Edexcel Specification focus:
‘The theory of plate tectonics, including key elements such as Earth’s internal structure, convection currents, subduction, ridge push, slab pull, sea floor spreading and palaeomagnetism.’
This topic explores the theory of plate tectonics, explaining Earth’s internal structure and the mechanisms that drive the movement of tectonic plates across the lithosphere.
Earth's Internal Structure and Tectonic Plates
Understanding plate tectonics begins with the internal structure of the Earth, which is divided into distinct layers:
Inner core: A solid ball of iron and nickel, with extremely high temperatures and pressures.
Outer core: A liquid layer of iron and nickel that surrounds the inner core; it generates Earth's magnetic field.
Mantle: A thick, semi-solid layer of silicate rock containing the asthenosphere, which is partially molten and allows tectonic plates to move.
Crust: The outermost solid shell, composed of continental and oceanic plates.
Asthenosphere: The ductile, partially molten portion of the upper mantle beneath the lithosphere that allows tectonic plates to move.
The lithosphere includes the crust and the rigid uppermost mantle. It is broken into tectonic plates that float atop the asthenosphere.
Cutaway diagram of Earth’s internal layers, showing how the rigid lithosphere and ductile asthenosphere relate to the deeper mantle and core. Source
The Theory of Plate Tectonics
The plate tectonics theory states that the Earth's lithosphere is divided into plates that move due to heat-driven processes in the mantle. It combines earlier ideas of continental drift and sea floor spreading, providing a comprehensive explanation for the distribution and movement of tectonic plates.
This theory emerged in the mid-20th century and is now supported by evidence from geology, geophysics, oceanography, and palaeomagnetism.
Convection Currents in the Mantle
Convection currents are caused by heat from the core and lower mantle. Hot material rises towards the lithosphere, cools, and sinks again, creating a continuous flow.
Diagram showing mantle convection cells beneath tectonic plates, with upwelling at ridges and downwelling at trenches. Source
These currents drag the overlying tectonic plates in different directions.
They are fundamental in driving plate movement at all types of plate boundaries.
Convection Current: A circular movement of heated material (such as mantle rock) rising due to lower density and sinking when cooled, driving plate motion.
Sea Floor Spreading and Palaeomagnetism
Sea floor spreading occurs at mid-ocean ridges where new oceanic crust is formed as magma rises and solidifies.
As the plates move apart, magma fills the gap and forms new lithosphere.
This creates symmetrical patterns of magnetic stripes on either side of the ridge.
A schematic of magnetic anomalies in newly formed oceanic crust, displaying alternating bands of normal and reversed polarity (labelled a, b, c). Source
Palaeomagnetism provides critical evidence for this process:
When magma solidifies, iron minerals align with the Earth’s magnetic field.
As the magnetic field reverses over time, alternating bands of magnetic polarity are preserved in oceanic crust.
Palaeomagnetism: The study of the record of Earth’s magnetic field preserved in rocks, especially as symmetrical magnetic stripes on the ocean floor.
These patterns confirm that oceanic plates are continuously forming and moving apart — a key element of the plate tectonics theory.
Slab Pull and Subduction
Subduction occurs at convergent plate boundaries, where one plate is forced beneath another into the mantle.
Oceanic crust, being denser, subducts beneath continental crust or another oceanic plate.
This process is associated with ocean trenches, volcanic arcs, and intense seismic activity.
Slab pull is one of the most significant driving forces of plate movement:
As a subducting plate sinks into the mantle, it pulls the rest of the plate behind it.
The weight of the descending slab helps maintain plate motion.
Slab Pull: A tectonic force caused by the gravitational sinking of a dense, subducting oceanic plate into the mantle, pulling the rest of the plate along.
Ridge Push
At divergent boundaries, ridge push contributes to plate motion:
Magma rising at mid-ocean ridges creates elevated areas of new crust.
Gravity causes the newly formed lithosphere to slide down the ridge, pushing the plate outward.
Although ridge push is weaker than slab pull, it still plays a supportive role in plate movement.
Ridge Push: A tectonic force where elevated mid-ocean ridges cause oceanic plates to slide away from the ridge due to gravity.
Summary of Key Mechanisms
Plate movement is a result of the combined effects of several mechanisms:
Convection currents in the mantle move plates laterally.
Slab pull acts at subduction zones, where denser plates sink and drag the rest of the plate.
Ridge push occurs at divergent boundaries, where gravity assists in plate movement away from mid-ocean ridges.
Sea floor spreading generates new oceanic crust and displaces older crust.
Palaeomagnetic evidence supports the theory by revealing symmetrical patterns of magnetic polarity across ocean ridges.
The Role of Plate Tectonics in Geography
Understanding plate tectonics is essential for explaining the distribution and formation of:
Earthquakes, volcanoes, and mountain ranges
Ocean trenches, island arcs, and rift valleys
Tectonic hazards such as tsunamis and volcanic eruptions
The theory of plate tectonics unifies the geological processes that shape the Earth’s surface and influence many human and environmental systems.
FAQ
Oceanic lithosphere is thinner (around 5–10 km), denser, and mainly composed of basalt. It is more likely to subduct beneath other plates at convergent boundaries.
Continental lithosphere is thicker (up to 70 km), less dense, and composed mainly of granite. It is more buoyant and resists subduction, often leading to mountain-building when it collides with other continental plates.
This difference in density and composition explains why oceanic plates subduct more readily, while continental plates often deform and uplift.
Subduction zones involve intense pressure and friction as one plate descends beneath another.
The descending plate grinds against the overriding plate, storing strain energy.
This energy is released suddenly as earthquakes.
The subducting plate melts due to increasing heat and pressure, forming magma.
The magma rises to form volcanic arcs, often explosive due to water content from the oceanic crust.
These zones are among the most tectonically active regions on Earth.
Slab pull is a gravity-driven process where the weight of a sinking plate pulls the rest of the plate with it. It operates mainly at subduction zones and is considered one of the most powerful tectonic forces.
Convection currents are driven by internal heat from the Earth’s core and mantle. They act across wider areas beneath the lithosphere, pushing plates apart or together.
Slab pull: Acts downward and laterally
Convection: Acts upward and outward beneath plates
Together, they contribute to complex plate movements.
Several lines of evidence support slab pull:
Plates with large subduction zones tend to move faster (e.g. Pacific Plate).
Seismic tomography shows dense, cold slabs penetrating deep into the mantle.
The velocity of plate movement often correlates with the size and depth of subducting slabs.
Oceanic plates, which are denser and colder, tend to subduct and drive slab pull more effectively.
These observations support slab pull as a dominant tectonic force, particularly for oceanic plates.
Palaeomagnetic studies revealed symmetrical magnetic stripes on either side of mid-ocean ridges.
These stripes reflect historical magnetic field reversals recorded in cooling basalt.
They are evenly spaced and mirror-imaged, indicating continuous formation of crust.
This pattern confirmed the concept of sea floor spreading.
Additionally, apparent polar wandering paths differed by continent, showing that landmasses had moved.
Palaeomagnetism provided crucial, measurable evidence for continental drift and plate motion.
Practice Questions
Question 1 (2 marks)
Define the term slab pull and explain how it contributes to plate movement.
Mark Scheme:
1 mark for a correct definition:
Slab pull is the force exerted by a sinking, subducting oceanic plate that pulls the rest of the plate behind it into the mantle.1 mark for explaining how it contributes to plate movement:
As the dense plate descends, it drags the remaining plate with it, driving the motion of tectonic plates.
Question 2 (6 marks)
Explain how both convection currents and ridge push contribute to the movement of tectonic plates.
Mark Scheme:
Up to 3 marks for convection currents:
1 mark: Heat from the Earth's core causes mantle material to rise.
1 mark: The material spreads beneath the lithosphere, dragging tectonic plates laterally.
1 mark: As it cools, the material sinks back down, completing the convection cycle.
Up to 3 marks for ridge push:
1 mark: Magma rises at mid-ocean ridges, forming new crust.
1 mark: The ridge is elevated due to the heat and buoyancy of the new crust.
1 mark: Gravity causes the newly formed lithosphere to slide away from the ridge, pushing the plate outward.
