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What is a constructive plate margin?

When two plates are moving apart.


What are constructive margins also known as?

Diverging plate boundaries.


What hazards can occur at constructive plate margins?

How is each formed?

Volcanes - mantle is under pressure from plates above, and when they move apart, the pressure is released at the margin. Mantle melts, forming magma which is less dense than the plate above - it rises and can erupt, forming a volcano.

Earthquakes - plates don't move apart in a uniform way - some move faster than others, so pressure builds up. This can cause the plate to crack, making a fault line and causing an earthquake.


What two landforms do constructive plate margins create?

Ocean ridges and rift valleys.


How are ocean ridges formed?

  • As tectonic plates diverge, basaltic lava rises up to fill gap created, then cools to form new crust.


  • New crust is dragged apart and even more new crust forms between it.


  • This is known as sea-floor spreading and can form a mid-ocean ridge.


What hazard can form on ocean ridges?

Give an example.

Underwater volcanoes erupt along mid-ocean ridges and they can build up to be above sea level.

Build up of underwater volcanoes along the Mid-Atlantic Ridge led to the formation of Iceland.


What is a fault line?

Where a plate has cracked under pressure.


Describe the formation of a rift valley:

  • At diverging plates, rising magma causes the continental crust to bulge and fracture, forming fault lines.
  • Plates keep moving apart, and crust between parallel faults drops down to form a rift valley.


At what type of plate margin are ocean ridges found?

Constructive margins.


At what type of plate margins are rift valleys formed?

Constructive margins.


Give an example of a rift valley:

East African Rift System - a series of rift vallyes that stretch from Mozambique to the Red Sea.

Formed due to Nubian plate and Somalian plates diverging. 


What type of hazard can be found in rift valleys?


Kilimanjaro and Mount Kenya found in East African Rift.


The area between two parallel rift valleys that forms an upstanding block.

A horst.


nm                           A horst.

The area between two parallel rift valleys that forms an upstanding block.


The line of the African Rift is thought to be what?

An emergent plate boundary - the beginning of the new ocean, as eastern Africa splits away from the remainder of the continent.


What is a destructive plate boundary?

Where two plates are moving towards each other


What is a destructive plate boundary also known as?

A converging margin.


What determines what happens at a destructive plate margin?

The types of plate involved in the boundary.


What hazards are associated with destructive margins?

Volcanoes and earthquakes.


What happens when an oceanic and continental plate converge?

  • More dense oceanic crust is forced under the less dense continental crust - it is subducted.


What landforms are created at destructive boundaries?

Deep sea trench or fold mountains (oceanic-continental).

Deep sea trench or island arcs (oceanic-oceanic).

Fold mountains (continental-continental).


What type of margin are deep sea trenches found at?



How is a deep sea trench formed?

When the denser oceanic crust subducts under the less dense continental crust.


How are fold mountains formed?

  • By two converging plates.
  • Edges and sediment bewteen the two plates are forced up into fold mountains.
  • There is little subduction and no volcanic activity.


On what type of margin are fold mountains formed?



How do volcanoes form on destructive margins?

  • Oceanic crust subducts under continental crust.


  • Oceanic crust is heated by friction and contact with the upper mantle, which melts it into magma.


  • Magma is less dense than the continental crust above and will rise back to the surface to form volcanoes.


How do earthquakes form on destructive margins?

  • As one plate moves under another it can get stuck.


  • This causes pressure to build up.


  • When pressure becomes too much, the plates jerk past each other, causing an earthquake.


What happens when oceanic and oceanic plates converge?

The denser one of the two is subducted.

Deep sea trenches form, and earthquakes and volcanic eruptions can be triggered.


How are island arcs formed?

  • During subduction, the descending plate encounters hotter surroundings, and this coupled with the heat generated from friction, begins to melt the plate.


  • As the material is less dense than the surrounding asthenosphere, it begins to rise towards the surface as plutons of magma.


  • These reach the surface and form volcanoes.


  • If eruptions take place off shore, a line of volcanic islands forms - known as island arcs.


What happens when continental and continental plates meet at a destructive margin?

Neither is subducted.