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Plate Boundaries Divergent (constructive) At Constructive margins such as the Mid-Atlantic Ridge two plates move away from each other. Molten rock which is magma immediately rises to fill any possible gaps and forms new oceanic crust. The Atlantic Ocean is widening by about 3cm a year which means that the Americas are moving away from Eurasia and Africa. Later eruptions elsewhere in Iceland caused the temporary evacuation of Heimaey in 1973 and the melting of the part of the Vatnajokull ice cap in 1996. Convergent (Destructive) Destructive margins occur where the plates consisting of Oceanic crust move towards plates of Continental crust.
The newly formed magma rises since it is lighter than the mantle and comes on to the surface to form volcanoes. Examples of this would be the West Indies, Japan and off the south coast of Alaska where at the destructive margins, the magma rises offshore to form island arcs which then became these countries. Convergent (Collisions) Collision margins are where the two plates which are continental crust move together. As Continental crust cannot be destroyed or sink the rocks between them are forced upwards. The Indian plate is moving into the Eurasian plate at a rate of 5cm a year.
As crust is neither being formed nor destroyed at this plate boundary new landforms are not created and there is no volcanic activity. However Earthquakes occur if the two plates stick. This is what is happening in California, where the San Andreas Fault marks the junction of the Pacific and North American plates. When these two plates stick instead of sliding past each other evenly, vast amounts of pressure is built up; one plate is jolted forward sending shockwaves in the surface. Shockwaves in San Francisco made the ground move 6 meters in 1906. There was a number of lives lost in the 5 hundreds and 28 000 buildings were destroyed. Short term calculations suggest that there could be further earthquakes in the San Francisco area. Plate Tectonics Yadeesh Thedchanamoorthy
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