What this lesson teaches
I can link where earthquakes and volcanoes occur to plate boundaries and hotspots.
Syllabus K229, Topic 3.2, Key Question 3. How might distribution of earthquakes and volcanoes influence the location of tectonic hazards?
Make a guess
Hawaii has active volcanoes, but it sits in the middle of the Pacific Plate, far from any boundary. How can that be?
- The plate is moving over a hotspot.
- The volcanoes must be extinct and harmless.
- A hidden subduction zone lies under Hawaii.
- Every Pacific island is on the Ring of Fire.
Show the answer
The plate is moving over a hotspot.
Hotspots can make volcanoes far from plate boundaries. As the Pacific Plate moved over one, it built a chain of ocean volcanoes.
Most earthquakes and volcanoes cluster near plate boundaries, but hotspots and far-travelling hazards are important exceptions.
Earthquakes happen at all three types of plate boundary, divergent, convergent and transform, because moving plates strain rock and make faults slip. The biggest cluster is the Pacific Ring of Fire. This is a broad belt around the edge of the Pacific Ocean, not one circular fault. Some earthquakes also happen inside plates.
Many volcanoes form where plates move apart, because rising mantle melts as its pressure falls. Others form at subduction zones, where water released from the sinking plate helps the mantle above it melt. Where continents collide or plates slide past each other, there are earthquakes but usually no line of volcanoes.
Hotspots can make volcanoes far from plate boundaries. Hawaii is a chain of ocean volcanoes that formed as the Pacific Plate moved over a hotspot. In Iceland, a hotspot sits on a plate boundary, so both explain its volcanoes.
Some hazards reach places far from their source. Shaking is usually strongest near the rupture, changed by local ground. Tsunamis can cross whole oceans, and wind can carry fine ash far away. So distant ports and flight routes can be disrupted.
To describe a pattern, state the main trend, name places that show it, and note exceptions. To explain it, link the places to the processes. For example, link the volcanoes around parts of the Pacific edge to subduction.
- Ring of Fire
- Belt of earthquakes and volcanoes around the Pacific.
- Hotspot
- Volcanoes away from boundaries, like Hawaii.
- Far-reaching hazards
- Tsunamis and wind-blown ash.
Step by step
Plate boundaries
Earthquakes at all types; volcanoes at ridges and subduction zones.
Hotspots
Volcanoes inside plates.
Travelling hazards
Tsunamis and ash can reach far-off places.
Real examples to use
Pacific Ring of Fire
Most of the world's active volcanoes and large earthquakes lie around the edge of the Pacific Ocean, along plate boundaries from Chile to Japan and Indonesia.
Hawaii and Iceland
Hawaii's volcanoes sit over a hotspot in the middle of the Pacific Plate. Iceland lies on the Mid-Atlantic Ridge, where two plates move apart.
Indian Ocean tsunami, 2004
An earthquake off Sumatra, Indonesia, sent a tsunami that killed people as far away as Sri Lanka, India and Somalia, thousands of kilometres from the source.
Worked example: Far from the volcano, still affected
Ash from an eruption can be blown across a distant flight route. A plane does not need to fly over the volcano to be affected. Eruption height and wind matter as well as distance.
Watch out for this
Places outside the Ring of Fire have no tectonic risk.
Other boundaries, hotspots and faults exist, and tsunamis or ash can travel far.
Check your understanding
A transform boundary has earthquakes but no volcanoes. Is this surprising?
- Yes. Earthquakes happen where plates converge, not where they slide.
- No. Earthquakes are rare along transform boundaries, so both are missing.
- No. Sliding plates cause earthquakes but make no magma.
Show the answer
No. Sliding plates cause earthquakes but make no magma.
Yes, that is expected. Volcanoes need melting, as at ridges and subduction zones.