Inside our planet

The Earth is made of layers. Heat from the core sets the outer layers moving — the engine behind every tectonic hazard:

Key idea: the crust is broken into tectonic plates that float on the semi-molten mantle. There are two kinds — thin, dense oceanic crust and thicker, less dense continental crust.

Which layer?

What makes plates move?

Heat from the core makes the mantle move in giant convection currents. Hot, less dense mantle rises; it cools, spreads out and sinks again. This slow churning drags the plates along above it.

Misconception check: plates do not float on molten liquid like boats on water — the mantle is solid rock that flows very slowly (semi-molten / plastic). "Convection currents drag the plates" is the phrasing examiners want.

The three main boundary types

Almost all earthquakes and volcanoes happen at plate boundaries — where two plates meet. What happens depends on how the plates move relative to each other:

• Constructive / divergent — plates move apart; magma rises to form new crust (mid-ocean ridges, gentle volcanoes). e.g. Mid-Atlantic Ridge.
• Destructive / convergent — plates move together; oceanic subducts under continental → explosive volcanoes + strong quakes. e.g. Nazca under South America.
• Conservative / transform — plates slide past; no volcanoes, but big earthquakes. e.g. San Andreas Fault.
• Collision — two continental plates meet; neither subducts, so crust crumples up into fold mountains. e.g. Himalayas (India + Eurasia).

Boundary ↔ feature

Tap a boundary on the left, then tap the feature it produces on the right.

Which plate sinks?

Where hazards happen

Earthquakes and volcanoes are not spread evenly — they cluster in narrow belts that trace out the plate boundaries. The most famous is the Pacific "Ring of Fire", a horseshoe of destructive boundaries around the Pacific Ocean.

Some volcanoes (e.g. Hawaii) sit in the middle of a plate, over a hot spot — a plume of rising magma — rather than at a boundary.

Composite vs shield volcanoes

The type of volcano depends on the boundary and the runniness of the lava:

Primary effects happen instantly during an eruption (lava flows, ash falls, pyroclastic flows, gases). Secondary effects follow on (lahars/mudflows, fires, contaminated water, disrupted farming and travel).

Name that volcano

Focus, epicentre & waves

An earthquake happens when stress built up along a fault is released suddenly, sending out seismic waves:

Measuring quakes — magnitude vs intensity: the Richter and moment magnitude scales measure the energy released (magnitude). The Mercalli scale measures the intensity — how much damage/shaking is felt at a place. One quake has one magnitude but many different Mercalli values depending on distance from the epicentre.

Name the point

Which scale?

Primary vs secondary effects

Examiners love this split. Get it right and you pick up easy marks:

• Primary effects — the immediate, direct result of the ground shaking or the eruption itself: buildings collapse, roads crack, people killed/injured by falling rubble, lava/ash destroys land.
• Secondary effects — knock-on effects that follow afterwards: fires from broken gas pipes, tsunamis, landslides, disease from contaminated water, homelessness, lost jobs and business.

Primary or secondary?

Tap a hazard effect, then tap the box it belongs in.

How cyclones form

Tropical cyclones (also called hurricanes or typhoons) are huge spinning storms. They only form where several conditions are met:

• Warm ocean water — sea surface temperature of at least 26.5–27 °C to a depth of ~50 m (fuel = evaporation).
• Latitudes roughly 5°–20° north or south of the Equator — far enough for the Coriolis effect to make the storm spin, but not on the Equator itself.
• Low wind shear (winds don't change much with height) so the storm can build vertically.

Misconception check: cyclones do not form on the Equator — there is too little Coriolis effect there for them to start spinning. They form a little to the north or south of it.

Structure of a cyclone

A cyclone has a calm centre surrounded by the most violent weather:

The main hazards are high winds, torrential rain causing flooding, and a storm surge — a wall of seawater pushed onshore that often causes the most deaths.

Cyclone conditions

The calm centre

Why live in a hazardous area?

It seems strange to live where the ground shakes or storms strike — yet millions do. The reasons are a mix of pull factors and having little choice:

• Fertile soils — weathered volcanic ash makes excellent farmland (e.g. slopes of Etna, Mount Merapi).
• Geothermal energy & tourism — hot springs, geysers and dramatic scenery bring jobs and money (e.g. Iceland).
• Minerals — volcanic areas are rich in valuable minerals.
• Family, community & work — people don't want to leave home, jobs or relatives.
• Poverty / few options — many cannot afford to move; hazards may be rare, so people take the risk.

Hazards become disasters only when they hit vulnerable people. The same magnitude quake can kill thousands in a poor country but only a handful in a wealthy, well-prepared one.

Pull to the danger

Prediction, protection, preparation

We can't stop tectonic hazards, but we can reduce their impact. Group your answers under the "three P's":

• Prediction & monitoring — watch for warning signs: seismometers, gas sensors and bulging ground for volcanoes; satellites and aircraft track cyclones so warnings can be issued. (Earthquakes still can't be predicted reliably.)
• Protection — build to survive: earthquake-proof buildings with deep foundations, cross-bracing and rubber shock absorbers; sea walls and cyclone shelters; lava-diversion barriers.
• Preparation / planning — get people ready: drills, evacuation routes, emergency kits, education and hazard mapping so risky land isn't built on.

Which strategy?

Typhoon Haiyan, Philippines (2013)

A named lower-income country hazard you can quote in exams:

• What: one of the strongest tropical cyclones ever recorded to make landfall, with winds around 310 km/h, striking the central Philippines on 8 Nov 2013.
• Primary effects: around 6,300 people killed; the city of Tacloban devastated by a 5-metre storm surge; over 1 million homes damaged or destroyed.
• Secondary effects: ~4 million people made homeless; contaminated water and disease; power, roads and jobs (fishing, farming) wiped out for months.
• Responses: mass evacuation before landfall saved many; international aid; slow rebuilding — a poorer country struggled to cope, showing why vulnerability matters.

Why it's a good exam case study: it shows how a hazard becomes a disaster where people are poor and densely packed on low-lying coasts.

Chile earthquake (2010) — a contrast

Compare Haiyan with a hazard in a higher-income country:

• What: a huge magnitude 8.8 earthquake off the coast of Chile on 27 Feb 2010, at a destructive boundary (Nazca plate subducting under South America).
• Effects: despite being far more powerful than many deadlier quakes, it killed about 500 people — Chile's strict earthquake-resistant building codes and preparation saved thousands of lives.
• Responses: a wealthy, prepared country restored power and water within days and rebuilt largely from its own funds.
• The contrast: the 2010 Haiti quake (magnitude 7.0, a poorer country) killed over 200,000 — far more, despite being far weaker. Wealth and preparation, not just magnitude, decide the death toll.

Exam gold: use the Chile-vs-Haiti pairing to argue that level of development is often more important than the size of the hazard itself.

Comparing impacts

The must-knows

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