Plate tectonics — AQA A-Level Geography
Test yourself on Plate tectonics with AQA A-Level practice questions.
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Plate tectonics explained
The Earth's structure comprises the inner core, outer core, mantle, and crust.
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Internal energy sources, primarily radioactive decay and residual heat, drive dynamic processes within the mantle. The plate tectonic theory of crustal evolution explains how the lithosphere is divided into tectonic plates that constantly move. This plate movement is driven by several mechanisms. Convection currents in the asthenosphere transfer heat, contributing to sea-floor spreading at constructive margins. Gravitational sliding occurs as lithosphere slides away from elevated mid-ocean ridges, known as ridge push. At destructive margins, the dense, subducting oceanic plate pulls the rest of the plate behind it, a dominant driving force termed slab pull.
Destructive, constructive and conservative plate margins. Characteristic processes: seismicity and vulcanicity. Associated landforms: young fold mountains, rift valleys, ocean ridges, deep sea trenches and island arcs, volcanoes.
Plate margins are boundaries where tectonic plates interact, driving characteristic processes and creating distinct landforms. At constructive margins, plates diverge, causing vulcanicity and shallow seismicity, forming ocean ridges (Mid-Atlantic Ridge) and rift valleys (East African Rift). Destructive margins involve convergence; subduction generates deep sea trenches (Mariana Trench), island arcs, explosive volcanoes, and intense seismicity, while continental collisions form young fold mountains (Himalayas). Conservative margins feature plates sliding past each other (San Andreas Fault), producing severe seismicity but no vulcanicity, as no magma is generated or released.
Magma plumes and their relationship to plate movement.
Magma plumes or hotspots are modelled as localised upwellings of anomalously hot mantle material that operate independently of most plate margins; some models trace deep plumes towards the core-mantle boundary. As mantle or plume material rises, falling pressure causes decompression melting and generates basaltic magma. Where that magma reaches the surface it builds volcanoes, commonly shield volcanoes. As a tectonic plate moves over a relatively stationary hotspot, older volcanoes are carried away and lose their magma supply, producing a chain whose ages generally increase with distance from the active centre, as in the Hawaiian-Emperor chain. The chain's position and ages provide evidence for past plate direction and rate, while bends can record changes in plate motion.
Your focus
- Describe the Earth's internal structure and identify the sources of its internal energy.
- Explain the mechanisms driving plate movement, including convection currents, ridge push, and slab pull.
- Evaluate the evidence supporting the plate tectonic theory of crustal evolution, such as sea-floor spreading.
Show all 9 objectives
- Categorise the three main types of plate margin and their characteristic directions of movement.
- Explain the processes of seismicity and vulcanicity specific to each type of plate margin.
- Link the formation of young fold mountains, rift valleys, ocean ridges, deep sea trenches, island arcs, and volcanoes to their respective plate margins.
- Define the concept of a magma plume and its origin within the mantle.
- Explain the mechanism by which magma plumes generate intra-plate volcanoes.
- Analyse how linear chains of volcanic islands provide evidence for the direction and speed of tectonic plate movement.
Plate tectonics exam tips
Marking Points
- Describe the Earth's internal structure, detailing the physical properties of the crust, mantle, and core.
- Explain the role of radioactive decay as the primary internal energy source driving mantle convection.
- Outline the process of sea-floor spreading and how it provides evidence for the plate tectonic theory of crustal evolution.
- Compare the relative importance of ridge push (gravitational sliding) and slab pull in driving plate movement.
- Explain the mechanics of constructive margins, noting decompression melting that forms basaltic magma, ocean ridges, and rift valleys.
- Detail destructive margins where subduction of denser oceanic crust creates deep sea trenches, island arcs, and explosive stratovolcanoes.
- Describe the formation of young fold mountains at destructive margins where two continental plates collide and undergo orogenesis.
- Clarify that conservative margins experience significant seismicity due to friction and locked faults, but lack vulcanicity.
- Define magma plumes as a theoretical model of anomalously hot mantle upwelling, with some deep-plume models extending towards the core-mantle boundary.
- Explain that decompression melting of rising mantle or plume material generates basaltic magma that can feed hotspot volcanoes.
- Describe how the continuous movement of a tectonic plate over a stationary plume creates a chronological chain of volcanic islands.
- Use the age and spatial distribution of island chains as evidence to calculate the direction and rate of historical plate movement.
Examiner Tips
- 💡Ensure you can clearly distinguish between the mechanisms of ridge push and slab pull, using appropriate geographical terminology.
- 💡When discussing internal energy sources, explicitly mention both primordial heat and radiogenic heat from radioactive decay.
- 💡Use annotated diagrams to illustrate the processes of sea-floor spreading, convection currents, and subduction.
- 💡Always link the specific landform (such as a deep sea trench) to the exact tectonic process (such as subduction and slab pull) operating at that margin.
- 💡Use named, real-world examples for every landform discussed, such as the Andes for young fold mountains or the Mid-Atlantic Ridge for ocean ridges.
- 💡When discussing seismicity, differentiate between the shallow focus earthquakes of constructive margins and the deep focus earthquakes of destructive margins.
- 💡Use the Hawaiian Islands as a primary case study, explicitly naming the active volcano and older extinct islands to illustrate plate movement.
- 💡Be prepared to calculate or interpret the rate of plate movement using the age of volcanic islands and their distance from the active hotspot.
- 💡Clearly distinguish between the stationary nature of the mantle plume and the dynamic movement of the overlying lithospheric plate.
Common Mistakes
- Stating that convection currents are the sole driver of plate movement. Correction: Explain that while convection currents are important, slab pull is now considered the dominant driving force for plate movement.
- Confusing the lithosphere with the crust. Correction: Define the lithosphere as the crust and the uppermost solid part of the mantle, which together form the tectonic plates.
- Describing ridge push as magma pushing the plates apart. Correction: Describe ridge push as gravitational sliding, where the elevated lithosphere at mid-ocean ridges slides downwards and outwards due to gravity.
- Stating that magma is created by friction at destructive margins; correct by explaining that flux melting occurs due to the release of volatiles from the subducting plate.
- Confusing the types of volcanoes at different margins; correct by associating shield volcanoes with constructive margins and composite volcanoes with destructive margins.
- Claiming conservative margins have volcanoes; correct by stating they only experience seismicity because there is no mechanism for magma generation or crustal destruction.
- Confusing magma plumes with constructive plate margins; correct by emphasising that plumes are intra-plate features occurring away from tectonic boundaries.
- Stating that the magma plume itself moves to create island chains; correct by explaining that the plume remains relatively stationary while the tectonic plate moves above it.
- Assuming all volcanoes in a hotspot chain are active; correct by noting that only the volcano currently directly above the plume is active, while the rest are extinct.