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Spec mapping (AQA 7037): Paper 1, §3.1.5 Hazards — "plate tectonics theory: Earth structure and internal energy sources; convection currents; sea-floor spreading; the structure of oceanic and continental crust; gravitational sliding; ridge push, slab pull; plate movement: constructive, destructive and conservative plate margins, characteristic processes; magma plumes and their relationship to plate movement." It is the physical-geography foundation that supports the volcanic and seismic case-study lessons, and it links synoptically to §3.1.1 (Earth's internal energy is the ultimate driver, analogous to the way solar energy drives the water/carbon-cycle systems) and to the global distribution patterns examined later. The assessment objectives engaged here are predominantly AO1 (knowledge of structure, mechanisms and evidence) and AO3 (interpreting the evidence base — palaeomagnetic stripes, the age-distance relationship of the ocean floor, hot-spot island ages — as data that tests the theory). AO2 appears when you apply boundary processes to explain why particular hazards occur where they do.
Plate tectonics is the unifying theory of Earth science. It explains the distribution of earthquakes, volcanic eruptions, mountain chains and ocean basins, and it provides the essential foundation for understanding tectonic hazards at A-Level. The theory evolved through contributions from numerous scientists over more than a century, and understanding this intellectual history is an important part of the AQA specification. The theory matters for hazard study because it does the explanatory heavy lifting: once you know where and why plates interact, the global pattern of tectonic risk — the ~90% of earthquakes and ~75% of volcanoes ringing the Pacific — ceases to be a list to memorise and becomes a logical consequence of boundary processes.
Key Definition: Continental drift is the hypothesis, proposed by Alfred Wegener in 1912, that the continents were once joined together in a single supercontinent and have since moved apart to their present positions.
Wegener, a German meteorologist and polar researcher, published Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans) in 1915. His evidence included:
| Evidence Type | Details |
|---|---|
| Continental fit | The coastlines of South America and Africa fit together like jigsaw pieces — particularly the continental shelves at the 500-fathom (900 m) contour, as later demonstrated by Bullard, Everett and Smith (1965) using computer mapping |
| Geological evidence | Identical rock sequences and mountain belts on continents now separated by oceans. The Caledonian mountain chain runs from Scotland through Norway and matches the Appalachian mountains in eastern North America |
| Palaeontological evidence | Identical fossils of the freshwater reptile Mesosaurus found in both Brazil and South Africa; the fern Glossopteris found across South America, Africa, India, Antarctica and Australia — all part of the former supercontinent Gondwana |
| Palaeoclimatic evidence | Glacial deposits (tillites) of Carboniferous-Permian age found in South America, Africa, India and Australia — regions that are now tropical or subtropical. These deposits make sense only if these landmasses were once clustered near the South Pole |
Despite compelling evidence, the geological establishment largely rejected Wegener's hypothesis during his lifetime:
Exam Tip: The story of Wegener illustrates an important point about the nature of science: a hypothesis can be supported by strong evidence but still be rejected if no plausible mechanism is identified. The mechanism — plate tectonics driven by mantle convection — was not established until the 1960s.
The strength of Wegener's case is worth appreciating as an exercise in evaluating evidence (an AO3 habit of mind). No single line of evidence was conclusive on its own — coastlines might fit by coincidence, fossils might have crossed on hypothetical "land bridges," and rock matches might be chance. What made the argument powerful was convergence: four independent strands (geometric fit, geological correlation, palaeontology and palaeoclimate) all pointed to the same reconstruction of Pangaea. The palaeoclimatic evidence is especially elegant — Carboniferous–Permian glacial tillites and striations in what are now tropical Brazil, central Africa, India and Australia, with ice-flow directions that only make sense if these landmasses were once clustered around the South Pole. The lesson for examiners and scientists alike is that a coherent multi-proxy reconstruction is far stronger than any one proxy, even when the underlying mechanism is unknown.
Harry Hess, a Princeton University geologist (and former naval officer who had used sonar to map the ocean floor during World War II), proposed the theory of seafloor spreading in a landmark paper he modestly described as "geopoetry."
Hess proposed that:
| Evidence | Researcher(s) | Date | Details |
|---|---|---|---|
| Magnetic striping | Vine and Matthews | 1963 | Symmetrical patterns of normal and reversed magnetic polarity in oceanic basalt on either side of mid-ocean ridges. As magma solidifies, iron-rich minerals align with Earth's magnetic field. Since the field reverses periodically, this creates a "barcode" pattern that confirms new crust is being created and spreading outward |
| Age of ocean floor | Various (from deep-sea drilling) | 1960s–1970s | Oceanic crust is youngest at mid-ocean ridges and progressively older toward the continental margins. The oldest oceanic crust is ~200 million years old (Jurassic), compared to continental crust up to 4 billion years old |
| Sediment thickness | Deep Sea Drilling Project | 1968 onwards | Sediment is thinnest at mid-ocean ridges (newly formed crust has had little time to accumulate sediment) and thickest near continents |
| Heat flow | Various | 1960s | Heat flow is highest at mid-ocean ridges (where hot magma rises) and lowest in deep-ocean trenches |
Worked AO3 skills exemplar — calculating a spreading rate from palaeomagnetic data. The magnetic-stripe and age-distance evidence can be turned into a quantitative skill. Spreading rate is simply distance over time, or half-spreading rate if measured from the ridge axis to one side:
v=td
(i) Describe: the magnetic-anomaly map shows symmetrical stripes either side of the Mid-Atlantic Ridge, the youngest at the axis. (ii) Manipulate: if a particular reversal boundary dated to 10 million years (10 Ma) lies 200 km from the ridge axis, the half-spreading rate is v=200,000,000 mm÷10,000,000 yr=20 mm/yr=2 cm/yr, giving a full spreading rate of ~4 cm/yr — consistent with the slow Atlantic. (iii) Explain: symmetry confirms new crust is created at the axis and carried outward on both plates, exactly as Hess predicted. (iv) Evaluate: the calculation assumes a constant spreading rate, which is an approximation — rates vary over geological time, and the reversal timescale itself carries dating uncertainty — so the figure is an average, not an instantaneous velocity. Compare with the East Pacific Rise (full rate up to ~16 cm/yr) to show how the same method discriminates between fast and slow ridges.
Understanding plate tectonics requires knowledge of Earth's internal structure:
graph TD
A["Crust<br/>5–70 km thick<br/>Rigid, brittle"] --> B["Upper Mantle (Lithosphere)<br/>to ~100 km<br/>Rigid, combined with crust"]
B --> C["Asthenosphere<br/>100–300 km<br/>Partially molten,<br/>ductile, convecting"]
C --> D["Lower Mantle (Mesosphere)<br/>300–2,900 km<br/>Solid but capable of<br/>slow convection"]
D --> E["Outer Core<br/>2,900–5,150 km<br/>Liquid iron-nickel<br/>Generates magnetic field"]
E --> F["Inner Core<br/>5,150–6,371 km<br/>Solid iron-nickel<br/>~5,500°C"]
| Layer | Composition | State | Thickness |
|---|---|---|---|
| Continental crust | Silica and alumina (SiAl); granite dominates; density ~2.7 g/cm³ | Solid, rigid | 25–70 km (up to 90 km under mountain ranges like the Himalayas) |
| Oceanic crust | Silica and magnesia (SiMa); basalt dominates; density ~3.0 g/cm³ | Solid, rigid | 5–10 km |
| Lithosphere | Crust + rigid upper mantle | Solid, rigid | ~100–250 km (thinner beneath oceans, thicker beneath old continental shields) |
| Asthenosphere | Peridotite (ultramafic rock) | Partially molten, ductile | ~100–300 km depth; behaves plastically over geological timescales |
The crustal density contrast also explains isostasy — the buoyant "floating" of the crust on the denser mantle. Thick, low-density continental crust stands high (and has deep "roots," up to ~90 km beneath the Himalayas, like an iceberg's submerged keel), whereas thin, dense oceanic crust sits low, forming the ocean basins. Mountain belts therefore have both topographic height and a deep low-density root, and as they erode the crust slowly rebounds isostatically. This is why the SiAl/SiMa distinction is not merely a composition label but the physical basis for the planet's first-order division into continents and oceans.
Key Definition: The lithosphere is the rigid outer shell of the Earth, comprising the crust and the uppermost mantle. It is broken into tectonic plates that move relative to one another on the ductile asthenosphere beneath.
The AQA specification asks specifically about Earth's internal energy sources, because these are the ultimate driver of the whole tectonic system. There are two principal sources: primordial heat left over from the planet's formation and core differentiation ~4.5 billion years ago, and radiogenic heat from the ongoing decay of long-lived radioactive isotopes — chiefly uranium-238, uranium-235, thorium-232 and potassium-40 — concentrated in the mantle and crust. Total heat loss from the Earth's surface is approximately 47 terawatts (TW), of which roughly half is radiogenic. It is this internal heat engine, not the Sun, that powers mantle convection, sea-floor spreading and ultimately every earthquake and eruption — an important contrast with the atmospheric hazards later in the option, which are solar-powered. This is why tectonic and atmospheric hazards have fundamentally different distributions: one follows plate boundaries (internal energy), the other follows latitude and circulation (external/solar energy).
We cannot drill to the mantle (the deepest borehole, the Kola Superdeep, reached only ~12 km), so almost all knowledge of Earth's interior is indirect, derived from seismic-wave behaviour. P-waves and S-waves refract and change velocity at boundaries where composition or state changes, producing the Mohorovičić discontinuity (crust–mantle boundary) and the Gutenberg discontinuity (mantle–core boundary). Critically, S-waves cannot travel through the liquid outer core, creating an S-wave shadow zone beyond ~103° from an earthquake's epicentre — the single most important piece of evidence that the outer core is liquid. The travel-time data also reveal the asthenosphere as a low-velocity zone where waves slow because the rock is near its melting point. This is a useful synoptic skills point: the same seismic waves that cause the hazard also provide the evidence for the structure that generates it.
The driving forces behind plate motion remain an area of active research. Several mechanisms contribute, and modern understanding recognises that no single force is sufficient — plate motion is driven by a combination of forces.
Arthur Holmes (1929) first proposed that convection currents in the mantle could be the mechanism driving continental drift. Hot material rises from the core-mantle boundary, spreads laterally beneath the lithosphere, cools, and sinks back down at subduction zones. Holmes' insight — decades ahead of the supporting data — was that radiogenic heat (the internal energy source discussed above) would set up slow, solid-state creep in the mantle: rock that behaves rigidly over seconds (transmitting seismic waves) flows like an extremely viscous fluid over millions of years. This is the same dual rigid/ductile behaviour that defines the lithosphere–asthenosphere distinction.
However, the simple "conveyor belt" model of convection has been superseded by more complex models:
Slab pull is now considered the dominant driving force for plate motion. At subduction zones, the cold, dense oceanic lithosphere sinks into the mantle under its own weight. The key is negative buoyancy: as oceanic lithosphere ages it cools and contracts, becoming denser than the asthenosphere beneath it, so once subduction begins gravity does the rest, with the descending slab dragging the rest of the plate behind it like a tablecloth pulled off a table. The pull is enhanced by phase changes in the sinking slab (minerals reorganising into denser forms at depth), which add to its negative buoyancy.
Evidence: plates attached to subducting slabs (e.g., the Pacific Plate) move faster (up to ~10 cm/year) than plates without subducting edges (e.g., the African Plate, ~2 cm/year). This velocity correlation — fast plates have long subducting margins, slow plates do not — is the single strongest argument that slab pull, not ridge push or basal drag, is the leading driver.
At mid-ocean ridges, newly formed lithosphere is elevated (because it is hot and less dense). As it moves away from the axis it cools, thickens and becomes denser, so the ridge forms a broad topographic high from which the older lithosphere effectively slides downhill under gravity — this is why ridge push is also called gravitational sliding, a term the AQA specification uses explicitly. The horizontal force arises from the integrated pressure difference between the hot, elevated ridge and the colder, deeper flanks. Ridge push is real but comparatively modest — estimated at only about 5–10% as effective as slab pull. Its existence is shown by the fact that even plates without a subducting edge (e.g. the African and Antarctic plates, largely ringed by ridges) still move, albeit slowly.
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