The internal structure of the Earth

The Earth is layered by density and physical state:

• Crust — thin, brittle outer skin; oceanic crust is denser and thinner than continental crust.
• Mantle — the thickest layer; solid rock that flows very slowly over time.
• Outer core — liquid iron and nickel; its motion generates Earth's magnetic field.
• Inner core — solid iron and nickel, kept solid by immense pressure despite being hottest.

Key distinction: the rigid lithosphere (crust + uppermost mantle) is broken into plates that ride on the weaker, partially molten asthenosphere below. Plate tectonics is about the lithosphere, not the whole crust.

Which layer is liquid?

Plate tectonics and its evidence

Plate tectonic theory holds that the lithosphere is broken into plates that move relative to one another. The evidence built up over the 20th century:

• Continental fit — coastlines such as South America and Africa fit together like a jigsaw.
• Palaeomagnetism & sea-floor spreading — symmetrical magnetic stripes either side of mid-ocean ridges record reversals of Earth's field as new crust forms.
• Matching fossils and rock/geological sequences found on now-separated continents.

Why it convinced sceptics: Wegener's continental drift lacked a mechanism. Sea-floor spreading and palaeomagnetism supplied one — new crust forming and spreading — turning a hypothesis into accepted theory.

What is the evidence?

What drives plate movement?

Several linked forces move the plates:

• Mantle convection — heat from the core and radioactive decay drives slow convection currents in the mantle that drag the plates.
• Ridge push — newly formed, elevated crust at a mid-ocean ridge slides down and away under gravity.
• Slab pull — a dense, subducting plate sinks into the mantle and pulls the rest of the plate behind it.

Current thinking: once seen as convection alone, plate motion is now attributed largely to slab pull and ridge push acting together with convection — an example of how tectonic theory keeps being refined.

The three plate boundary types

• Constructive (divergent) — plates move apart; magma rises to form new crust at mid-ocean ridges or continental rift valleys.
• Destructive (convergent) — plates collide; oceanic crust subducts, or continental plates crumple into fold mountains.
• Conservative (transform) — plates slide past; crust is neither made nor destroyed, but friction causes earthquakes.

Hotspots: not all volcanism is at boundaries. Mantle plumes create hotspots in the plate interior; as the plate moves over a fixed plume, a chain of volcanoes forms.

Which plate boundary?

Tap a feature, then tap the boundary type it belongs to.

At a constructive boundary

Earthquakes, waves and magnitude

An earthquake starts at the focus (hypocentre) underground; the point directly above on the surface is the epicentre. Energy radiates as seismic waves:

• P (primary) waves — fastest, travel through solids and liquids.
• S (secondary) waves — slower, travel through solids only.
• Surface waves — slowest but usually most destructive at the surface.

Magnitude vs intensity: the Richter and moment-magnitude scales measure the energy/amplitude released; the Modified Mercalli scale measures the intensity (shaking and damage) experienced.

Why it matters: because the scale is logarithmic, a magnitude 6 is not "20% bigger" than a magnitude 5 — it is ten times the amplitude and roughly 32 times the energy.

Your turn — amplitude ratio

Your turn — energy factor

Magnitude vs intensity

Volcanoes and their hazards

Eruption style depends largely on magma:

• Effusive eruptions — runny, low-gas basaltic lava flows gently (typical of constructive boundaries and hotspots).
• Explosive eruptions — sticky, gas-rich magma at subduction zones erupts violently.

The Volcanic Explosivity Index (VEI) ranks eruptions and, like magnitude, is logarithmic — each step up is roughly a ten-fold increase in erupted material.

Primary hazards come directly from the eruption: lava, tephra, pyroclastic flows and gases. Secondary hazards are triggered consequences: lahars (mudflows) and tsunamis.

Exam tip: classify hazards as primary or secondary and link the style of eruption back to plate setting — explosive andesitic volcanoes at destructive margins, effusive basaltic ones at constructive margins and hotspots.

Match the volcanic hazard

Tap a description on the left, then its matching hazard on the right.

Primary or secondary?

Managing hazards

Geographers use several models to understand hazard response:

• Park model (disaster response curve) — plots quality of life over time through relief, rehabilitation and reconstruction; the curve dips then recovers.
• Hazard management cycle — mitigation, preparedness, response and recovery as a repeating loop.
• Pressure and Release (PAR) model — a disaster results when a natural hazard meets vulnerability built up from root causes, dynamic pressures and unsafe conditions.

Responses combine prediction, preparedness (drills, warning systems), mitigation (building design, land-use planning) and building long-term resilience.

Big idea: the PAR model reframes disasters as socially produced, not purely natural — the hazard is the trigger, but vulnerability decides the impact.

The Park model

Your turn — percentage change

Why impacts vary with development

Two hazards of similar physical magnitude can have very different human impacts. The difference is largely vulnerability, shaped by development:

• Governance and preparedness — enforced building codes, warning systems, drills and emergency services.
• Wealth and infrastructure — resilient buildings, insurance and the capacity to respond and rebuild.
• Population and land use — dense, informal settlements on hazardous ground raise exposure.

Evaluation: lower-income countries often suffer higher death tolls, while higher-income countries may record higher economic losses but fewer deaths. Impact is a product of hazard and human vulnerability, not magnitude alone.

Same magnitude, different toll

The big ideas to know

You have worked through Hazardous Earth, from the planet's interior to why the same hazard hits people so unequally. Press Finish to see your score.