Plates and plate movement

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Plates and plate movement

There are 2 main types of crust that lie on plates and each has its own characteristics:

Oceanic Crust (Sima) Young (under 200 million years), thin, dense and heavy, will sink. Always being created and destroyed. For example, Basalt.
Continental Crust (Sial) Old, light, thick (up to 150km under mountains) permanent, does not sink. For example, Granite.

Movement of plates is in one of 3 ways:

Towards each other: Convergent (destructive or collision).

Away from each other: Divergent.

Alongside each other: Transform or transcurrent.

There is a further subdivision for Convergent plates, which is dependent on the properties of the plates moving towards each other (oceanic / continental crust).

Convergent plate boundaries (together)

The two types of margins that occur here are Destructive Margins or collision Zones. The chart and the diagrams below outline their main features:

Destructive margins Oceanic crust moves towards continental crust. Oceanic crust sinks, and slowly destroyed. Results in deep ocean trenches, island arcs and fold mountains. Can be referred to as subduction zones.
Collision Zones Two continental crusts meet, neither sinks, fold mountains result. For example, the Alps.

Divergent plate boundaries (away)

Also known as a Constructive margin. Plates move away from each other, for example, N. American and Eurasian plates, cresting mid-ocean ridges such as the Mid Atlantic Ridge. New material appears at the ocean ridge. Click play on the diagram below to see this in action:

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Transform/transcurrent boundaries

Here plates simply move alongside each other, land is neither created nor destroyed.

Examples of features created and hazards caused by plate movement are outlined below:

Destructive plate margins (For example, Nazca and south American plates)
A Peru - Chile Trench as oceanic crust is subducted.
B Earthquakes, due to the heat created as the Nazca plate is destroyed.
C Fold Mountains such as the Andes.
D Volcanoes if heat from the subduction zone has the opportunity to reach the surface, for example, Cotopaxi.
E Island arcs and volcanic islands, for example, Japan.

Collision zones: Indian Plate and Eurasian Plate created the Himalayas.

Constructive margins: North American plate moving away from the Eurasian plate resulting in the mid Atlantic ridge.

Transform faults: earthquakes such as those linked with the San Andreas Fault.

Hot spots

There are examples where volcanic activity is not linked to plate margins as in the case of the Hawaiian Islands. This is believed to be due to the presence of 'hot spots' - places of localised heat under the earth's crust that then find their way to the surface.