Plate Tectonic Theory in Detail

The theory of plate tectonics is the unifying framework for understanding the distribution and behaviour of tectonic hazards. For AQA A-Level Geography, you need a thorough understanding of the evidence supporting the theory, the mechanisms driving plate motion, and the Wilson cycle of ocean basin evolution.

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Evidence for Plate Tectonics

Continental Drift (Wegener, 1912)

Alfred Wegener proposed that continents were once joined in a supercontinent called Pangaea. His evidence included:

• Geological fit — the coastlines of South America and Africa align closely, including matching rock sequences and mountain belts (e.g., the Appalachians and Caledonian mountains)
• Fossil evidence — identical fossils of Mesosaurus (a freshwater reptile) found in both Brazil and South Africa; Glossopteris fern across India, Africa, Australia, and Antarctica
• Palaeoclimatic evidence — glacial deposits (tillites) found in tropical regions such as India and equatorial Africa, suggesting these landmasses were once at higher latitudes
• Palaeomagnetism — iron-rich minerals in volcanic rocks align with the Earth's magnetic field at the time of cooling; studying ancient rocks reveals that continents have shifted latitude over geological time

Wegener's ideas were rejected initially because he could not explain the mechanism for continental movement.

Sea-Floor Spreading (Hess, 1962)

Harry Hess proposed that new oceanic crust forms at mid-ocean ridges and spreads laterally. Key evidence:

• Magnetic striping — symmetrical patterns of normal and reversed magnetic polarity on either side of mid-ocean ridges (e.g., the Mid-Atlantic Ridge), confirmed by the Vine-Matthews-Morley hypothesis (1963)
• Age of ocean floor — oceanic crust becomes progressively older with distance from the ridge; the oldest oceanic crust is approximately 200 million years old (far younger than continental crust at up to 4 billion years)
• Sediment thickness — sediment cover increases with distance from the ridge, consistent with older, more distant crust having accumulated more material

Modern GPS Evidence

Global Positioning System measurements confirm plate motion in real time:

• The North Atlantic is widening at approximately 2.5 cm/year
• The Pacific Plate moves north-westward at roughly 7–10 cm/year
• India continues to converge with the Eurasian plate at about 4–5 cm/year

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Driving Mechanisms of Plate Motion

Mantle Convection

Radioactive decay of uranium, thorium, and potassium in the mantle generates heat. This creates convection currents — hot material rises at constructive margins and cooler material sinks at destructive margins. However, convection alone cannot fully explain plate velocities.

Slab Pull

The most significant driving force. At subduction zones, dense oceanic lithosphere sinks into the asthenosphere under gravity. The descending slab drags the rest of the plate with it. Plates attached to subducting slabs (e.g., the Pacific Plate) tend to move faster than those without (e.g., the African Plate).

Ridge Push

At mid-ocean ridges, newly formed lithosphere sits at a higher elevation. Gravity causes it to slide downslope away from the ridge, pushing the plate laterally. Ridge push is a weaker force than slab pull but contributes to plate motion.

Basal Drag

Friction between the base of the lithosphere and the flowing asthenosphere may either drive or resist plate motion depending on the relative velocity and direction of flow.

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Plate Boundaries

Boundary Type	Motion	Features	Example
Constructive (divergent)	Plates move apart	Mid-ocean ridges, rift valleys, shield volcanoes, shallow earthquakes	Mid-Atlantic Ridge; East African Rift
Destructive (convergent)	Plates move together	Subduction zones, deep ocean trenches, fold mountains, explosive volcanoes, deep earthquakes	Nazca–South American; Pacific–Philippine
Conservative (transform)	Plates slide past	No volcanic activity; shallow but powerful earthquakes; fault lines	San Andreas Fault; North Anatolian Fault
Collision	Continental–continental convergence	Fold mountains; no subduction; shallow earthquakes	Indo-Australian–Eurasian (Himalayas)

Hotspots

Some volcanic activity occurs away from plate boundaries at mantle plumes — columns of abnormally hot material rising from deep within the mantle. As a plate moves over the stationary hotspot, a chain of volcanic islands forms (e.g., the Hawaiian Islands; the plate moves north-west while the hotspot remains fixed, creating a trail of progressively older islands).

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The Wilson Cycle

J. Tuzo Wilson described the cyclical opening and closing of ocean basins over hundreds of millions of years:

1. Embryonic — continental rifting begins (East African Rift)
2. Young — narrow sea forms as continents separate (Red Sea)
3. Mature — wide ocean basin with mid-ocean ridge (Atlantic Ocean)
4. Declining — subduction begins at ocean margins, basin shrinks (Pacific Ocean)
5. Terminal — ocean basin nearly closed (Mediterranean Sea)
6. Relic — continents collide, forming fold mountains; suture zone marks the former ocean (Himalayas)

The Wilson cycle helps explain the distribution of ancient mountain belts, ophiolite sequences, and the repeated assembly and break-up of supercontinents (Pangaea, Rodinia).

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Key Summary

• Multiple lines of evidence — palaeomagnetism, sea-floor spreading, GPS — confirm plate tectonic theory
• Slab pull is the dominant driving force; mantle convection and ridge push contribute
• Four main boundary types produce distinct hazard profiles
• Hotspots provide evidence for absolute plate motion
• The Wilson cycle explains long-term ocean basin evolution

Exam Tip: AQA expects you to link evidence for plate tectonics to specific plate boundary processes. Use named examples and quantitative data (plate velocities, ages) to strengthen your answers.