Earthquakes and Volcanoes » What Happens at Each Plate Boundary

From plate movement to hazards

You know the four types of plate boundary and which way the plates move at each. Now we ask a bigger question: what actually happens at each boundary to cause an earthquake or a volcano? The answer is different every time, so each boundary needs its own set of steps. In the exam, write them as clear steps in the right order.

Key terms:

• Magma: hot, melted rock below the surface of the Earth.
• Lava: magma that has reached the surface.
• Friction: the force that makes two surfaces grip or stick as they rub together.
• Subduction: when a denser plate sinks beneath another plate and is destroyed in the mantle.
• Ocean trench: a very deep, long valley in the ocean floor where a plate sinks.
• Fold mountains: mountains made when rock layers are squeezed and crumpled upwards.

Divergent (constructive) boundary

Here the plates move apart and new crust is made. The earthquakes are usually small and the eruptions are mostly gentle.

1. Two plates move apart.
2. A gap opens and the pressure on the hot mantle below drops.
3. Magma rises up into the gap.
4. The magma erupts as lava, then cools and hardens to form new crust.
5. The plates keep moving, so the process repeats and a ridge or volcano builds up.

The lava is runny, so gas escapes easily and the eruptions are mostly gentle. The plates pull apart rather than grip, so only small earthquakes happen as the crust cracks.

Convergent (destructive) boundary

This is where an oceanic plate meets a continental plate (or another oceanic plate). The oceanic plate is denser, so it is forced down. This is the most dangerous kind of boundary, with both powerful earthquakes and explosive volcanoes.

Where the oceanic plate bends down, it drags the ocean floor into a deep ocean trench, such as the Peru-Chile Trench off the west coast of South America. Because the plate keeps sinking, earthquakes can happen at many depths, from shallow to deep, and they are often very powerful.

Collision boundary

Two continental plates are both light and thick, so neither one sinks easily. They push against each other instead.

1. Two continental plates move towards each other.
2. They are too light and thick to sink into the mantle, so there is no subduction.
3. The rock layers between them are squeezed and crumpled upwards into folds.
4. This builds high fold mountains, such as the Alps and, as you saw in Types of Plate Boundary, the Himalayas.
5. Friction and pressure build up in the crust and are suddenly released, causing earthquakes.

Conservative boundary

Here two plates slide past each other. Crust is not made and it is not destroyed.

1. Two plates try to slide past each other, either in different directions or at different speeds.
2. Friction makes the plates lock together.
3. The plates keep trying to move, so pressure builds up.
4. The rock suddenly breaks and the plates slip, releasing the pressure as an earthquake.

The San Andreas Fault in California, which you met in Types of Plate Boundary, is an example. There are no volcanoes because no plate sinks and melts, and no plates pull apart to let magma rise. Only earthquakes happen.

Hotspots: volcanoes away from boundaries

A few volcanoes are nowhere near a plate boundary. A hotspot is a place where a plume of very hot rock rises through the mantle, melts and breaks through the crust above it. The Hawaiian Islands, from Types of Plate Boundary, are in the middle of the Pacific plate. As the plate slowly moves over the hotspot, new volcanoes form one after another and make a chain of islands.

Comparing the four boundaries

Boundary	Earthquakes	Volcanoes	Key process
Divergent	Small	Yes, mostly gentle	Magma rises and makes new crust
Convergent (destructive)	Powerful, shallow to deep	Yes, explosive	Subduction and melting
Collision	Yes	Few or none	Folding into fold mountains
Conservative	Yes	None	Plates lock, then slip

Exam-style question

Fig. 1.1 shows made-up information about three plate boundaries, P, Q and R.

Boundary	Plates	Movement	Earthquakes	Volcanoes
P	Two continental plates	Together	Yes	None
Q	Oceanic and continental	Together	Powerful, shallow to deep	Many, explosive
R	Two plates	Sliding past each other	Yes	None

(a) Identify the type of boundary at Q. [1]

(b) Explain why there are earthquakes but no volcanoes at boundary R. [3]

(c) Explain how earthquakes and volcanoes are caused at boundary Q. [6]