What evidence built the theory of plate tectonics, and what keeps the plates moving?
Plate tectonics theory and evidence: the development of the theory from Wegener's continental drift, through Hess's sea-floor spreading, to plate tectonics; the evidence of palaeomagnetism and the symmetrical magnetic striping of the ocean floor; the increasing age of oceanic crust away from mid-ocean ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the Eduqas Geology statement on the development of plate tectonics. Covers Wegener's continental drift evidence and why it was rejected, Hess's sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, a worked spreading-rate calculation, and the driving forces of mantle convection, ridge push and slab pull.
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What this dot point is asking
Eduqas wants you to trace the development of plate tectonics from Wegener's continental drift, through Hess's sea-floor spreading, to the unified theory; to explain the evidence of palaeomagnetism and the symmetrical magnetic striping of the ocean floor; to explain why the age of oceanic crust increases away from mid-ocean ridges; and to describe the three driving mechanisms (mantle convection, ridge push and slab pull).
The answer
Continental drift (Wegener)
In 1912 Alfred Wegener proposed that the continents had once been joined in a supercontinent (Pangaea) and had since drifted apart. He assembled four main lines of evidence:
- The jigsaw fit of continental coastlines, especially the matching margins of South America and Africa.
- Matching fossils of the same species (for example the freshwater reptile Mesosaurus and the plant Glossopteris) on continents now separated by wide oceans.
- Matching rock types, mountain belts and structures across the oceans (for example the Appalachians lining up with the Caledonides of Scotland and Scandinavia).
- Palaeoclimatic evidence: glacial deposits now in present-day tropical regions, and tropical coal now in cold regions, showing the continents had moved through different climate zones.
Despite this evidence, the theory was rejected because Wegener could offer no convincing mechanism: there seemed to be no force capable of ploughing solid continents through the solid ocean floor, so the idea was dismissed for decades.
Sea-floor spreading (Hess)
The missing mechanism was found on the ocean floor. In the early 1960s Harry Hess proposed sea-floor spreading: new oceanic crust forms where basaltic magma rises at the mid-ocean ridges, then moves outwards symmetrically on both sides as the ridge keeps erupting. Old crust is destroyed at subduction zones, so the ocean floor is a continuously recycled conveyor belt.
Palaeomagnetism and magnetic striping
The decisive evidence that confirmed spreading was palaeomagnetism.
Because the Earth's magnetic field periodically reverses polarity, basalt erupting at a ridge records a succession of normal and reversed periods. As the crust spreads outwards equally on both sides, this produces stripes of alternating magnetic polarity that are symmetrical about the ridge axis, a mirror image either side of the centre line. The symmetry could only arise if crust is created at the ridge and carried away equally on each side, so it is a direct fingerprint of sea-floor spreading. The width of each stripe records the length of that polarity period, so matching the pattern to the known reversal timescale dates and measures the spreading.
Driving mechanisms
A modern plate (a slab of lithosphere, not just crust) moves because of three linked forces:
- Mantle convection. Heat from the core and radioactive decay drives slow convection currents; hot mantle rises beneath ridges and cool mantle sinks at subduction zones, dragging on the base of the plates.
- Ridge push. New crust at a ridge is hot and stands high; as it cools and thickens away from the axis it slides down the gentle slope under gravity, pushing the plate ahead of it.
- Slab pull. A cold, dense slab sinking into the mantle pulls the rest of the plate behind it. Slab pull is thought to be the strongest force, which is why plates with long subducting margins move fastest.
Examples in context
Example 1. The Mid-Atlantic Ridge. Symmetrical magnetic stripes and the steady increase of crustal age away from the axis make the Mid-Atlantic Ridge the classic demonstration of sea-floor spreading, opening the Atlantic at a few centimetres per year.
Example 2. Slab pull at the Pacific margins. The deep subduction of old, cold, dense Pacific crust generates strong slab pull, which is why the fastest plate motions are associated with long subducting margins.
Try this
Q1. State two pieces of evidence Wegener used for continental drift, and the reason his theory was rejected. [3 marks]
- Cue. Any two of: the jigsaw fit of the continents; matching fossils (for example Mesosaurus); matching rocks and mountain belts across oceans; palaeoclimatic evidence (glacial deposits now in the tropics). It was rejected because he could not supply a convincing mechanism to move the continents.
Q2. Explain why the magnetic stripes on the ocean floor are symmetrical about a mid-ocean ridge. [3 marks]
- Cue. New basalt forms at the ridge and records the field's polarity as it cools; as crust spreads equally outwards on both sides while the field periodically reverses, the same sequence of normal and reversed stripes is laid down on each side, giving a mirror-image pattern about the axis.
Q3. Name the three forces that drive plate movement and state which is thought to be strongest. [2 marks]
- Cue. Mantle convection, ridge push and slab pull; slab pull is thought to be the strongest force.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksExplain how palaeomagnetism and the pattern of magnetic stripes on the ocean floor provide evidence for sea-floor spreading.Show worked answer →
Build from how rocks record the field to the symmetry of the stripes about a ridge.
- How rocks record the field
- When basaltic magma erupts at a mid-ocean ridge and cools below its Curie temperature, magnetic minerals such as magnetite align with the Earth's magnetic field at that moment and lock in that direction permanently. The basalt becomes a fossil compass.
- Reversals
- The Earth's magnetic field reverses polarity at irregular intervals, so basalt formed during a period of normal polarity is magnetised in the opposite sense to basalt formed during a period of reversed polarity.
- The symmetrical stripes
- As new crust forms continuously at the ridge axis and moves outwards on both sides, it records the succession of normal and reversed periods. This produces a pattern of stripes of alternating polarity that is symmetrical about the ridge axis.
- Why this proves spreading
- The symmetry shows that crust is created at the ridge and carried away equally on each side, and the width of each stripe records how long that polarity period lasted. This is direct evidence for sea-floor spreading.
Top-band answers link cooling below the Curie point, field reversals, and the symmetry of the stripes about the ridge to the continuous creation and outward movement of crust.
Eduqas 20214 marksMagnetic surveys show that ocean floor now lying 30 km from a ridge axis formed 1.5 million years ago. Calculate the average half-spreading rate in centimetres per year, showing your unit conversion, and state the total rate at which the two plates move apart.Show worked answer →
Use the rate relation, convert the units carefully, then double for the total opening rate.
The calculation. The half-spreading rate is the distance from the axis divided by the age of that crust:
Convert the distance from kilometres to centimetres: . The time is .
The total opening rate. That is the half-spreading rate (one side of the ridge). New crust is added on both sides, so the two plates move apart at about , a typical mid-ocean ridge value.
Markers reward a correct conversion ( to ), the half-rate of , and doubling it to give the total opening rate.
Related dot points
- Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the mechanical layers (lithosphere and asthenosphere); the seismic evidence for the layering from changes in P and S wave velocity at boundaries such as the Moho; the P and S wave shadow zones as evidence for a liquid outer core; the use of meteorites and density as evidence for the composition of the core and mantle.
A focused answer to the Eduqas Geology statement on Earth structure. Covers the crust, mantle, outer and inner core, the lithosphere and asthenosphere, the seismic evidence from P and S wave velocity changes and the Moho, the shadow zones proving a liquid outer core, the meteorite analogy for the core, and locating an earthquake epicentre from the P-S travel-time gap.
- Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.
A focused answer to the Eduqas Geology statement on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-ocean, ocean-continent and continent-continent sub-types, the Benioff zone, subduction and decompression melting, and the diagnostic rocks, structures, earthquakes and volcanoes that identify each margin in the exam.
- Earthquakes and seismic waves: the focus and epicentre; the elastic rebound mechanism; the P, S and surface waves and their properties; the difference between magnitude (the logarithmic Richter scale and its saturation, and the moment magnitude scale) and intensity (the Modified Mercalli scale); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the Eduqas Geology statement on earthquakes. Covers the focus and epicentre, the elastic rebound mechanism, P, S and surface waves and their properties, the difference between magnitude (Richter saturation and moment magnitude) and intensity (Modified Mercalli), a worked example using the P-S travel-time gap, and how triangulation from three stations locates an epicentre.
- Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.
A focused answer to the Eduqas Geology statement on volcanic activity. Covers how silica content, viscosity and dissolved gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, the products (lava, tephra and pyroclastic material), the landforms (shield, stratovolcano, caldera and fissure), and the link to plate setting.
- The lithosphere, mantle plumes and hotspots: the structure and composition of oceanic and continental lithosphere; the principle of isostasy and isostatic adjustment; mantle plumes and hotspots as a cause of intraplate volcanism; hotspot tracks and the age progression of volcanic island chains (for example Hawaii); the basis of the Component 3 geology of the lithosphere option.
A focused answer to the Eduqas Geology statement on the lithosphere, mantle plumes and hotspots. Covers oceanic and continental lithosphere, the principle of isostasy and isostatic rebound, a worked density calculation, mantle plumes and hotspots as a cause of intraplate volcanism, hotspot tracks and the age progression of island chains such as Hawaii, and the Component 3 lithosphere option.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)