Continental Drift (1912): Alfred Wegener's Moving Continents

Continental drift is the theory that Earth’s continents have slowly moved across the globe over hundreds of millions of years. The German scientist Alfred Wegener first presented it in a lecture in Frankfurt on 6 January 1912, arguing that the continents had once formed a single landmass. Rejected by most geologists for decades, his idea was vindicated in the 1960s by the theory of plate tectonics, now the foundation of the Earth sciences.

Background: a fixed Earth

In the early 20th century most geologists believed that continents and oceans had stayed broadly in the same places throughout Earth’s history. To explain why similar fossils appeared on continents separated by oceans, many proposed “land bridges”, strips of land that had once linked the continents and later sank beneath the sea.

Others thought the Earth was slowly cooling and shrinking, wrinkling its crust like the skin of a drying apple to form mountains. Neither theory explained the evidence well, but they fitted the assumption that the continents themselves did not move. There were also earlier speculations about moving continents. Writers since the 16th century had remarked on the fit of the Atlantic coasts, and in 1910 the American geologist Frank Bursley Taylor proposed that continents had crept across the globe. None, however, assembled a body of evidence comparable to Wegener’s.

Alfred Wegener

Alfred Wegener (1880–1930) was not trained as a geologist. He earned a doctorate in astronomy in Berlin in 1905 and became a meteorologist and polar explorer. He made balloon flights to study the atmosphere, once staying aloft for more than 52 hours with his brother Kurt, and took part in a Danish expedition to north-east Greenland in 1906–1908.

He was also a lecturer at the University of Marburg. Around 1910, while looking at a world atlas, he was struck by how closely the coastlines on either side of the Atlantic seemed to fit together. In 1911 he came across scientific reports of similar fossils on separate continents, which convinced him that the idea deserved serious study.

The 1912 lecture and book

On 6 January 1912 Wegener presented his theory to the Geological Association at the Senckenberg Museum in Frankfurt am Main. He published two papers on the subject later that year. His research was then interrupted by another Greenland expedition, in which he and Johan Peter Koch crossed the ice sheet in 1912–1913, and by the First World War, in which he was wounded.

While recovering he expanded his ideas into a book, Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans), published in 1915. Revised editions followed in 1920, 1922 and 1929. In them he described an ancient supercontinent, which he first called the Urkontinent (“original continent”) and from the 1920 edition named Pangäa, now usually spelled Pangaea, from Greek words meaning “all lands”. An English translation appeared in 1924 and brought the theory to a wide audience.

Wegener’s evidence

Wegener’s great strength was that he drew on many sciences at once. His main lines of evidence were:

• Matching coastlines. The bulge of Brazil fits into the Gulf of Guinea in West Africa, and the fit is even better along the edges of the continental shelves.
• Fossils. Remains of the same plants and animals appear on continents now far apart: the small freshwater reptile Mesosaurus in both South America and southern Africa, and the fern-like plant Glossopteris in South America, Africa, India and Australia (and later also found in Antarctica). These organisms could not have crossed wide oceans.
• Rocks and mountains. Rock types and mountain belts appear to continue across oceans, for example between the Appalachians of North America and the mountains of Scotland and Scandinavia.
• Ancient climates. Working with his father-in-law, the climatologist Wladimir Köppen, Wegener showed that traces of glaciers about 300 million years old appear in now-tropical regions such as India and southern Africa, while coal, formed from lush vegetation, is found in Antarctica.

Rejection

Many European scientists took Wegener’s ideas seriously, but the response in Britain and especially North America was largely hostile. At a symposium of the American Association of Petroleum Geologists in New York in 1926, most speakers rejected the theory.

The main objection was the lack of a mechanism. Wegener suggested that continents ploughed through the ocean floor, pushed by forces linked to Earth’s rotation and the tides. The British geophysicist Harold Jeffreys showed that these forces were far too weak. Critics also attacked some of Wegener’s measurements, including his claim that Greenland was moving away from Europe at a fast rate, which proved to be greatly exaggerated.

A few scientists defended drift. In 1929 the British geologist Arthur Holmes suggested that slow convection currents in Earth’s mantle could carry continents, and the South African geologist Alexander du Toit gathered further evidence in his 1937 book Our Wandering Continents. But for most geologists drift remained a fringe idea.

Historians such as Naomi Oreskes have shown that the rejection was not only a matter of evidence. American geologists in particular valued careful field observation and distrusted sweeping theories, and Wegener was an outsider, a meteorologist challenging the core assumptions of geology. Some critics also disliked his style of argument, which they felt gathered evidence selectively to support a conclusion he had already reached.

Death in Greenland

In 1930 Wegener led a German expedition to study the climate of the Greenland ice sheet. In late 1930 he and the Greenlander Rasmus Villumsen set out from the central station, Eismitte, to return to the coast in severe cold. Neither survived. Wegener’s body was found in May 1931, carefully buried; he had probably died of heart failure from exhaustion. Villumsen’s body was never found.

Vindication: plate tectonics

The evidence Wegener lacked came from the oceans and from rock magnetism. In the 1950s, studies of the magnetism preserved in old rocks showed that the continents had changed position relative to the magnetic poles. Mapping of the ocean floor revealed a vast system of mid-ocean ridges.

In the early 1960s Harry Hess and Robert Dietz proposed that new sea floor forms at the ridges and spreads outwards. In 1963 Frederick Vine and Drummond Matthews, and independently Lawrence Morley, explained the matching stripes of magnetism on either side of the ridges as records of reversals in Earth’s magnetic field. By the late 1960s these findings had been combined into the theory of plate tectonics.

Plate tectonics differs from Wegener’s version: the continents do not plough through the ocean floor but ride on large, rigid plates that move a few centimetres a year. Yet his central claim, that continents move and were once joined, was correct. Today satellite measurements using GPS can track the motion of the continents directly. They show, for example, that the Atlantic Ocean is widening by a few centimetres each year, roughly the rate at which fingernails grow.

Timeline

• 1880, 1 November: Alfred Wegener is born in Berlin.
• 1912, 6 January: Wegener presents continental drift in a lecture in Frankfurt.
• 1915: The Origin of Continents and Oceans is first published.
• 1924: The first English translation appears.
• 1926: Geologists at a New York symposium largely reject the theory.
• 1929: Arthur Holmes proposes mantle convection as a possible mechanism.
• 1930, November: Wegener dies on the Greenland ice sheet.
• 1962: Harry Hess publishes his theory of sea-floor spreading.
• 1963: Vine and Matthews explain the magnetic stripes of the sea floor.

Why it matters

Wegener’s theory changed how we see the planet: not as fixed and unchanging, but as a surface in constant slow motion. The story of its rejection and vindication is also a lesson in how science works. Strong evidence can be resisted until a convincing mechanism is found, as the later study of the breakup of Pangaea and plate tectonics would show.