What are the processes associated with plate tectonics?

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Evidence to support the theory of plate tectonics

1) Continents seem to fit together like a jigsaw

2) Cynognatus fossils were found in both Africa and South America

3) The same glacial deposits left by glaciers in different continents (eg. Antarctica, South Africa, India)

4) The magnetic allignment of iron minerals have the same pattern- New molten rock containing iron alligns according to the Earth's current magnetic field.

5) Volcanoes and earthquakes occur at the plate boundaries

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What drives plate tectonics?

Radioactive decay in the core generates heat. This heat is transferred into the mantle, resulting in the formation of convection currents. The direction of the convection currents beneath the crust determine the direction of crustal movement.

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Constructive (divergent) plate bounaries

At constructive plate boundaries, two plates move apart from each other. 

Convection currents within the mantle, caused by radioactive decay at the Earth's core,  rise in the form of mantle plumes. The convection curents force molten magma in the aesthenosphere (upper layer of the mantle) upwards.

This forces the earth's crust to rise, forming a ridge. This is crustal upwarping. The Mid Atlantic Ridge is an example which seperates the Eurasian and North American plates.

As the crust is forced up to form this ridge, fracturing occurs att the boundary, which creates a void. This is then filled with magma below, which forms new crustal material. Volcanic eruptions are common along constructive plate boundaries, particularly when there are active convection cells (known as hot spots).

On the Mid Atlantic Ridge, these eruptions have built up to form volanic islands such as Iceland. In Iceland, many of the eruptions deisplay a linear nature. For example, Krafla last erupted in 1984 and occurred on a 14km long fissure.

As the magma cools, the two plates move away from the ridge and create new crustal material. The land is dropping between the fissures, known as rifting. Sea floor spreading is when the crustal material is formedand moving away from the ridge (eg. the Atlantic Ocean).

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Constructive (divergent) plate bounaries

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Destructive (convergent) plate boundaries- Oceanic

  • At these collision zones, denses oceanic crust cannot override continental crust and is therefore subducted beneath it. This leads to the formation of a subduction zone and an ocean trench, such as the Peru- Chile trench off the west coast of South America where the Nazca plate is subducted beneath the South American plate.
  • Often, the oceanic plate may become stuck as ist is subducted down past the continental plate. This can create powerful earthquakes.One of these hit Chile in May 1960 and was the most powerfl earthquake ever recorded (9.5 RS). The zone where the plates stick is known as the Benioff Zone.
  • The heat produced by the friction of the two plates moving against eachother also causes partial melting of the overriding plate, creating magma. Being less dense than the surrounding mantle, this magma will rise up through the lithosphere to form volcanoes such as Puyehue in Chile, which last erupted in 2011.
  • Volcanoes along these plte boundaries are violent and explosive due to the fact that the magma is made of melted crustal material and not magma from the mantle. It may be mixed with sediment from the sea bed and water. The magma rising up through the continental crust may cool and block vents, causing a build up of pressure, leading to violent eruptions. Strong tectonic activity is a feature of this boundary.
  • The convergence of the two plates causes the young rocks of the lithosphere to fold as a result of compressional forces (eg. the Andes).
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Destructive (convergent) plate boundaries- Oceanic

These collision zones operate in the same way as the oceanic/continental collisions. However, they result in a series of volcanic islands known as an isand arc.


  • The Carribean
  • The Philippines
  • Japan 
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Destructive (convergent) plate boundaries- Contine

When two continental crusts collide, neither is subducted. Therefore, there is no volacanic activity asssociated with this type of plate boundary. The two colliding crusts push against eachother to form high fold mountains. Powerful earthquakes are common at this plate boundary.

The Himalayas have formed at this type of plate boundary. 

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Conservative (transform) plate boundaries

At these locations, two neighbouring plates slide past eachother. This process often caused the plates to lock against eachother, resulting in a build up of tectonic pressure. When pressure is released, powerful eathquakes occur as movement occurs along the fault (eg, the San Andreas fault in California).

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