Continental Drift Theory — Wegener, Pangaea & Evidence

4.1 A Restless Globe — Why Continents Wander

Look at any modern world map. Continents occupy roughly 29 per cent of the Earth's surface; the remaining 71 per cent lies under oceanic water. The familiar arrangement we see today — the Americas in the west, Eurasia and Africa in the east, Antarctica anchoring the south — feels permanent. It is not. The continents and ocean basins have shifted constantly through geological time, and they are still moving today. The questions naturally follow: where were they before, why do they move, and how do scientists know any of this when no one was around to take photographs of the ancient Earth?

4.2 Early Hints — Did the Atlantic Once Close?

Anyone who has traced the Atlantic coastline on a globe has felt a tug of recognition: the bulge of South America seems to slot neatly into the curve of Africa; the bumps and bays on either side of the ocean look uncannily like pieces of one giant jigsaw. Many scientists noticed this symmetry and wondered whether the two Americas, Europe and Africa had once been welded together.

The earliest written record belongs to Abraham Ortelius, a Dutch map-maker who, in 1596, suggested that the continents had been pulled apart. Centuries later, Antonio Pellegrini drew a map showing three of the continents joined. But it was a German meteorologist, Alfred Wegener, who in 1912 framed the idea into a comprehensive scientific argument and gave it a name — the Continental Drift Theory.

4.3 The Birth of Pangaea — and Its Slow Break-up

According to Wegener, the entire continental crust of the Earth had once been clustered into a single landmass. He named it Pangaea — Greek for "all earth". A single global ocean, Panthalassa ("all water"), washed all around it. The break-up began about 200 million years ago. Pangaea first split into two large continental masses: Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere. Each in turn fragmented further, drifting slowly to the positions we recognise today as North America, Eurasia, South America, Africa, Antarctica, Australia and the Indian sub-continent.

4.4 Six Pieces of Evidence for Continental Drift

Wegener did not arrive at his idea by intuition alone. He marshalled an impressive array of evidence drawn from geography, geology, palaeoclimatology and palaeontology. Six lines stood out.

4.4.1 The Matching of Continents (Jig-Saw-Fit)

The shorelines of Africa and South America facing each other across the Atlantic show a striking and almost unmistakable match. In 1964, Sir Edward Bullard used a computer programme to find the best fit of the Atlantic margins. The match — taken at the 1,000-fathom line rather than the present-day shoreline — proved extraordinarily precise. The two coasts had once been one.

4.4.2 Rocks of Same Age Across the Oceans

Modern radiometric dating allows geologists to compare rock formations on different continents directly. A belt of ancient rocks 2,000 million years old runs along the coast of Brazil and matches an identical belt in western Africa. Likewise, the earliest marine deposits along the South American and African coasts date from the Jurassic age. If these rocks are the same age and chemistry, the ocean between them must have opened after they formed.

4.4.3 Tillite — Glacial Footprints on Six Continents

Tillite^? is a sedimentary rock formed from the deposits of glaciers. The Gondwana system of sediments in India is found, layer for layer, in Africa, the Falkland Islands, Madagascar, Antarctica and Australia — all in the Southern Hemisphere. At the base of every section sits a thick tillite bed, recording an extensive and prolonged ice age. The remarkable resemblance of these Gondwana-type sediments shows that the six landmasses had nearly identical climatic histories — proof of both ancient palaeoclimate and continental drift.

4.4.4 Placer Deposits — Gold That Travelled

The coast of Ghana in west Africa is famous for its rich placer deposits^? of gold. Yet geologists have searched in vain for the source rock that should have produced them. The gold-bearing veins are found instead on the Brazilian plateau, on the other side of the Atlantic. The neat explanation: the two continents once lay side by side, and the gold of Ghana was originally washed out from the Brazilian veins.

4.4.5 Distribution of Fossils

When identical species of plants and freshwater animals are found on either side of an ocean, biologists face a puzzle. Take three examples cited by Wegener:

• Lemurs — found in India, Madagascar and Africa. To explain this, some scientists once imagined a vanished landmass called Lemuria joining the three regions.
• Mesosaurus — a small reptile that lived in shallow brackish water. Its skeletons are known from only two places: the southern Cape province of South Africa and the Iraver formations of Brazil. The two sites today lie 4,800 km apart, with a deep ocean between.
• Glossopteris flora — fossil ferns of the Gondwana region — appear consistently across the southern continents.

None of these animals could have swum or floated 4,800 km. The simplest explanation is that the continents were once joined.

4.5 Forces of Drift — Wegener's Engine

Wegener could see that the continents had moved; he was less sure about why. He proposed two forces.

Both forces are real, but most contemporary scholars considered them far too weak to move giant continents through solid rock. This was the chief reason Wegener's theory was rejected during his lifetime — it lacked a credible engine.

4.6 Post-Drift Studies — A Fresh Look at the Ocean Floor

Wegener died in 1930 with his theory under heavy criticism. But after the Second World War, three independent lines of new evidence revived interest: convection-current theory, ocean-floor mapping, and the discovery of palaeomagnetic stripes (the last is taken up in Part 2).

4.6.1 Convection Currents — Arthur Holmes (1930s)

Most of the evidence Wegener used came from continents — fossils, tillite, gold deposits. The new ocean-floor data soon shifted the spotlight elsewhere. In the 1930s, the British geologist Arthur Holmes proposed that convection currents operate within the mantle. He argued that radioactive elements in the mantle generate heat. The hot, soft rock rises towards the crust, spreads sideways, cools, and sinks back down — exactly the way water circulates in a heated pot. A whole system of such convection cells, said Holmes, could supply the missing engine of continental motion.

4.6.2 Mapping of the Ocean Floor

Detailed expeditions in the post-war period revealed that the ocean floor is far from a featureless plain. It is instead full of relief — submerged mountain ranges, deep trenches near the continental margins, and broad flat plains in between. Most of the volcanic activity in the deep ocean was found to be concentrated along the mid-oceanic ridges. Equally surprising, the rocks of the oceanic crust turned out to be much younger than continental rocks. Rocks on either side of the crest of the ridges, at equal distances from it, showed remarkable similarities in chemistry and age — a clue that would later prove crucial.

4.6.3 Three Major Divisions of the Ocean Floor

Based on depth and relief, the ocean floor can be divided into three major zones: continental margins, deep-sea basins and mid-ocean ridges.

4.7 Where the Earth Trembles & Spits Fire

The maps of seismic activity and volcanic eruptions tell their own story. Plot every recorded earthquake on a globe and a curious pattern emerges: a thin ribbon of dots runs down the central Atlantic, almost parallel to the coastlines, and continues into the Indian Ocean. South of the Indian sub-continent the line splits — one branch swings into East Africa, the other heads east through Myanmar and onwards to New Guinea. This belt of dots is the same as the line of mid-oceanic ridges. The earthquakes there occur at shallow depths.

A second, broader belt of seismic activity coincides with the Alpine–Himalayan mountain system and the rim of the Pacific Ocean. Here the earthquakes are deep-seated. The rim of the Pacific is sometimes called the Ring of Fire because it also hosts a remarkable density of active volcanoes. These two distributions — earthquakes and volcanoes — are not random; they trace the boundaries of moving plates.

4.8 The Reception — Bold Idea, Cold Welcome

When Wegener first published his theory in 1912, the geological establishment was unimpressed. The fit of the coastlines was dismissed as coincidence; the fossil distributions were explained away by hypothetical land bridges that had since "sunk"; and, fatally, no one could believe that Wegener's two forces could move a continent. He died in 1930 on an expedition to Greenland, his ideas widely rejected.

Yet the bones of continental drift survived. Each line of evidence Wegener gathered — jig-saw fit, matching rocks, tillite, placer gold, fossils — turned out to be correct; only the mechanism needed replacing. Holmes' convection currents, the post-war ocean-floor surveys, and (in Part 2) Hess's sea-floor spreading and the unifying theory of plate tectonics together rescued and transformed his vision.