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TYPES OF PLATE BOUNDARIES & THEIR ASSOCIATED PROCESSES / LANDFORMS

Most large scale geological features are found at plate boundaries, as well as earthquakes and volcanoes. To understand these features we need to look at the processes operating at each of the plate boundaries.

IMPORTANT TO REMEMBER:

• Continental crust (low density / permanent) - does not sink; Ocean crust (denser and can sink) - it is therefore oceanic crust that is continually created and destroyed;
• Contiental plates, may consist of both oceanic and continental crust (e.g. Eurasian Plate)

EXAMPLES OF PLATE BOUNDARIES TO LEARN:

Constructive Plate Margins (also known as Divergent Boundaries)

Example: Mid-Atlantic Ridge - where the American plates are moving away from the Eurasian and African plates

Description: 2 plates diverging (moving away from each other)

• These boundaries occur where two plates move away from each other above the rising limbs of convection currents in the mantle;
• doming of the surface, may result from the upwelling of magma at these points, creating a ridge;
• as the plates continue to move apart, faults are created through which upwelling magma is able to reach the surface and cool and solidiy, creating new crust as material becomes new lithosphere added to the edges of diverging plates
• Volcanoes occur along constructive margins, either as fissure eruptions or as individual volcanic vents;
• New crust spreads laterally away from the ridge until it reaches an island-arc or mountain belt where it descends into an oceanic trench at the subduction zone of a destructive boundary (see notes below)
• This process of oceanic crust development is known as sea-floor spreading.
• Ridges tend to have an irregular pattern which are offset by transform faults which are at 90o to the plate boundary and which are created by tensional forces forces during the divergence of the two plates;
• Shallow focus earthquakes occur at constructive boundaries, associated with the rising magma, as well as due to the friction between the moving plates at the transform faults along the boundary. The earthquakes here are often low magnitude but high frequency as pressure is easily released.
• Divergence may also occur between two plates of continental crust, creating a rift valley - e.g. the East African Rift Valley.

See this excellent animation on the processes at constructive plate boundaries as well as this animation on the formation of rift valleys (both links - Wycombe High School)

Destructive Plate Margins (also known as Convergent Boundaries)

Example: NAZCA (Oceanic Crust) and SOUTH AMERICAN (Continental Crust) plate boundary (creating the Andes)

Oceanic Nazca plate - moving eastwards at approx 12cm/yr-1, converging with and subducting beneath the South American plate which is moving west at 1cm/yr-1.

Description: 2 plates (oceanic and continental crust) converging, resulting in the subduction of the oceanic plate

• Two places converge at a destructive boundary due to the existence of the descending limbs of convection currents. The oceanic plate, which is denser than the contienntal crust, is subducted underneath the continental plate and descends into the asthenosphere;
• At the point of subduction, an oceanic trench is created on the ocean floor
• The descending ocean plate is heated by heat from the interior and from friction created at the subduction zone and melts due to the heat and increasing pressure;
• The newly formed magma (melted plate material) is less dense than the surrounding material and therefore rises through weaknesses towards the surface - where it will either being intruded into the crust, or will break through the surface, creating volcanic landforms inland or if offshore, will create an island arc.
• The forces of collision of the two plates, cause the continental crust to be uplifted and folding and faulting results in the creation of fold mountains (e.g. the Andes range which rises 7000 m above sea level);
• The forces of collision and subduction also trigger major earthquakes within the subduction zone, along the benioff zone (boundary between the overlying crust and the plate undergoing subduction).
• The earthquakes vary from shallow focus (at the ocean trench) to deeper focus earthquakes (although only up to a depth of 700km, beyond which friction is insignficant). Major earthquakes are often associated with destructive plate boundaries.
• Where there are two oceanic plates converging, the older, denser plate is subducted and a simillar set of processes occurs, although an island arc is formed along the subduction zone (chain of volcanic islands)

See this excellent animation on the processes at destructive plate boundaries (Wycombe High School Link)

Collision Plate Margins

Example: Indian and Eurasian Plate - formed the HIMALAYAS

Description: involves the convergence (movement together) of two plates of continental crust

• 2 plates of continental crust move together - both plates are rigid, buoyant and of low density and no subduction occurs;
• The forces of collision causes the crust to deform and any ocean sediments between the two plates are forced up;
• This results in OROGENESIS (mountain building) as the folding and faulting creates Fold Mountains
• No volcanoes occur here due to the absence of a magma source due to the lack of subduction.
• Deep focus earthquakes do occur (intensity felt at surface not great due to depth of focus)

See this excellent animation on the processes at collision plate boundaries (Wycombe High School Link)

Conservative Plate Margins (Transform Margin)

Example: North American Plate & Pacific Plate (San Andreas Fault)

Description: 2 plates sliding past each other horizontally at a Transform Fault

• Two plates moving laterally past each other - crust is neither created nor destroyed
• Plates have been sliding past each other for millions of years and the rocks facing each other are often therefore of different types and ages;
• The movement is not a smooth one and friction builds up, resulting in a 'stick-slip' process with the build up of pressure and friction being released in the form of earthquakes.
• The earthquakes experienced are shallow-focus earthquakes - where the pressure is released easily, frequent, low magnitude tremours are experienced. However, more violent, high magnitude earthquakes may result after the a significant build up of pressure is suddenly released.

A Summary of Plate Margin Movements and Associated Geological Processes and Landforms:

A good article on the different types of plate boundaries and plate tectonics

Understanding plate Motions - an excellent page from the USGS - part of their "Dynamic Earth" web

You need to know the processes associated with the different types of plate boundaries and this includes earthquakes. As well as the St Ivo School Weblinks section on earthquakes, there are more links specifically relating to earthquakes on the Geography pages site written by Alan Parkinson (Head of Geography at King Edward VII School - Kings Lynn) - thank you Alan!