Continental drift and Pangaea

Look at a map of the world. Find South America and Africa.

Their edges look like puzzle pieces! Long ago, they really were joined together.

If you look closely at a world map, the east coast of South America and the west coast of Africa look as if they could fit together like puzzle pieces. People noticed this a long time ago. The U.S. Geological Survey (USGS) explains that in 1596, the Dutch mapmaker Abraham Ortelius suggested the Americas had been torn away from Europe and Africa.

Today scientists know the fit is no accident. The continents really were joined, in one huge supercontinent called Pangaea. Its name means 'all lands' in Greek.

The jigsaw fit of the Atlantic coasts invited the idea of moving continents long before anyone could test it. USGS's This Dynamic Earth records that Abraham Ortelius, in his Thesaurus Geographicus of 1596, suggested the Americas were 'torn away from Europe and Africa', and that in 1858 the geographer Antonio Snider-Pellegrini drew maps showing the American and African continents fitted together and then separated.

The supercontinent at the center of the story is Pangaea, from the Greek for 'all lands'. Continental drift, the theory built around it, was the forerunner of plate tectonics, which USGS describes as Earth's outermost layer being broken into a dozen or more large and small plates moving relative to one another.

Long ago, the land was all in one giant piece.

Slowly, very slowly, it cracked and pulled apart. The pieces became the continents.

The continents have been moving for hundreds of millions of years. National Geographic explains that about 480 million years ago, most continents were scattered pieces of land near or below the Equator. Slowly they came together, and by 240 million years ago almost all the land was joined in one giant continent.

Then it began to break up. National Geographic says this started by about 200 million years ago; USGS gives about 225 to 200 million years ago. Scientists still work out exact dates, so different sources give slightly different numbers. One big piece became Europe, Asia and North America. Another split into South America, Africa, Antarctica, Australia and India.

Dates for Pangaea depend on the source and on what is being dated (assembly, peak or break-up). National Geographic Education gives scattered continents about 480 million years ago, nearly all land joined by 240 million years ago, and break-up beginning by about 200 million years ago. USGS dates the start of break-up to about 225 to 200 million years ago, and says Wegener himself placed it around 200 million years ago. The University of California Museum of Paleontology says Pangaea existed during the Triassic (251.0 to 199.6 million years ago) and began to break apart in the Middle Triassic.

The Berkeley museum and USGS (citing Alexander Du Toit) describe the first split into a northern landmass, Laurasia (North America and Eurasia), and a southern one, Gondwana (South America, Africa, India, Antarctica and Australia), which broke into smaller continents.

A scientist named Alfred Wegener looked for clues.

He found the same fossils on lands that are now far apart. He thought the lands must once have touched.

In 1912, a German scientist named Alfred Wegener, who studied weather, published the idea of continental drift. USGS explains that he gathered clues. The same fossil plants and animals were found on the matching coasts of South America and Africa, now separated by the Atlantic Ocean. Those living things could not have swum or been carried across such a wide ocean.

He found climate clues too. Coal, made from tropical plants, had been found in frozen Antarctica, so Antarctica must once have been closer to the Equator. And signs of ancient glaciers were found in parts of Africa that are dry today.

USGS describes how Alfred Wegener, a 32-year-old German meteorologist, introduced continental drift as a scientific theory in two articles in 1912, and developed it while recovering from a war injury into the 1915 book The Origin of Continents and Oceans. Beyond the coastal fit, he assembled geological and paleontological evidence: matching fossil species on the African and South American coasts, which Wegener saw as the most compelling evidence that the two continents were once joined.

He also pointed to climatic mismatches: coal deposits of tropical plants in Antarctica, fossils of the fern Glossopteris in now-polar regions, and glacial deposits in present-day arid Africa, such as South Africa's Vaal River valley. In Wegener's view, drifting continents explained both the matching fossils and the evidence of dramatic climate changes.

At first, most scientists did not believe Wegener.

He could not say what pushed the continents. But he kept looking for clues for the rest of his life.

Most scientists of Wegener's time believed that the continents and oceans had always stayed in the same places. They had a big question he could not answer: what could be strong enough to push whole continents across the Earth?

Wegener thought the continents plowed through the ocean floor, but the English scientist Harold Jeffreys showed that solid rock could not do that without breaking up. Wegener kept gathering evidence anyway. He died in 1930 on an expedition across the Greenland ice cap, long before his main idea was accepted.

USGS notes that when Wegener proposed drift, the scientific community firmly believed continents and oceans were permanent. His critics' most basic objection was the mechanism: he suggested continents plowed through the ocean floor, and the geophysicist Harold Jeffreys argued correctly that solid rock could not do so without breaking up. Many leading geologists also regarded him as a meteorologist meddling in their field.

Wegener defended the theory for the rest of his life and froze to death in 1930 on an expedition across the Greenland ice cap. Continental drift was debated for decades and largely dismissed. USGS points out the irony that one of plate tectonics' chief open questions is still the one Wegener could not resolve: what exactly drives the plates.

Later, scientists explored the bottom of the sea.

They found a long line of underwater mountains. New sea floor is made there. It slowly spreads out on both sides.

The answer came from the ocean floor. In the 1950s, ships mapping the sea floor found a huge underwater mountain range that winds around the whole planet, more than 50,000 kilometers long. In 1961, scientists began to suggest that the sea floor is splitting along these ridges.

Hot melted rock rises there, cools and makes new sea floor, which slowly spreads away on both sides. This is called seafloor spreading. Along the Mid-Atlantic Ridge, USGS says, the sea floor spreads about 2.5 centimeters a year. Over millions of years, that has widened the Atlantic Ocean from a narrow inlet into a vast ocean.

Four developments, USGS explains, revived drift and produced plate tectonics: the ruggedness and youth of the ocean floor; repeated reversals of Earth's magnetic field recorded in rocks; seafloor spreading and the recycling of oceanic crust; and the concentration of earthquakes and volcanoes along trenches and submarine ranges. In the 1950s, surveys revealed the global mid-ocean ridge, more than 50,000 km long, and zebra-like magnetic stripes on either side of it.

From 1961, scientists proposed that ridges are weak zones where new crust forms and spreads away. Robert S. Dietz coined the term seafloor spreading, and Harry Hess reasoned that crust must be consumed elsewhere, beneath trenches. The Mid-Atlantic Ridge spreads at about 2.5 cm a year, or 25 km in a million years, and over the past 100 to 200 million years this has grown the Atlantic from a tiny inlet into an ocean.

The continents are still moving today.

Each year, they move only a tiny bit, just a few centimeters. Far in the future, the map of the world will look different.

The plates are still moving. National Geographic says North America and Europe are moving apart by about 2.5 centimeters a year. In Iceland, which sits right on the Mid-Atlantic Ridge, USGS explains that the island is splitting as North America moves west compared with Eurasia.

In the far future, National Geographic suggests, part of California might separate from North America and become an island, and Africa might split in two along the Great Rift Valley. It is even possible that a new supercontinent could form someday.

Plate motion continues at centimeters per year. Iceland, straddling the Mid-Atlantic Ridge, lets scientists study spreading on land as it splits along the boundary between the North American and Eurasian plates, USGS notes. National Geographic gives the separation of North America and Europe as about 2.5 cm (1 inch) per year.

Forecasts are possibilities, not certainties. National Geographic suggests part of California could become an island and Africa might split along the Great Rift Valley, and that another supercontinent may form someday. USGS adds that scientists still debate how plate tectonics operated, if at all, early in Earth's history, and whether similar processes act on other planets.