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Mechanisms of Plate Motion

Types of Collisions

Ocean-Ocean Collisions

When two oceanic plates collide one oceanic plate is eventually subducted under the other. Where one plate slides under the other is referred to as the 'subduction zone'. As the subducting plate descends into the mantle where it is being gradually heated a benioff zone is formed. This benioff zone is a zone of shallow,intermediate and deep focused earthquakes. Some deep focused earthquakes that occur at ocean ocean- collision boundaries can be as deep as 670 kilometres. As the subducted plate descends into the mantle it is gradually heated allowing the formation of magma. The magma that forms is andesitic in composition and begins to form when the subducted plate reaches a depth of 100 kilometres. This andesitic magma is formed from the partial melting  of the asthenosphere just above the subduction zone. This partial melting of the subducting plate is due to the loss of water as it descends into the mantle. The andesitic magma is now less dense than the surrounding material so it rises through the crust and erupts to form an arc of volcanoes called an island arc. The distance between the trench and the island arc depends greatly upon where the subducting plate reaches the 100 kilometer depth. If the subduction angle is steep then the distance between the arc and the trench will be short.If the suduction angle is shallow the distance is longer. The main features are indicated in the diagram below. The swell is seen by a bulge in the in the downgoing plate where it is subducted into the mantle.where the plate subducts into the mantle is known as the trench.the forearc ridge contains highly deformed sedimentary and metemorphic rock. The backarc region is located behind the arc and can be compressed or extended.

Fig.2.1. Ocean-ocean collision zone

Ocean-Continent Collisions

When an oceanic and a continental plate collide, eventually the oceanic plate is subducted under the continental plate due to the high density of the oceanic plate. Once again a benioff zone forms where there are shallow intermediate and deep focus earthquakes. As the oceanic plate is subducted sediment is scraped off to form an acretionary wedge at the point of collision between the two plates. When the oceanic plate is subducted due to partial melting of the asthenosphere magma with an andesitic composition is formed. The magma formed is less dense than the surrounding material so it rises to the surface to form a magmatic arc on the edge of the continent which the oceanic plate is subducted under. Over time the continental margin, due to compression forms into a folded mountain belt. As time goes on the hot magma rising upward from the subduction zone causes further compression of the mountain belt. Deep mountain roots form and are gradually metamorphosed and intruded with granitic plutons.Explosive volcanic activity is commonly associated with this type of collision boundary. This is shown in the diagram below.The major componants of an oceanic - continental collision zone and a accretionary wedge are also shown in the diagrams below.

Fig.2.2 Ocean-continent collision zone

Fig.2.3. Deformation at an ocean-continent collision zone

Continent-Continent Collision

When two continental plates collide neither plate can be subducted due to their high bouyancy. With this type of collision there are no features such as a subduction zone, trench or acretionary wedge. The collision of two continental plates occurs when a sea becomes narrower until both plates collide. After collision the oceanic lithosphere breaks off and sinks into the mantle. The subduction zone eventually becomes inactive  The two continents become welded together as they are compressed together over time. The crust is thickened by the underthrusting of one continent under the other. These regions have both shallow focus earthquakes and deep focus earthquakes as the oceanic lithosphere is subducted under the mounatin range. Thrust faults and highly metamorphised granites are common charcteristics of these collision zones. The most well known example of this collision zone is the Himalayas where India has collided with Asia.

Fig.2.4. Continent-continent collision zone
Mechanisms of Plate Motion
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