Types, Processes and Landforms of Plate Margins (AQA A Level Geography)

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

Types of plate boundary

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

Constructive 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

Formation of a rift valley

• • 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

Destructive plate 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

Collision plate boundary

• 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

Conservative plate boundary

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