Tectonic Hazards » Plate Margins: Constructive, Destructive and Conservative

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.1.1.2

• What happens at constructive, destructive and conservative plate margins
• How each type of margin leads to earthquakes and volcanic activity
• How to describe each margin in words, as if you were labelling a diagram

Why the type of margin matters

Iceland sits on a constructive margin, where plates move apart and magma rises

Earthquakes and volcanic eruptions are the result of physical processes. The processes depend on how two plates move against each other at a plate margin. There are three types of margin you need to know: constructive, destructive and conservative. Each one makes different hazards. If you can say what the plates are doing, you can work out what hazard to expect.

Key terms:

• Constructive margin: a plate margin where two plates move apart and new crust is made.
• Destructive margin: a plate margin where two plates move towards each other and one is destroyed.
• Conservative margin: a plate margin where two plates slide past each other and no crust is made or destroyed.
• Magma: molten rock beneath the Earth's surface. When it erupts at the surface it is called lava.
• Subduction: when a denser plate sinks beneath another plate.
• Ocean trench: a deep, narrow valley in the sea floor where a plate is sinking.
• Fault: a crack in the crust where rocks have moved.

Constructive margins: plates move apart

At a constructive margin, two plates pull away from each other. This is also called a divergent margin. Here is what happens, step by step:

1. The two plates move apart, which leaves a gap.
2. Magma rises from the mantle to fill the gap.
3. The magma cools and solidifies, making new crust.
4. This happens again and again, so a ridge of new rock builds up along the margin.
5. Some of the magma reaches the surface. The lava is runny, so it flows a long way before it sets. This builds a wide volcano with gentle slopes, called a shield volcano.

Earthquakes also happen here. As the plates pull apart, the crust cracks and shifts. These earthquakes are usually small, because the plates are not locked together under great pressure.

Destructive margins: plates move together

At a destructive margin, two plates move towards each other. The best example for you to know is where an oceanic plate meets a continental plate. The Nazca Plate (oceanic) is being pushed beneath the South American Plate (continental) along the west coast of South America.

Remember from Plate Tectonics and Where Hazards Happen that oceanic crust is denser than continental crust. This decides which plate sinks:

1. The denser oceanic plate is forced down beneath the lighter continental plate. This is subduction.
2. An ocean trench forms where the plate sinks.
3. The plates do not slide smoothly. Friction makes them stick, and pressure builds up. When the plates suddenly jerk free, the energy is released as an earthquake. These can be very powerful.
4. As the sinking plate goes deeper, it melts and is destroyed. The melted rock becomes magma.
5. The magma is lighter than the rock around it, so it rises. It forces its way up to the surface and erupts as a volcano.

Volcanoes at destructive margins tend to be steep-sided cones. The magma is thick and sticky, so it traps gas and erupts violently. The continental plate is also crumpled upwards, which forms fold mountains such as the Andes.

Conservative margins: plates slide past

At conservative margins plates slide past each other, causing earthquakes but no volcanoes

At a conservative margin, two plates slide past each other. They may move in the same direction at different speeds, or in different directions. The San Andreas Fault in California is an example. The Pacific Plate and the North American Plate slide past each other here.

1. The plates try to slide past each other, but friction makes them stick.
2. Pressure builds up along the fault as the plates keep trying to move.
3. Eventually the pressure is more than the rocks can hold. The plates suddenly jerk past each other.
4. The energy is released as shock waves, which we feel as an earthquake.

Crust is neither made nor destroyed here. There is no magma, so conservative margins do not produce volcanoes. Their only tectonic hazard is earthquakes, but these can be very strong.

Comparing the three margins

Exam-style question

(a) Name the type of plate margin where two plates slide past each other. [1 mark]

(b) Explain why earthquakes happen at this type of margin. [3 marks]

(c) Describe what happens at a destructive plate margin to cause a volcano to form. [3 marks]