The development of plate tectonic theory from continental drift, and the evidence for it (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and palaeomagnetic stripes).

What this dot point is asking

WJEC wants you to outline how plate tectonic theory grew out of Wegener's continental drift, and to marshal the evidence (geological, biological, climatic, and the decisive ocean-floor evidence) that confirmed it. This is the unifying theory of geology, so the evidence and its history are firmly examinable.

The answer

From continental drift to plate tectonics

In 1912 Alfred Wegener proposed continental drift: that the continents had once been joined in a supercontinent (Pangaea) and had since moved apart. He assembled strong evidence but could offer no convincing mechanism, so the idea was rejected for decades. From the 1950s and 1960s, the discovery of sea-floor spreading and the recognition of mantle convection supplied the mechanism, and continental drift was absorbed into the broader theory of plate tectonics: the lithosphere is broken into rigid plates that move over the asthenosphere.

The early evidence (Wegener's case)

Continental fit: the coastlines (better, the edges of the continental shelves) of South America and Africa fit together like a jigsaw. Matching geology: rock types, ages and mountain belts (such as the Appalachian-Caledonian belt) continue from one continent to another across the ocean. Matching fossils: identical fossils of land animals and plants (such as Mesosaurus and Glossopteris) occur on continents now separated by oceans they could not have crossed. Palaeoclimate: ancient glacial deposits occur in now-tropical regions and coal (tropical swamp) in now-cold regions, showing the continents have moved across climate belts.

The decisive evidence: sea-floor spreading and palaeomagnetism

The breakthrough came from the ocean floor. At mid-ocean ridges, new basaltic crust is created and spreads outward on both sides, so the sea floor is youngest at the ridge and oldest far from it (confirmed by dating). As the new basalt cools through its Curie point, iron-bearing minerals lock in the direction of the Earth's magnetic field. Because the field reverses polarity over time, the spreading crust records alternating normal and reversed magnetic stripes. These stripes are symmetrical about the ridge and identical on both sides, exactly as sea-floor spreading predicts, which provided the decisive confirmation of plate tectonics.

Examples in context

The Caledonian-Appalachian belt. The same mountain-building event is preserved in Scotland, Scandinavia and the eastern United States, a belt split by the later opening of the Atlantic, matching geology across an ocean. Glossopteris across Gondwana. This fossil plant occurs in South America, Africa, India, Antarctica and Australia, impossible unless those landmasses were once joined. The Vine-Matthews stripes. The interpretation of symmetrical magnetic stripes either side of the Mid-Atlantic Ridge in the 1960s was the observation that converted the geological community to plate tectonics.

Try this

Q1. What did Wegener call the ancient supercontinent? [1 mark]

• Cue. Pangaea.

Q2. Why was continental drift rejected when Wegener proposed it? [2 marks]

• Cue. He could not provide a convincing mechanism (he wrongly suggested continents ploughed through the ocean floor), so the theory was not accepted until sea-floor spreading supplied one.

Q3. Explain how cooling basalt at a ridge records the Earth's magnetic field. [2 marks]

• Cue. As basalt cools through its Curie point, iron-bearing minerals lock in the direction (polarity) of the Earth's magnetic field at that time, so the crust preserves a record of normal and reversed periods.