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Plate tectonic theory – WJEC Constructive plate boundaries

Earthquakes and volcanic eruptions affect people all over the world. They are caused by the movement of tectonic plates. Tectonic hazards can destroy buildings, infrastructure and cause deaths.

Part of GeographyTectonic landscapes and hazards

Constructive plate boundaries

At a constructive plate boundary - also known as a - the plates are moving apart from one another. When this happens, the magma from the mantle rises up to make (or construct) a new crust. The movement of the plates over the mantle can cause earthquakes. Rising magma can also create .

Magma rises through the gap between the two plates, pushing them apart. A volcano forms on the Earth's crust at this point.
Figure caption,
Magma rises through the gap between the two plates, pushing them apart. A volcano forms on the Earth's crust at this point. Plate A would be the North American plate which is moving westwards, where plate B would be the Eurasian plate which is moving eastwards

Landforms associated with constructive plate boundaries

Large scale landforms at a constructive plate boundary include:

  • ocean ridges - such as the Mid-Atlantic ridge, where the Eurasian plate and the North Atlantic plate are moving apart from each other under the Atlantic Ocean
  • rift valleys - such as the East African Rift Valley where the African plate is tearing itself apart
  • shield volcanoes

Small scale landforms at constructive plate boundaries are lava tubes and geysers.

Large scale landforms (Constructive plate boundary)Small scale landforms (Constructive plate boundary)
Shield volcanoesLava tubes
Rift ValleyGeysers
Large scale landforms (Constructive plate boundary)Shield volcanoes
Small scale landforms (Constructive plate boundary)Lava tubes
Large scale landforms (Constructive plate boundary)Rift Valley
Small scale landforms (Constructive plate boundary)Geysers
A plume of water bursts into the air with tourists watching.
Image caption,
The Great Geyser, Strokkur, Iceland

Conservative plate boundaries

At a , the plates slide past each other in opposite directions, or in the same direction but at different speeds. As the plates try and move, friction occurs and plates become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released it sends out huge amounts of energy, causing an earthquake. The earthquakes at a conservative plate boundary can be very destructive as they occur close to the Earth's surface. There are no volcanoes at a conservative plate boundary.

At a conservative plate margin, plates slide past each other.

A large scale landform found along a conservative plate boundary is a fault line. One of the most active fault lines in the world is the San Andreas fault line in California, USA.

A vast mountainous landscape under a clear blue sky
Image caption,
The San Andreas fault