-Chile deep sea trench.

Collision Plate Margins This is Continental crust against Continental crust. Two plates move together and collide, the land moves upwards and its opposite reaction is to move back down. If the land was at the bottom of the sea and it moved up it would still have shells on it. Earthquakes give off 3 types of shock waves: Primary, Secondary, Longitudinal. E.g. Earthquakes, Taiwan 1999 Sept, E.g. Volcanoes, Mount St Helens 1980, May.

Conclusion In conclusion we can see that constructive plate margins create more land. As the plates split apart and magma rises up from beneath and creates mid-ocean ridges. The Mid-Atlantic Ridge being the most famous. Rift valleys are also features of constructive plate margins. Destructive plate margins, however, destroy the land. As one plate sinks beneath the other the disappearing land is turned into magma, features include subduction zones and deep sea trenches. Collision plate margins don’t destroy or create land. They collide and move together, this causes earthquakes and volcanos.

Links Books: Geography in Place 1, Michael Raw & Sue Shaw Sites: GCSE Bitesize