Continental Drift

Continental drift is the slow movement of Earth's continents across the planet's surface over millions of years. Today the continents look fixed in place, but they have actually moved thousands of kilometres in the past and are still drifting at a few centimetres per year. Around 250 million years ago all the world's land was joined into one giant supercontinent called Pangaea. Since then, the continents have been steadily breaking apart and rearranging themselves into the world we know today. The idea was first proposed in 1912 by German scientist Alfred Wegener, but it took over 50 years for most other scientists to accept it.

  • Drift speed2 to 10 cm/yearComparable to fingernail growth
  • Pangaea existedApprox. 250 million years agoSingle huge supercontinent
  • Atlantic wideningApprox. 2.5 cm/yearAbout the speed your fingernails grow
  • Africa-Europe distanceClosing at approx. 2 cm/yearWill eventually destroy the Mediterranean
  • Wegener proposed theory1912Was widely rejected for 50 years
  • Theory accepted1960sAfter seafloor evidence proved it

The story of Pangaea

Around 335 million years ago, the continents drifted together to form one giant landmass we now call Pangaea ("all earth"), surrounded by a single huge ocean called Panthalassa. Pangaea lasted for around 100 million years. Then about 175 million years ago, Pangaea began to break apart.

The break-up happened in stages.

  • First, Pangaea split into two: Laurasia (north) and Gondwana (south).
  • Laurasia later broke into modern North America and Eurasia.
  • Gondwana broke into modern South America, Africa, Antarctica, Australia and India.
  • India spent millions of years drifting north before crashing into Asia approx. 50 million years ago, raising the Himalayas.

The evidence for continental drift

By the early 1900s, scientists had collected lots of strange clues that suggested the continents had once been joined.

  • Matching coastlines: the east coast of South America and the west coast of Africa fit together like puzzle pieces.
  • Matching fossils: identical land-animal fossils (like the small reptile Mesosaurus) found in South America, Africa, India and Antarctica. They could not have swum across oceans.
  • Matching rocks: the same rock formations appear at the joining edges of continents we now know were once stuck together.
  • Old climate evidence: signs of past glaciation in places that are now tropical (like India and central Africa), and signs of ancient tropical forests in places that are now Arctic.
  • Magnetic stripes on the ocean floor (discovered in the 1960s): symmetrical bands of normal and reversed magnetism on either side of mid-ocean ridges proved that new seafloor was being created.

Why scientists rejected it for so long

Alfred Wegener's 1912 book The Origin of Continents and Oceans presented strong evidence, but other scientists dismissed it because Wegener could not explain how the continents moved. He suggested they ploughed through solid ocean crust, which is physically impossible. Without a mechanism, the evidence alone was not enough to convince most geologists.

The mechanism turned out to be different from what Wegener imagined. The continents do not plough through the seafloor; they ride on top of larger tectonic plates that are themselves moving as new seafloor is created at mid-ocean ridges and old seafloor is destroyed at deep trenches. Once this picture became clear in the 1960s, continental drift became the cornerstone of the wider theory of plate tectonics. Wegener was finally proved right, 30 years after his death.

Fact Continental drift is still going on. Some current movements:

  • The Atlantic Ocean is widening by approx. 2.5 cm per year, pushing North America and Europe apart.
  • Africa is moving north, slowly closing the Mediterranean Sea. In approx. 50 million years it will collide with Europe and the Mediterranean will be gone.
  • India is still pushing north into Asia, making the Himalayas taller by a few millimetres per year.
  • Australia drifts north by approx. 7 cm per year, faster than most continents.

What will happen in the future

Tectonic plates will keep moving for hundreds of millions of years to come. Looking ahead:

  • In approx. 50 million years, Africa will collide with Europe and close the Mediterranean.
  • In approx. 150 million years, the Atlantic may stop widening and start closing again.
  • In approx. 250 million years, the continents may join into a new supercontinent (sometimes called Pangaea Ultima or Amasia), in roughly the opposite arrangement of the original Pangaea.

These predictions are necessarily uncertain. Tectonic motion can change over time, and 200 million years is a very long time.

Did you know? Alfred Wegener never got to see his theory accepted. He died in 1930 on a research expedition to Greenland, after celebrating his 50th birthday in a tent on the ice. He was looking for evidence of continental movement when he died. His body was buried in the Greenland ice; tectonic and ice movements have probably carried his remains many kilometres from where he was first laid.
Deeper dive: how we measure plate movement today

In Wegener's day, the only evidence for continental movement was indirect: matching fossils, rocks and coastlines. Today, scientists can measure plate movement directly, in real time, to within a few millimetres per year.

The tools they use include:

  • GPS: a network of stationary GPS receivers around the world records its own position to incredible precision. Over years, the receivers slowly drift, exactly tracking the motion of the tectonic plate they sit on.
  • Very Long Baseline Interferometry (VLBI): astronomers use radio telescopes on different continents to measure the exact distance between them by timing signals from very distant quasars. Continents that are drifting apart show measurably increasing distances.
  • Satellite Laser Ranging: high-precision lasers bouncing off satellites measure the position of ground stations.
  • InSAR (Interferometric Synthetic Aperture Radar): satellite radar can detect tiny changes in ground position before and after an earthquake.

Together these techniques have produced detailed maps of how every plate is moving, down to a precision of a few millimetres per year. They show, for example, that the San Andreas Fault is moving at approximately 5 cm per year, that the Indian Plate is pushing into Asia at about 4 cm per year, and that the Atlantic is widening at about 2.5 cm per year. These measurements are now used to assess earthquake risk and to refine our understanding of how Earth works.

For more, see tectonic plates and fault lines.