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Schemes of work

Introduction to plate tectonics

Destructive plate margins

Collision margins

Constructive margins

Rift valleys

Faults

Hot spots

Conservative margins

Tectonic summary

Vulcanicity

Impact of volcanic activity

Volcanic hazards

Montserrat

Etna

An introduction to earthquakes

The effect of earthquakes

The impact of tectonic activity

 

 

 

Destructive margins

Watch this as an introduction Destructive margins

To the west of South America, the Nazca Plate (oceanic crust, basalt 3.0) is moving towards the American Plate (continental crust, granite 2.7) at about 15 cm per year. Where the plates meet, the denser Nazca Plate is forced downwards to form a subduction zone with an associated deep-sea trench (the Peru-Chile trench). The increase in pressure, where the plate is forced downwards, can trigger off severe earthquakes. This zone of earthquakes is called the Benioff Zone . These earthquakes can be mapped quite clearly as the basalt plate descends into the mantle. The deepest earthquakes are found here, with their foci up to 700 km deep.
The descending plate takes water which is within the ocean crust with it. As the crust continues to descend, it takes with it water and other volatiles, this lowers the melting point, thus inducing partial melting. The newly-formed magma, from the destroyed oceanic crust, is lighter than the mantle. Some of it will rise to the surface to form volcanoes and long chains of fold mountains (Andes). These volcanoes typically form about 100 km from the trench. The chemistry of the magma changes to amore acidic andesite or rhyolite which has a higher viscosity thanthe basic basalt. This stops the gases getting out and causes the magma to solidify quickly. Thus giving cone shaped volcanoes which will explode dangerously.
Usually they appear at destructive margins where volcanic eruptions are at their most violent. Melting snow, caused by the eruption of Nevado del Ruiz in Colombia in 1985, created mudflows which killed 21,000 inhabitants of the town of Armero.

Some of the material that was deposited under the deep ocean is not subducted and becomes attached to the continental plate. The pressure on the continental plate causes folds to form, these rarely are simple folds with synclines and anticlines, but often the folds become 'overfolded', (recumbant) and faulted. These can form ' nappes '. Within the fold mountains are large intrusive igneous bodies as well as metamorphic rocks.



One sub-type is where the two crusts which converge are both oceanic: because densities are similar, speed of movement controls which one of the two subducts. The faster one is destroyed and the molten crust material that rises up to the ocean floor as lava builds up island arcs . Examples of this are found in the western Atlantic Ocean where the basalt of the Atlantic Ocean meets the basalt of the Carribean Plate. The former is descending, giving rise to such islands as Montserrat . Probably the most famous eruption of the last few years was the 1991 eruption of Pinatubo .

If the descending basalt plate is 'carrying' continental rock then the result will again be fold mountains. But gradually subduction will cease as the two plates becoms welded together. This is called a Collision Zone .