Geography · Physical geography
Tectonic plates
Earthquakes and volcanoes.
What are tectonic plates?
Earth's outer shell is cracked into huge slabs called tectonic plates. These plates float on hot, semi-molten rock beneath called the mantle. Plates move very slowly – only a few centimetres each year. Where plates meet, dramatic events happen: earthquakes shake the ground and volcanoes erupt. Most earthquakes and volcanoes occur along plate boundaries (edges where plates touch).
Earthquakes at plate boundaries
Earthquakes happen when tectonic plates suddenly jolt past each other. Plates can get stuck as they try to move. Pressure builds up over many years. Eventually the plates slip, releasing huge amounts of energy as shock waves. These waves travel through the ground, shaking buildings and land. Scientists measure earthquake strength using the Richter scale or moment magnitude scale.
Volcanoes and eruptions
Volcanoes form where molten rock (magma) pushes up from deep underground. At some boundaries, one plate dives beneath another in a process called subduction. The sinking plate melts, creating magma that rises and erupts. At other boundaries, plates pull apart and magma wells up to fill the gap. When magma reaches the surface, we call it lava.
The Ring of Fire
The Pacific Ring of Fire is a massive horseshoe of volcanoes and earthquake zones. It circles the Pacific Ocean, stretching from New Zealand through Asia to the Americas. About 75% of the world's active volcanoes sit in this ring. Frequent earthquakes shake countries like Japan, Indonesia, Chile and California. All this activity happens because many tectonic plates meet around the Pacific.
Worked examples
1. Why do earthquakes happen more often in Japan than in the United Kingdom?
- Japan sits where several tectonic plates meet and push against each other.
- The UK lies in the middle of a plate, far from any plate boundaries.
- Earthquakes mainly occur at plate boundaries where plates move and collide.
- Therefore Japan experiences many earthquakes, while the UK has very few.
2. Explain how a volcano forms at a subduction zone.
- Two plates collide and one plate slides beneath the other (subduction).
- The descending plate melts as it sinks into the hot mantle below.
- This melting creates magma, which is less dense than surrounding rock.
- Magma rises through cracks in the crust and erupts at the surface as a volcano.
3. Describe what the Richter scale measures.
- The Richter scale measures the strength or magnitude of an earthquake.
- It records the size of seismic waves (vibrations) travelling through the ground.
- Each number increase represents about 30 times more energy released.
- A magnitude 7 earthquake is much more powerful and destructive than magnitude 5.
Try it yourself
Look at a map of the Pacific Ring of Fire. Why do so many volcanoes and earthquakes happen in Chile and Japan?
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