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.
Plate Boundaries
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)
Processes: 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
Example: South American and Nazca Plates (forming the Andes and a deep sea trench (Peru-Chile trench)
2. CONSTRUCTIVE BOUNDARY (also known as a divergent boundary)
Movement: two plates moving away from each other
Processes: 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
Example: North American and Eurasian Plate - (forming the Mid-Atlantic Ridge)
3. COLLISION BOUNDARY
Movement: two plates moving towards each other (both continental crust)
Processes: 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)
4. CONSERVATIVE BOUNDARY
Movement: two plates moving alongside each other
Processes: 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.
Example: San Andreas Fault - North American and Pacific Plates
HOTSPOTS
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.
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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