Types, Processes and Landforms of Plate Margins (AQA A Level Geography): Revision Note
Constructive Plate Margins
Types of Plate Boundary
At each plate boundary, different processes take place and different landforms are found
There are three main types of plate boundary:
Constructive (divergent) - plates move apart or diverge
Destructive (convergent) - plates move together or converge
Conservative (transform) - plates sliding past each other

At a constructive (divergent) boundary the plates are moving apart
Both volcanic eruptions and earthquakes can occur at this type of plate boundary

Constructive margins create two different landforms:
An ocean ridge forms when the diverging plates are under the ocean
As the plates move apart, magma rises up to fill the gap and this accumulates over time to become taller and wider
The Mid Atlantic Ridge is an example of an ocean ridge
A rift valley forms when the diverging plates are beneath the land
As the plates move apart, the crust stretches and fractures
Areas of crust drop down between faults to create a valley
The East African Rift Valley is an example of a rift valley

Destructive Plate Margins
At a destructive (convergent) plate boundary, the plates are moving towards each other
Oceanic plate and continental plate meet
Deep sea trenches are long, narrow depressions in the ocean floor with depths of over 6km and up to 11km
Trenches are found adjacent to land areas and associated with island arcs
The trench is usually asymmetric, with the steep side towards the land mass
The boundary between the Nazca plate and the South American plate is an example
The zone of earthquakes along the subduction slab is known as the Benioff Zone
The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
This forms deep ocean trenches in the subduction zone
Both violent volcanic eruptions and earthquakes occur at this type of plate boundary
Volcanoes tend to be steep-sided, composite volcanoes with andesitic and rhyolitic lava flows
Oceanic and continental convergent plate boundaries are also responsible for fold mountains
The Andes are an example of fold mountains at an oceanic–continental destructive boundary

Oceanic - continental destructive boundaries create two distinctive landforms:
Deep sea trenches are formed as the oceanic plate bends and subducts
An example of a deep sea trench is the Peru-Chile trench
Fold mountains are formed when the continental plate is compressed and deformed, buckling and folding due to pressure
Fold mountains form the highest of the world’s mountain ranges
They are long, relatively narrow belts of mountains, with parallel ridges and valleys
The main fold range is made up of a series of smaller ranges, with flatter areas forming plateaus in the mountains
Oceanic plate and oceanic plate meet
The older and denser oceanic plate subducts beneath the younger less dense plate
Both volcanic eruptions and earthquakes occur at this type of plate boundary
The convergence of two oceanic plates creates two distinctive landforms:
Deep sea trenches form due to subduction
An example of a deep-sea trench at an oceanic–oceanic boundary is the Mariana Trench
Island arcs may form where submarine volcanic eruptions lead to crust building up and rising above sea level
Japan is an example of an island arc
Continental plate and continental plate meet (also known as a collision boundary)
Two plates of similar density move towards each other
Neither is dense enough to subduct so the land is pushed upwards
This forms fold mountains such as the Himalayas
Earthquakes are the main hazard at this type of plate boundary
There is no volcanic activity because there is no subduction

Worked Example
Outline processes which lead to the formation of fold mountains
[4 marks]
Remember, this answer is point marked with 1 mark for each valid point made with extra marks for developed points (d)
The command word is ‘outline’
The focus of the question is ‘fold mountains’
You will gain marks for outlining the stages of fold mountain formation – try to add development at each stage e.g. if you say that two plates are converging, develop this by explaining what is causing them to converge
Answer:
Fold mountains are product of the convergence of tectonic plates (1).
Continental and / or ocean plates are forced together (1). This may be as a result of opposing convection currents or as a result of concepts associated with slab pull and ridge push (d)
Some may refer to continental fold mountain formation which crust is less dense, forced together with a crumpling effect (1) whereby continental mass is warped and forced upwards (d).
Others may consider the relationship between continental and ocean crust whereby the more dense ocean curst is forced into the mantle (1) leading to uplift of the continental crust(d). These fold mountains such as the Andes are also associated with volcanic activity (d).
Conservative Plate Margins
At a conservative (transform) margin the plates move past each other in opposite directions or in the same direction at different speeds
Earthquakes are the only hazard at this type of boundary
An example of a conservative margin is the San Andreas Fault

Process and magnitude
The processes which occur at the plate boundaries impact on the magnitude of the eruption or earthquake
At constructive margins:
Earthquakes tend to be mild and shallow
Volcanic eruptions tend to be less explosive
At destructive margins:
Friction and pressure build up in the Benioff zone (the area within the subduction zone where most friction and pressure build up occurs) causing strong earthquakes
Volcanic eruptions tend to be explosive as the magma is forcing its way to the surface
At conservative margins:
Plates can stick causing a significant build up of pressure and powerful earthquakes
Examiner Tips and Tricks
It is important that you are able to explain the processes that occur at the different plate boundaries but also that you are able to explain how these processes impact on the magnitude of the earthquake or the volcanic eruption.
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