Earth structure and internal energy; plate tectonic theory, continental drift, sea-floor spreading and palaeomagnetism; mantle convection, slab pull and ridge push; plate margins; hot spots; and the global distribution of earthquakes and volcanoes.

What this dot point is asking

AQA section 3.1.5 begins the hazards topic with the theory of plate tectonics: Earth's internal structure and energy, the evidence for plate movement (continental drift, sea-floor spreading, palaeomagnetism), the drivers of movement (convection, slab pull, ridge push), the types of plate margin, hot spots, and the resulting global distribution of earthquakes and volcanoes. It underpins the volcanic and seismic hazard dots that follow.

Earth structure and internal energy

Earth is layered by density: a solid inner core and liquid outer core (iron and nickel), the mantle (the largest layer, hot and slowly convecting), and the thin crust (continental, thick and less dense; oceanic, thin and denser). The rigid lithosphere (crust plus uppermost mantle) sits on the plastic asthenosphere. The internal energy that drives tectonics comes from radioactive decay in the mantle and residual primordial heat, which together power mantle convection.

Plate tectonic theory and its evidence

The evidence built up over the twentieth century:

• Continental drift (Wegener): the jigsaw fit of continents, matching fossils and rock sequences across oceans, and ancient climate evidence.
• Sea-floor spreading: mid-ocean ridges where new oceanic crust is created and spreads outwards, with the youngest rock at the ridge.
• Palaeomagnetism: as basalt cools at a ridge it locks in Earth's magnetic polarity; because the field periodically reverses, the sea floor records symmetrical magnetic stripes either side of the ridge, the decisive proof of spreading.

The drivers of plate movement

Three processes move the plates. Mantle convection carries heat upward and drags the plates along, though it is now seen as a secondary driver. Ridge push (gravitational sliding away from the elevated, hot ridge) adds force. Slab pull, the sinking of dense, cold subducting slabs that drags the rest of the plate behind, is now regarded as the dominant driver. Sea-floor spreading creates crust; subduction destroys it.

Plate margins, hot spots and distribution

Three margin types produce the world's tectonic hazards:

• Constructive (divergent): plates move apart; magma rises by decompression melting to form basaltic, effusive volcanoes (mid-ocean ridges, Iceland) with shallow, low-magnitude earthquakes.
• Destructive (convergent): denser oceanic crust subducts, dehydrating the mantle wedge to feed explosive composite volcanoes, with deep, powerful earthquakes along the Benioff zone. Two continental plates collide to form fold mountains (Himalayas) with major earthquakes but little volcanism.
• Conservative (transform): plates slide past each other, locking and releasing to cause earthquakes (San Andreas Fault) but no volcanoes, because no crust is created or destroyed.

Hot spots are isolated plumes of rising magma that produce volcanoes away from margins (Hawaii); as the plate moves over a fixed plume, a chain of progressively older volcanoes forms.

Try this

Q1. Name the three drivers of plate movement and state which dominates. [3 marks]

• Cue. Mantle convection, ridge push and slab pull; slab pull is now regarded as the dominant driver.

Q2. Explain why explosive volcanoes occur at destructive margins. [3 marks]

• Cue. Subducting oceanic crust dehydrates the mantle wedge, generating viscous, gas-rich magma that erupts explosively as composite volcanoes.

Q3. Explain how palaeomagnetism supports sea-floor spreading. [3 marks]

• Cue. Cooling basalt locks in Earth's polarity; periodic field reversals produce symmetrical magnetic stripes either side of the ridge, showing crust is created and spreads outwards.