The big idea: The Earth's rigid outer shell is broken into tectonic plates that float on the soft asthenosphere and move a few centimetres a year, driven by convection currents in the mantle.
Margins control the hazard: Most earthquakes and volcanoes cluster along plate boundaries (margins) — so the type of margin controls where these hazards happen and what they are like.
The core Option D skill: link a margin type to the character of its hazard
- Its lava
- Its explosiveness
- Its earthquake depth and magnitude
Key terms
- Tectonic plate — a rigid slab of the Earth's crust and upper mantle that moves over the asthenosphere.
- Plate margin (boundary) — where two plates meet; almost all earthquakes and volcanoes occur here.
- Constructive (divergent) — plates move apart; new crust forms (e.g. Mid-Atlantic Ridge).
- Destructive (convergent) — plates move together; one is subducted (e.g. the Andes).
- Conservative (transform) — plates slide past each other (e.g. the San Andreas Fault).
- Hotspot — a mantle plume producing volcanoes away from any margin (e.g. Hawaii).
- Focus / epicentre — where an earthquake starts underground / the point directly above it on the surface.
Margin type decides the hazard: Destructive margins give the most violent hazards — explosive volcanoes and large, deep earthquakes.
Constructive margins give gentle hazards — runny lava and small, shallow earthquakes.
Conservative margins give no volcanoes but can give large, shallow earthquakes.
What moves the plates: Plates move because the Earth is losing internal heat. That heat drives convection in the mantle, and gravity acts on the plates themselves at ridges (ridge push) and trenches (slab pull).
Two kinds of crack in the plates
- Rifting — a plate stretched over rising mantle splits apart; on land this makes a rift valley, as in the East African Rift, which may one day become a new ocean.
- Transform faults — short faults that offset an ocean ridge into sections; the plates slide past each other there, giving shallow earthquakes but no volcanoes.
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How this is tested: The figure you read here is usually a hazard-zone map — a region shaded by volcanic risk around the vent, often with spot heights and a scale bar.
| Command term | What it asks for |
|---|---|
| Estimate | A ground distance from the scale |
| Determine | A height difference between two spot heights |
| Identify | A physical effect, e.g. of liquefaction |
| Outline | The map evidence that an area is volcanic |
Read the key first: Read the key first and quote the units the question asks for.
Read the key first. Which zones are darkest (highest risk) and how does risk change away from the vent?
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
IB-style questionDescribe[2 marks]
Describe the distribution of volcanic hazard risk shown on the map.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
The three plate margins and what each produces
| Margin type | Plate movement | Volcanoes | Earthquakes |
|---|---|---|---|
| Constructive (divergent) | Move apart; magma rises to fill the gap | Frequent, gentle (effusive); runny basaltic lava builds shield volcanoes / ridges | Frequent but small and shallow |
| Destructive (convergent) | Move together; oceanic plate subducts | Explosive (high VEI); sticky andesitic lava builds steep cone (composite) volcanoes | Large, can be deep (along the subduction zone); cause the biggest quakes |
| Conservative (transform) | Slide past one another | None - no magma is created | Large but shallow; sudden release of built-up strain |
Why lava type follows the margin
- Constructive margins melt the mantle directly -> low-silica, basaltic magma -> runny, low-viscosity lava -> gas escapes easily, so eruptions are gentle (effusive) and the lava flows far across a wide area (shield volcanoes).
- Destructive margins melt subducted oceanic crust + sediment -> high-silica, andesitic magma -> thick, sticky, high-viscosity lava -> gas is trapped until it bursts out, so eruptions are explosive and build steep composite cones.
- This is why a shield volcano's lava spreads out so widely while a composite volcano is steep and dangerous.
Reading a volcano map: Estimate a distance: measure the gap, then multiply by the scale (on a 1:100 000 map, 1 cm = 1 km).
Determine a height difference: subtract the lower spot height from the higher one.
Identify volcanic evidence: look for circular contours (a cone or dome), a named crater, or lava fields near the coast.
IB-style questionOutline[2 marks]
A topographic map of an Icelandic volcano shows the surrounding landscape. Outline one feature on the map that shows the area is volcanic, and develop how it does so.
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An earthquake is the sudden release of strain energy that has built up as plates lock and grind against each other.
Why the largest quakes are rare: The bigger the strain stored, the bigger the quake - which is why the very largest earthquakes are rare: it takes a long time to build the huge stored energy a high-magnitude event releases.
How a tsunami forms (the sequence)
- 1. Trigger - a large undersea earthquake (usually at a subduction zone), eruption or submarine landslide suddenly displaces the sea floor.
- 2. Water displaced - the whole column of water above is pushed up or dropped, displacing a vast volume of ocean.
- 3. Waves radiate - long, low waves spread out from above the focus across the open ocean at high speed (hundreds of km/h).
- 4. Waves grow at the coast - as the waves reach shallow water they slow and pile up into a tall, destructive wall of water that floods low-lying coasts.
Tohoku, Japan (2011) - destructive margin: A magnitude-9.0 earthquake off NE Japan (the Pacific plate subducting beneath Japan) suddenly lifted the sea floor and generated a tsunami over 10 m high.
Tohoku - why so severe
A destructive margin stores enormous strain, so it produced one of the largest quakes ever recorded - and the undersea displacement is exactly the tsunami mechanism. The waves overtopped sea walls and triggered the Fukushima nuclear crisis.
Haiti (2010) - conservative margin
A magnitude-7.0 earthquake struck near Port-au-Prince on a conservative (transform) fault.
Why so deadly: the focus was shallow and close to a poorly built capital city, so even a moderate magnitude caused catastrophic damage and liquefaction (saturated ground behaving like a liquid). It shows that vulnerability, not just magnitude, shapes the disaster.
Volcanic events - Eyjafjallajokull (2010) & Nevado del Ruiz (1985)
Eyjafjallajokull (Iceland, constructive margin) erupted under ice; its ash cloud grounded European flights for days - showing volcanic hazards reach far beyond the vent.
Nevado del Ruiz (Colombia, destructive margin) killed ~23 000 when hot gas melted its ice cap and sent a lahar (volcanic mudflow) into the town of Armero - a deadly secondary hazard.
IB-style questionOutline[2 marks]
Very high-magnitude earthquakes happen only rarely. Outline one reason for this and develop how that process controls the frequency.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
IB-style questionSuggest[3 marks]
A tsunami struck a coast shortly after an offshore earthquake. Suggest how the earthquake produced the tsunami.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
More on plate movement, volcanoes and earthquakes
- What moves plates — heat from the core and radioactive decay drives convection in the mantle; plumes rise under hotspots; slab pull at subduction zones and rifting where plates split (the East African Rift) move plates apart.
- Cinder cones — small, steep cones of loose fragments from short, gassy eruptions; Parícutin in Mexico grew from a cornfield in 1943.
- Pyroclastic flows — avalanches of hot gas and ash moving at over 100 km/h; they destroyed Plymouth, Montserrat, in 1997.
- Earthquake waves — P waves (fastest, push-pull), S waves (side to side, through solids only) and surface waves (slowest, most damaging).
- Human triggers — filling large reservoirs and injecting wastewater from oil and gas wells can trigger earthquakes; Oklahoma’s earthquakes rose sharply in the 2010s with wastewater injection.
How this is tested - the [10] Examine essay: Paper 1 Option D ends with a 10-mark Examine essay, marked on markbands. The recurring version asks how the type of movement at different plate margins controls the character and distribution of volcanoes OR of earthquakes.
Top band needs
- Accurate terms
- A contrast of constructive, destructive and conservative margins linked to hazard character (lava type/explosiveness/landform, or earthquake depth/magnitude/location)
- An example
- The point that some hazards occur away from margins (hotspots / intraplate quakes)
- A clear conclusion
IB-style questionExamine[10 marks]
Examine how the type of movement at different plate margins controls the characteristics and distribution of volcanic activity.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Swap volcanoes for earthquakes: The same essay is set about earthquakes: destructive margins give the largest, deepest quakes (subduction), conservative margins give large but shallow quakes (San Andreas, Haiti), constructive margins give small, shallow ones - plus intraplate quakes occur away from margins. Same structure, swap the content.