Sunday, January 07, 2007

Plate Boundaries

Plate Boundaries

The point at which two tectonic plates meet is called a plate boundary. It is at these locations where tectonic activity results in earthquakes, volcanoes and the formation of mountain ranges due to the movement of the plates. The diagram below shows the major plates and their boundaries. The arrows indicate the direction of movement at each plate. It is the direction of movement as well as the difference in crust which determine the variations in processes and landforms at the different plate boundaries. (animation from USGS)

There are a number of different types of plate boundaries. For each plate boundary you will need to be able to describe (i) the movement (ii) processes which occur and (iii) an example

1. DESTRUCTIVE BOUNDARY (also known as a convergent boundary)

Movement: Two plates moving towards each other (continental and oceanic crust)
(note where two oceanic plates meet one will be subducted and an island arc will form)

The denser oceanic crust is subducted underneath the continental crust forming a subduction zone and oceanic trench. As it is subducted it melts due to heat and pressure. The heat sources are friction between the two plates and from the earth's interior. Melting of the subducting plates creates magma which is lighter than the mantle and therefore rises resulting in the formation of volcanoes. Earthquakes also occur at this type of boundary due to the friction and pressure during subduction.

Landforms Created:
Fold Mountains and Ocean Trench

South American and Nazca Plates (forming the Andes and a deep sea trench (Peru-Chile trench))

: click here to see an animation of the processes at a destructive plate boundary (with thanks to Wycombe High School)

2. CONSTRUCTIVE BOUNDARY (also known as a divergent boundary)
Movement: two plates moving away from each other (see animation opposite - courtesy of USGS)

As the two plates separate, hot magma is able to rise to fill the 'gap' creating new crust. As magma continues to build up, new mountain ranges form under the sea creating a mid-oceanic ridge. Where rising magma continues to build up above the ocean surface, a volcanic island is formed (for example Surtsey, Iceland). Both earthquakes and volcanoes occur at this type of boundary.

Landforms Created:
Ocean Ridge; Volcanic Islands

North American and Eurasian Plate - (forming the Mid-Atlantic Ridge)

click here to see an animation of the processes at a constructive plate boundary


Movement: two plates moving towards each other (both continental crust)

As both plates consist of continental crust they both resist subduction and buckle and fold, being forced upwards to create fold mountains, such as the Himalayas. Although there is no volcanic activity at these locations, due to the forces of collision major earthquakes often occur here.

Example: Indo-Australian and Eurasian Plate (forming the Himalayas)
Animation: Click here to see an animation of the processes at a collision boundary

Movement: two plates moving alongside each other

crust is neither created or destroyed here but as both pressure and friction results during the movement of the plates side by side, a 'stick-slip' motion results in the creation of significant earthquakes. Pressures builds up due to friction between the plates and when the plates break apart the energy is sent through the earth as seismic waves in the form of an earthquake.

San Andreas Fault - North American and Pacific Plates

See this BBC visualisation of the San Andreas Fault conservative boundary

You should be aware that whilst most volcanoes / earthquakes occur along plate boundaries, there are exceptions. For example the volcanic Hawaiian islands which can be found in the middle of the Pacific Plate are formed due to a Hotspot. Hotspots are plumes of molten rock which rise underneath a plate causing localised melting and the creation of magma resulting in volcanic activity. See this animation for further explanation of hotspot activity.

Follow up Links
An excellent site from the USGS called "Understanding Plate Motions" provides further information
A Flash animation showing the major plate boundaries.
Plate Tectonics on Wikipedia also provides a good overview of the processes at plate boundaries

Key Term Check
Constructive Boundary (Divergent) - where two plates move away from each other resulting in new crust being formed.
Destructive Boundary (Convergent) - where two plates move towards each other - in the case of a plate consisting of continental crust meeting a plate consisting of oceanic crust, the oceanic crust will be subducted and destroyed as it is less dense.
Conservative Boundary - where two plates move alongside each other - although crust is neither created or destroyed here, earthquakes usually occur here.
Collision Boundary - where two plates of continental crust move towards each other creating fold mountains.
Volcano - a vent through which lava, ash etc. is erupted (often, but not always cone-shaped)
Earthquake - a sudden ground movement

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