Analyse the lines of evidence for continental drift and sea-floor spreading that led to the theory of plate tectonics, including but not limited to palaeomagnetism, fossil and rock matches and the Australian geological record

What this dot point is asking

NESA wants you to set out the evidence that built the theory of plate tectonics: the older evidence for continental drift (fit, fossils, rocks, ancient climate) and the later evidence for sea-floor spreading (palaeomagnetism, age of ocean floor). You should explain how each line of evidence works and connect it to Australia's place in the ancient supercontinent Gondwana.

The answer

Plate tectonics is the unifying theory of Earth science: the lithosphere is broken into rigid plates that move over the ductile mantle, driven by convection. The theory was assembled over the twentieth century from independent lines of evidence, first for continental drift and later for the sea-floor spreading that explains how continents move.

Continental fit

The coastlines of continents, and even better their continental shelves, fit together like puzzle pieces. South America and Africa are the classic match, but the southern continents, including Australia, Antarctica, India and Africa, also reassemble into the supercontinent Gondwana. Fit alone is suggestive rather than conclusive, which is why other evidence matters.

Matching fossils and rocks

The same distinctive fossils appear on now-separated continents. The fern Glossopteris and the reptile Lystrosaurus are found across Australia, Antarctica, India, Africa and South America, an impossible distribution unless those landmasses were once joined. Mountain belts and rock sequences also line up: the rocks and ages of parts of Western Australia match those of India and Antarctica, and glacial deposits of the same age are found across all the Gondwanan continents.

Ancient climate evidence

Rocks carry the fingerprint of the climate in which they formed. Glacial deposits and scratches (striations) of late Palaeozoic age occur in now-tropical India and Africa as well as in Australia, showing these regions once sat near the South Pole together. Coal seams, formed in warm swamps, are found in places now far too cold, recording the drift of continents through different climate zones.

Palaeomagnetism and sea-floor spreading

The decisive evidence came from the ocean floor. As basalt erupts at mid-ocean ridges and cools, iron-bearing minerals lock in the direction of the Earth's magnetic field at that moment. Because the field reverses polarity at irregular intervals, the sea floor records a pattern of magnetic stripes, symmetrical on both sides of the ridge. This symmetry shows that new crust forms at the ridge and spreads outwards, the mechanism of sea-floor spreading. Ocean floor is also youngest at ridges and progressively older away from them, and nowhere older than about 200 million years, because old crust is recycled at subduction zones. Palaeomagnetic data from continents also trace apparent polar wander paths that only make sense if the continents themselves moved.

The Australian context

Australia was part of Gondwana until it rifted from Antarctica and began drifting north roughly 45 to 50 million years ago, a separation recorded in matching rock and fossil sequences on the two continents and in the age of the sea floor of the Southern Ocean between them. Australia continues to move north-east at around seven centimetres a year, one of the fastest-moving large plates, which is why GPS reference frames for the continent must be periodically updated.

Try this

Q1. Explain how the distribution of Glossopteris fossils supports continental drift. [3 marks]

• Cue. Identical fossils of a plant that could not cross oceans occur on now-separated continents, so those continents must once have been joined.

Q2. Describe how magnetic stripes on the ocean floor provide evidence for sea-floor spreading. [4 marks]

• Cue. New basalt records the field's polarity as it cools; reversals produce symmetrical stripes either side of the ridge, showing crust forms at the ridge and spreads outward.