What evidence built the theory of plate tectonics, and what drives the plates?
Plate tectonics: the development of the theory from continental drift (Wegener's evidence) through sea-floor spreading to plate tectonics; the evidence of palaeomagnetism and symmetrical magnetic striping at mid-ocean ridges; the increasing age of oceanic crust away from ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the OCR H414 dot point on the development of plate tectonics. Covers Wegener's continental drift evidence, sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, and the driving mechanisms of mantle convection, ridge push and slab pull.
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What this dot point is asking
OCR 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 symmetrical magnetic striping and the increasing age of oceanic crust away from ridges, and to describe the 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. His evidence:
- The jigsaw fit of continental coastlines, especially South America and Africa.
- Matching fossils (for example Mesosaurus, a freshwater reptile) on now-separated continents.
- Matching rock types, mountain belts and structures across oceans (for example the Appalachians and Caledonides).
- Palaeoclimatic evidence: glacial deposits in present-day tropics, and tropical coal in present-day cold regions.
His theory was rejected because he could offer no convincing mechanism: there seemed no force capable of moving continents through the solid ocean floor.
Sea-floor spreading (Hess)
The missing mechanism came from the ocean floor. Harry Hess proposed sea-floor spreading: new oceanic crust is created by basaltic magma rising at mid-ocean ridges, then moves outwards on both sides as the ridge keeps erupting. Supporting evidence:
- The age of oceanic crust increases with distance from the ridge (youngest at the axis, oldest near the continents), and the ocean floor is far younger than the continents.
- High heat flow and a central rift valley at the ridge.
Palaeomagnetism and magnetic striping
The decisive evidence was palaeomagnetism:
Because the Earth's field periodically reverses polarity, basalt forming 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 polarity that are symmetrical about the ridge axis. The symmetry proves crust is created at the ridge and carried away equally on each side, confirming spreading.
Driving mechanisms
The plates move because of three linked forces:
- Mantle convection. Heat from the core and from radioactive decay drives slow convection currents in the mantle, which drag the plates along.
- Ridge push. New, hot, elevated crust at a ridge slides down the slope away from the axis under gravity.
- Slab pull. A cold, dense subducting slab sinks into the mantle and pulls the rest of the plate behind it; this is thought to be the strongest force.
Examples in context
Example 1. The Mid-Atlantic Ridge. Symmetrical magnetic stripes and the increasing age of crust 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 Pacific oceanic crust generates strong slab pull, which is why the fastest plate motions are associated with subducting margins.
Try this
Q1. State two pieces of evidence Wegener used for continental drift. [2 marks]
- Cue. Any two of: the jigsaw fit of continents; matching fossils (for example Mesosaurus); matching rocks and mountain belts across oceans; palaeoclimatic evidence (glacial deposits in the tropics).
Q2. Explain why oceanic crust is youngest at a mid-ocean ridge. [2 marks]
- Cue. New crust forms by magma rising and crystallising at the ridge axis, then moves outwards, so the crust gets older with distance from the ridge.
Q3. Name the three forces that drive plate movement. [2 marks]
- Cue. Mantle convection, ridge push and slab pull.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20196 marksExplain how palaeomagnetism and the pattern of magnetic stripes on the ocean floor provide evidence for sea-floor spreading.Show worked answer →
A level-of-response answer; build from how rocks record magnetism to the symmetrical stripes.
- How rocks record the field
- When basaltic magma erupts at a mid-ocean ridge and cools below the Curie temperature, magnetic minerals (for example magnetite) align with the Earth's magnetic field at that time and lock in that direction permanently.
- Reversals
- The Earth's magnetic field reverses polarity at intervals, so basalt formed during a normal-polarity period is magnetised in the opposite sense to basalt formed during a reversed period.
- The symmetrical stripes
- As new crust forms at the ridge and moves outwards on both sides, it records the succession of normal and reversed periods. This produces stripes of alternating polarity that are symmetrical about the ridge axis.
- Why this proves spreading
- The symmetry shows crust is created at the ridge and carried away equally on each side, and the width of each stripe records how long that polarity 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.
OCR H414/01 20174 marksWegener proposed continental drift but it was not widely accepted at the time. State two pieces of evidence Wegener used, and explain why his theory was initially rejected.Show worked answer →
Give specific evidence, then the reason for rejection.
Evidence (any two). The jigsaw fit of the continents, especially the coastlines of South America and Africa. The matching of fossil species (for example Mesosaurus) on continents now separated by oceans. The matching of rock types, mountain belts and geological structures across the ocean (for example the Appalachians and Caledonides). Palaeoclimatic evidence, such as glacial deposits now found in tropical regions and coal (tropical) deposits now in cold regions.
Why it was rejected. Wegener could not provide a convincing mechanism: he could not explain what force could move the continents through the solid ocean floor. Without a driving mechanism, geologists at the time did not accept the theory; it was only later, with sea-floor spreading and mantle convection, that a mechanism was found.
Markers reward two valid lines of evidence plus the lack of a credible mechanism as the reason for rejection.
Related dot points
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- Earthquakes: focus and epicentre; the elastic rebound mechanism; the types of seismic wave (P, S and surface waves) and their properties; magnitude (the logarithmic Richter and moment magnitude scales) versus intensity (Modified Mercalli); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the OCR H414 dot point on earthquakes. Covers focus and epicentre, elastic rebound, P, S and surface waves, the difference between magnitude (Richter and moment magnitude) and intensity (Modified Mercalli), and how P and S wave travel times and triangulation locate an epicentre.
- Volcanism: the control of magma composition (silica content), viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions and andesitic or rhyolitic explosive eruptions; volcanic products (lava, tephra, pyroclastic flows and gases); volcanic landforms (shield volcanoes, stratovolcanoes, calderas and fissures); the link between volcanism and plate setting.
A focused answer to the OCR H414 dot point on volcanism. Covers how silica content, viscosity and gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, volcanic products (lava, tephra, pyroclastic flows, gases), the landforms (shield, stratovolcano, caldera, fissure), and the link to plate setting.
- Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the contrasts between oceanic and continental crust; the seismic evidence for the layering (changes in wave velocity at boundaries such as the Moho); the P and S wave shadow zones as evidence for a liquid outer core; the link between the core and the Earth's magnetic field.
A focused answer to the OCR H414 dot point on Earth structure. Covers the crust, mantle, outer core and inner core, oceanic versus continental crust, the seismic evidence for the layering (velocity changes and the Moho), the P and S wave shadow zones as evidence for a liquid outer core, and the link between the core and the magnetic field.
- The rock cycle: the continuous transformation between igneous, sedimentary and metamorphic rocks; the processes that link them (crystallisation, weathering, erosion, transport, deposition, lithification, metamorphism, melting, uplift and exposure); the role of plate tectonics in driving the cycle; recognising that any rock type can be converted into any other.
A focused answer to the OCR H414 dot point on the rock cycle. Covers the continuous transformation between igneous, sedimentary and metamorphic rocks, the processes that link them (crystallisation, weathering, transport, lithification, metamorphism, melting and uplift), the role of plate tectonics in driving the cycle, and how any rock type can become any other.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)