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.
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 is to link a margin type to the character of the hazard it produces (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.
How this is tested: Paper 1 Option D opens with a data-response read off a volcano map or hazard diagram — you Estimate a ground distance using the scale, Determine a height difference between two spot heights, Identify a physical effect (e.g. of liquefaction), or Outline the map evidence that an area is volcanic. Always work carefully from the figure and quote the units the question asks for.
| 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.
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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. 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.
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.
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), and a clear conclusion.
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.