3.1.4 Types of Plate Margins Flashcards
Constructive Plate Margins
A constructive margin occurs where two plates are moving apart (diverging).
What landforms do constructive plate margins create?
- Oceanic Ridges
- Rift Valleys
[Landforms at constructive margins]
Oceanic Ridges
1) Where diverging plates are underwater, an ocean ridge forms. For example, the Mid-Atlantic Ridge is where the Eurasian plate and North American plate are moving apart.
2) Underwater volcanoes erupt along mid-ocean ridges and they can build up to be above sea level. For example, Iceland has been formed by the build-up of underwater volcanoes along the Mid-Atlantic Ridge.
[Landforms at constructive margins]
Rift Valleys
1) Where plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines.
2) As the plates keep moving apart, the crust between parallel faults drop down to form a rift valley.
3) Volcanoes are found around rift valleys.
How do volcanoes and earthquakes happen at constructive plate margins?
- The mantle is under pressure from the plates above. When they move apart, the pressure is released at the margin.
- The release of pressure causes the mantle to melt, producing magma.
- The magma is less dense than the plate above, so it rises and can erupt to form a volcano.
- The plates don’t move apart in a uniform way some parts move faster than others. This causes pressure to build up. When the pressure becomes too much the plate cracks, making a fault line and causing an earthquake. Further earthquakes may also occur along the fault line once it’s been created.
Destructive Plate Margins
A destructive margin occurs where two plates are moving towards each other (converging). What happens at these margins depends on the type of plates converging.
What happens at destructive plate margins when two continental plates meet?
Where two continental plates move towards each other, neither is subducted so there aren’t any volcanoes - but the pressure that builds up between them can cause earthquakes.
Fold mountains form when continental crusts converge. For example, the Himalayas.
What happens at destructive plate margins when two oceanic plates meet?
Where two plates of oceanic crust are moving towards each other, the denser of the two will be subducted, forming a deep sea trench and triggering earthquakes and volcanic eruptions.
Volcanic eruptions that take place underwater (e.g. when two plates of oceanic crust converge) create island arcs - clusters of islands that sit in a curved line, e.g. Hawaii
What happens at destructive plate margins when one oceanic and one continential plates meets?
Where continental crust and oceanic crust converge, thre more dense oceanic crust is forced under the less dense continental crust (it is subducted). This forms a deep sea trench.
Fold mountains also form where the plates meet. They’re made up of sediments that have accumulated on the continental crust, which are folded upwards along the edge of the continental crust.
The oceanic crust is heated by friction and contact with the upper mantle, which melts it into magma.
The magma is less dense than the continental crust above and will rise back to the surface to form volcanoes.
As one plate moves under the other they can get stuck. This causes pressure to build up. When the pressure becomes too much the plates jerk past each other, causing an earthquake.
Conservative plate margins
Where two plates are moving past each other.
The two plates get locked together in places and pressure builds up. As with destructive margins, this causes the plates to jerk past each other (or to crack, forming fault lines), releasing the energy as an Earthquake.
For example, the Pacific plate is moving past the North American plate. Many earthquakes occur along this margin and along its fault lines, e.g. along the San Andreas fault in California.
What is a magma plume?
A vertical column of extra-hot magma that rises up from the mantle.
Volcanoes form above magma plumes.
How are volcanic island chains formed?
1) Volcanoes form above magma plumes.
2) The magma plume remains stationary over time, but the crust moves about it.
3) Volcanic activity in the part of the crust that was above the magma plume decreases as it moves away.
4) New volcanoes form in the part of the crust that is now above the magma plume.
5) As the crust continues to move, a chain of volcanoes is formed.
Give an example of a chain of volcanic islands?
The chain of island that make up Hawaii was formed by a magma plume.
