The Continental Jigsaw

Why Nobody Believed the Continents Could Move

In the autumn of 1911, while browsing in the university library at Marburg, Germany, meteorologist and balloonist Alfred Wegener noticed a footnote in a paleontological compilation. The monograph listed identical fossilized skeletons and needle-like teeth of Mesosaurus—a one-meter-long Permian reptile adapted exclusively to shallow freshwater lakes—excavated from black shales in southern Brazil and the Karoo basin of South Africa. Separated by 5,000 kilometers of deep saltwater Atlantic Ocean, the small freshwater swimmer could never have crossed the open sea.

To early twentieth-century geologists, transoceanic fossil twins presented no mystery. Earth science was governed by the contractionist doctrine formulated by Eduard Suess and championed in North America by Bailey Willis. According to contraction theory, as the Earth cooled from a molten globe, its interior shrank and its outer crust wrinkled like a drying apple: high arches remained as permanent continents while adjacent segments collapsed into permanent ocean basins. Whenever identical fossils turned up across an ocean, paleontologists drew hypothetical prehistoric "land bridges" spanning the Atlantic or Indian Ocean and asserted that once the organisms walked across, the bridges sank into the seafloor.

Trained in physics rather than geology, Wegener spotted a fatal flaw in sunken land bridges. Gravity measurements had established isostasy: Earth's outer crust consists of two distinct layers. Continents are built of light granitic rock (sial, density 2.7 grams per cubic centimeter), which floats buoyantly like an iceberg atop denser basaltic rock (sima, density 3.0 to 3.3 grams per cubic centimeter) forming the ocean floor. A three-thousand-mile bridge of lightweight continental granite could no more sink spontaneously into denser basaltic mantle rock than a timber barge could sink into liquid mercury.

If land bridges never existed, the continents themselves must once have been joined directly together before breaking apart and drifting horizontally. Matching the continents along the edge of their submerged continental shelves at a depth of 200 meters, Wegener found that the bulge of South America slotted cleanly into the Bight of Biafra in West Africa.

Between his first lecture in Frankfurt in January 1912 and his 1915 book Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans), drafted while recovering from combat wounds suffered on the Western Front, Wegener assembled four lines of evidence that modern continents had rifted from a single Late Paleozoic supercontinent he named Pangaea:

• - Structural geology: When the Atlantic was closed, the folded Appalachian mountains in Newfoundland linked directly into the Caledonian ranges of Ireland, Scotland, and Norway, while the Cape Fold Belt in South Africa aligned rock-layer for rock-layer with the Sierra de la Ventana of Argentina.
• - Paleobotany: Heavy seeds of the Permian seed fern Glossopteris—too bulky to blow across oceans or float in saltwater—formed a continuous belt across South America, South Africa, Madagascar, India, Antarctica, and Australia.
• - Paleoclimatology: Permo-Carboniferous glacial boulder beds from 300 million years ago covered subtropical Brazil, southern Africa, India, and Australia; when reassembled into southern Pangaea, every bedrock glacial striation radiated outward from a single polar ice cap.
• - Geodesy: Meanwhile, thick coal seams formed from tropical rainforests lay buried beneath the Arctic tundra of Spitsbergen.

The geological community rejected Wegener's synthesis at the November 1926 symposium of the American Association of Petroleum Geologists in New York. Yet the orthodoxy had a hard quantitative calculation on its side. Wegener had proposed that lighter sial blocks plowed directly through the solid basaltic sima of the ocean floor like icebreakers, driven by the Polfluchtkraft (a tiny centrifugal force pushing masses toward the equator) and tidal drag. Cambridge geophysicist Sir Harold Jeffreys calculated the exact shear stresses in his 1924 treatise The Earth: centrifugal and tidal forces generated a horizontal stress of barely $10^2$ to $10^3$ Pascals, whereas solid oceanic basalt possessed a yield strength of $10^8$ Pascals—a hundred thousand times stronger than Wegener's proposed motor. Expecting brittle granite to plow through solid basalt under tidal drag was like expecting wet clay to plow through armor steel.

Seeking sharper geodetic measurements in Greenland in 1930, Wegener skied 400 kilometers inland through minus-50-degree cold to resupply the Eismitte station, set out toward the coast on his fiftieth birthday on November 1, and died of cardiac exertion on the ice sheet. Only in the 1960s, when naval magnetometers towed across the Mid-Atlantic Ridge by Harry Hess, Fred Vine, and Drummond Matthews revealed symmetrical magnetic stripes frozen into newly erupted seafloor basalt, was Jeffreys's objection resolved: continents do not plow through the ocean floor, but ride passively atop rigid lithospheric plates driven by mantle thermal convection.