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Formation of Rift Valley’s, Ocean Trenches and Fold Mountains

Examples of Fold Mountains can be found all over the world. A famous example would be the Andes Mountains which are located across the Eastern side of South America. This location is where the South American Plate made of continental crust meets the Nazca Plate made oceanic crust. Sites like these are known as destructive plate margins. The Nazca Plate (oceanic crust) moving west is forced under the South American Plate (continental crust) moving east. The theory of the formation of fold mountains is believed to be result of the destructive plate margin.

The oceanic plate is orced under the continental crust deep down into the Earth’s surface. This movement of oceanic crust downwards creates a subduction zone. The disappearing crust is turned back into magma due to the high amount of friction that acts on it. The newly formed magma is less dense than the magma it lies on which is found much deeper in the Earth and is therefore tries to rise to the Earth’s surface. Where the magma is trapped and can’t reach the Earth’s surface a volcano is formed. The Volcano will erupt over and over again making the mount created by previous eruptions larger and arger.

Also as the lava rises it cools and is stored in the new landform and turns to granite. Eventually after long periods of times large fold mountains are created on land such as the Andes. Deep down in the Andes remains the large batholith which is like a large storage tank of magma which would have originally have contained magma that created the base of the Andes mountains. The batholith would have been created due to the magma being more acidic at this destructive margin than that at a constructive margin and therefore flows less easily which solidifies more easily and reate an intrusive feature such as a batholith.

This is one piece of evidence which suggests that this is how the fold mountains were formed. Another landform is the saline lakes located at the top of the Andes which are believed to have been the remnants of former oceans. An example of this is Lake Titicaca which is located in the Andes mountain range and covers an area of 3200 square miles. This lake suggests that a large area of a previous ocean has risen, and the theory of fold mountains being created beneath this area of ocean creates an answer to why this lake is situated here.

Ocean Trenches Ocean Trenches are located on destructive margins, the same place where fold mountains are found. The Example I will focus on is again on the Eastern side of South America. Just of the east coast of South America where the oceanic and continental crust create a destructive margin, being the Nazca plate and the South American plate respectfully, deep sea trenches can be found which go down as far as 35,000 feet. Where the Nazca plate is forced to dip downwards underneath the South American plate a large v-shaped trench is formed which runs parallel to the destructive argin.

It is created here because of the subduction zone created underneath the continental plate which is where the oceanic crust is destroyed. These trenches can also be located near island arcs which are islands created in the same way which fold mountains are and therefore trenches are created running parallel to the destructive margin again. An example of this is off the west side of Japan near Tokyo where a deep sea trench has been created by destructive margin between the oceanic crust of the Pacific plate and the continental crust of the Eurasian plate.

This has created the island arc more commonly known as Japan. The trench plunges more than 6,000 metres below the oceans surface. The fact that all fold mountains and deep sea trenches are located where there is a destructive margin along a coastline suggests that they must have all been created in a similar way and the theory of plate tectonics and oceanic plates submerging under continental plates satisfies how they were created and why they are located so close to each other. Rift Valleys Rift valleys are created along faults in some tectonic plates.

In East Africa the brittle crust has fractured and has caused sections to move apart. The central portion of the fault has given way and fallen creating the Great African Rift. This creates large wall type cliffs on either side with heights of 600 metres. The rift is created by the movement of the African plate where in the south it is believed to be diverging away in two separate directions pulling both east and west. The plate movement is because deep down in the Earth currents in the mantle are separating the plate into two creating the rift valley.

Of course with this comes the ormation of volcanoes which have developed on the eastern and western areas of land along the side of the subsided valleys. The volcanoes have formed because of the rising of mantle through the faults created in the plate. Examples of these volcanoes are Ruwenzori on the eastern ridge and Kilimanjaro on the western ridge. The two valleys on either side of the central plateau have occupied large lakes which have formed due to the downward movement of land. Lake Tanganyika is located in the eastern valley which covers 12,700 square miles and is the second largest freshwater lake n the world.

As well as lakes in the two valleys, the middle of the central plateau has subsided enough to create Lake Victoria. The fact that volcanoes have been created on either side of the rift valley suggests that the land which has subsided in the valley has been converted to magma and then this magma has risen through the faults of the land or has created the volcanoes that appear there today. This continuation of the African plate being pulled apart may eventually lead to Africa splitting into two islands instead of the current single island.