What actually moves the plates

Earth is layered by composition (crust, mantle, core) and by mechanical behaviour — and it is the second layering that matters for tectonics. The rigid lithosphere (crust + uppermost mantle) is broken into plates that move over the hotter, ductile asthenosphere.

• Oceanic crust — thin (of the order of 6–10 km), basaltic, dense, young (constantly recycled).
• Continental crust — thick (tens of kilometres), granitic, less dense, and ancient: it is too buoyant to subduct, which is why it survives.
• Mantle convection — heat from the core and from radioactive decay drives slow convection. This was the classic explanation, but it is now seen as a contributor rather than the whole story.
• Slab pull — the dominant force. Cold, dense oceanic lithosphere at a subduction zone sinks under its own negative buoyancy and drags the plate behind it.
• Ridge push (gravitational sliding) — newly formed, hot lithosphere at a ridge is elevated; as it cools and thickens it slides away downslope. Weaker than slab pull.

Examiner's eye: "convection currents drag the plates" on its own is a GCSE answer. A-level explanation ranks the forces: plates attached to long subducting slabs (e.g. Pacific) move fastest — direct evidence that slab pull dominates.

Ranking the driving forces

Boundaries explain the global distribution

Plot the world's earthquakes and volcanoes and they form narrow linear belts — because they are boundary phenomena. The exceptions (hot spots, intra-plate quakes) are the ones you must be able to explain separately.

Benioff zone. Earthquake foci get progressively deeper away from the trench because they mark the descending slab. It is the single cleanest piece of evidence for subduction — and a favourite of resource-based questions.

Which boundary is it?

Tap a place or feature, then tap the boundary type it belongs to. Getting these automatic saves you thinking time in the exam.

Magma type decides eruption style

Everything about a volcano's behaviour follows from its magma's silica content, and therefore its viscosity and its ability to hold dissolved gas.

• Basaltic — low silica, low viscosity, hot. Gas escapes easily, so eruptions are effusive: lava fountains and fluid flows building broad shield volcanoes. Found at divergent boundaries and hot spots.
• Andesitic / rhyolitic — high silica, high viscosity, cooler. Gas cannot escape, pressure builds, and the eruption is explosive: ash columns, pyroclastic flows, lahars, building steep composite cones. Found at destructive boundaries, because subducted water and sediment contaminate the melt.
• VEI (Volcanic Explosivity Index) — a logarithmic 0–8 scale based on erupted volume, column height and duration. VEI ≥ 5 events are rare but capable of global effects.

Hazards to name precisely: lava flows (rarely lethal, slow), pyroclastic density currents (fast, hot, overwhelmingly the biggest killer), tephra and ash fall (roof collapse, aviation, health), lahars (ash + water; can occur years after an eruption), volcanic gases, and jökulhlaups where an eruption melts an ice cap.

Why the distinction earns marks: effusive eruptions can be enormous in volume yet cause few deaths, because people can walk away from a lava flow. Explosive eruptions kill because pyroclastic flows arrive in minutes. Magnitude and impact are not the same variable — the theme of this whole topic.

Reading the volcano

Elastic rebound, waves and measurement

Plates do not slide smoothly. Friction locks a fault while the plates keep moving, so the rock deforms elastically and stores strain energy. When the stress exceeds the frictional strength, the fault ruptures and the rock snaps back — elastic-rebound theory. The energy radiates as seismic waves.

• Focus (hypocentre) — where rupture begins, at depth. Epicentre — the point on the surface directly above it. Shallow-focus quakes are the most damaging for a given magnitude.
• P-waves — primary, fastest, compressional, travel through solids and liquids. S-waves — secondary, slower, shear, solids only. Surface waves (Love, Rayleigh) — slowest, largest amplitude, and responsible for most structural damage.
• Moment magnitude (M_w) — measures the energy released (from rupture area × slip × rock rigidity). It is logarithmic and it does not saturate at large sizes, which is why it has replaced the Richter scale for big events.
• Modified Mercalli — measures intensity: what the shaking actually did at a given place. One earthquake has one magnitude but many intensities, decaying with distance and varying with ground conditions.

The distinction that separates grades: magnitude is a property of the event; intensity is a property of a place. A moderate quake directly beneath a city on soft sediment can produce far higher intensities — and far more deaths — than a great quake offshore.

Your turn — a logarithmic scale

Your turn — energy, not amplitude

Amplitude and energy scale differently. Each whole magnitude step releases about 31.6× more energy (that is 10^1.5). This is why the largest few earthquakes of a century release more energy than all the rest combined.

The hazard behind the hazard

In most large earthquake disasters the shaking itself is not what does the killing. The secondary hazards are:

• Liquefaction — shaking causes saturated, loosely packed sediment to lose its grain-to-grain contact; pore-water pressure rises and the ground temporarily behaves as a fluid. Buildings tilt and sink; buried tanks and pipes float upwards. Reclaimed land, deltas and river floodplains are the classic sites.
• Landslides and rockfalls — especially where steep slopes meet weak regolith, and often the dominant killer in mountainous regions.
• Fire — ruptured gas mains and downed power lines; compounded when the water mains have also ruptured.
• Disease and displacement — sanitation failure in crowded emergency shelters.

Tsunami generation. A shallow-focus megathrust earthquake at a subduction zone displaces the seabed vertically, and with it the entire water column above. In the deep ocean the wave has small amplitude but enormous wavelength and travels at high speed. As it enters shallow water it slows, the wavelength shortens and the energy is compressed into height — shoaling. Tsunami can also be generated by volcanic flank collapse and submarine landslides, which is why not every tsunami has a big earthquake attached to it.

Watch the wording. Strike-slip (conservative) earthquakes rarely generate large tsunami, because the motion is horizontal — very little water is displaced vertically. Mechanism, not magnitude, is the discriminator.

Why liquefaction?

Name that process

Tap a description on the left, then the term it defines. Precise terminology is the cheapest mark in this topic.

Hazard is not disaster

Edexcel builds the whole second half of the topic on one idea: a hazard only becomes a disaster where it meets a vulnerable population. The Degg model makes this visual — a disaster is the intersection of a hazard event with a vulnerable population; a huge eruption in an empty wilderness is not a disaster at all.

Same magnitude, different disaster

The Park model — and how to bend it

Park's hazard-response curve plots quality of life (or level of functioning) against time through five phases: pre-disaster → disruption → relief (hours–days: search and rescue, medical care) → rehabilitation (weeks–months: temporary shelter, restoring services) → reconstruction (months–years: rebuilding, ideally better).

• The depth of the trough = how badly the event disrupts — reduced by mitigation, building codes and land-use zoning.
• The speed of recovery = how quickly the curve climbs back — driven by capacity to cope: emergency planning, insurance, aid, governance.
• The end point matters most for evaluation. A curve that returns above the original line means build back better; one that never regains it means the disaster has permanently reduced quality of life — often the story in low-income contexts.

Modification strategies map neatly onto three targets: modify the event (lava diversion, slope stabilisation — limited scope), modify vulnerability (prediction, warning, education, aseismic design), and modify loss (aid, insurance, emergency response).

Prediction has hard limits. Volcanoes give precursors — tiltmeters detect swelling, seismometers pick up harmonic tremor, gas monitoring detects rising SO₂ — so useful forecasting and evacuation are realistic. Earthquakes, by contrast, cannot currently be predicted with useful precision in time. Seismic-gap analysis gives probabilistic forecasts over decades, not warnings over days. Saying "earthquakes can be predicted" is a straightforward error.

Prediction, forecasting or warning?

Why impacts differ across the development spectrum

Edexcel expects you to compare tectonic events in contrasting settings. Structure the comparison around the causes of differential vulnerability rather than a list of statistics:

• Governance — the master variable. Are building codes written, and are they enforced? Corruption in construction converts a code on paper into a collapsed school.
• Development and income — wealth buys aseismic engineering, insurance, hospitals, and a rapid organised response. But wealth is not sufficient: high-income countries with poor planning still suffer.
• Urbanisation and density — rapid, unplanned urban growth concentrates people in poorly built housing, often on the worst ground (steep slopes, reclaimed land, floodplains). Megacities on plate boundaries are the great emerging risk.
• Multiple-hazard zones and disaster hotspots — places where tectonic and hydro-meteorological hazards overlap (e.g. the Philippines: typhoons, floods, landslides, earthquakes and eruptions). Repeated shocks prevent recovery, so vulnerability ratchets upwards.
• Complex disasters — a hazard striking a place already destabilised by conflict or a weak state, where relief cannot be delivered safely.

Nuance that gets Level 4: resist "rich = safe, poor = unsafe". Argue instead that governance and preparedness mediate the relationship between development and impact — and note the trade-off between economic loss (typically far larger in absolute terms in high-income countries) and mortality (typically far higher in low-income and fragile ones).

Writing the evaluative essay

Edexcel Paper 1 assesses AO1 (knowledge), AO2 (application — the bulk of the marks in extended answers) and AO3 (using data/sources). For "Evaluate…" and "To what extent…" questions:

• Decide before you write. State a line of argument in the introduction, then defend it. An essay that only "discusses" caps itself.
• Use a model as a spine — PAR, Degg, Park, or the risk equation. It gives you structure and it is exactly the conceptual language the mark scheme rewards.
• Two contrasting case studies, minimum. Compare like with like (similar magnitude, contrasting development/governance) so the comparison actually isolates the variable.
• Weigh factors explicitly. "Governance matters more than magnitude, because…" is AO2. Listing five factors equally is AO1 pretending to be AO2.
• Conclude with a conditional judgement. Say to what extent, and on what your judgement depends (scale, timescale, whether you measure loss in lives or dollars).

Try it: "Evaluate the view that the impacts of tectonic hazards depend more on human factors than on the magnitude of the event." Plan: (1) magnitude and mechanism do matter — shallow focus, tsunamigenic megathrust, explosive vs effusive; (2) but vulnerability decides who dies — density, building quality, ground conditions; (3) and capacity to cope decides who recovers — Park curve end point. Judgement: human factors dominate mortality, physical factors set the ceiling of what is possible.

Sharpening the judgement

The big ideas to know

That is Edexcel Topic 1 — and the risk framework you will reuse in every hazard question. Press Finish to see your score.