Plate Tectonics Theory

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.

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Continental Drift: The Origins of the Theory

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.

Alfred Wegener (1880–1930)

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

Why Was Wegener Rejected?

Despite compelling evidence, the geological establishment largely rejected Wegener's hypothesis during his lifetime:

• He could not explain the mechanism by which continents moved through the rigid oceanic crust
• His proposed mechanisms — tidal forces and centrifugal force from Earth's rotation — were shown to be quantitatively inadequate by the physicist Harold Jeffreys (1924)
• He was a meteorologist, not a geologist, and suffered from disciplinary prejudice
• Wegener died on a Greenland expedition in 1930, and his ideas remained largely dormant until the 1960s

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.

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Seafloor Spreading

Harry Hess (1962)

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:

1. New oceanic crust is created at mid-ocean ridges (MORs) by upwelling magma from the mantle
2. The oceanic crust moves away from the ridge on either side, like a conveyor belt
3. Old oceanic crust is destroyed at deep-ocean trenches, where it sinks back into the mantle (subduction)
4. The continents do not plough through the oceanic crust (as Wegener proposed) but are carried passively on the moving plates

Supporting Evidence

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

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The Structure of the Earth

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"]

Key Distinctions

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

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.

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Mechanisms of Plate Movement

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.

Mantle Convection