Definition
Continental drift is the scientific theory that continents now separated by oceans were once joined as a single giant supercontinent called Pangaea. Over a very long time, they slowly split apart and moved to where they are today.
The Giant Puzzle Pieces on the Map
Look closely at a world map and you'll spot something odd. The east coast of South America and the west coast of Africa, on opposite sides of the Atlantic Ocean, look like two puzzle pieces that fit together.
Around 1910, a German meteorologist named Alfred Wegener started from that simple bit of curiosity and began collecting remarkable evidence. In 1912 he first presented the theory of continental drift. Fossils of Mesosaurus, a freshwater reptile that couldn't cross an ocean, turned up on both continents. Rock layers in mountain ranges thousands of kilometers apart, and even traces of glaciers, lined up as if drawn in one line.
Based on this evidence, Wegener argued that about 300 million years ago, all the continents formed one huge supercontinent. He gave that ancient landmass the name Pangaea, from Greek words meaning "all land."
But He Couldn't Explain "How"
Wegener's idea shook the scientific world, but it also met fierce criticism and ridicule. It had a fatal weakness. If giant continents moved on their own, what pushed those heavy slabs of land? He couldn't point to a clear source of force.
Wegener suggested that Earth's rotation and the pull of the Moon and the Sun made the continents plow through the solid ocean floor like icebreakers. Physicists replied that those forces were nowhere near strong enough to push huge continents of rock.
In the end, Wegener never clearly proved what drives the continents. His theory was ignored by the scientific community for a long time. Wegener himself sadly died during an expedition to Greenland.
To Be More Precise: The Comeback as Plate Tectonics
Decades after Wegener died, sonar technology for mapping the deep sea improved, and everything changed. Scientists found that new magma rises at undersea mountain ranges, so the sea floor itself is spreading, and they found traces of Earth's magnetic field recorded in the rocks.
A new picture came together. Hot mantle deep underground slowly churns, a process called convection. Plates form at mid-ocean ridges, then sink back into the mantle at ocean trenches as they slowly move. Besides mantle convection, a key force is the pull of a heavy, sinking plate dragging the rest of the plate along. Continents weren't floating across the ocean after all. Rigid plates, made of the crust and upper mantle, were moving as whole pieces.
Today, scientists have built on Wegener's idea to complete the theory of plate tectonics. His early explanation had limits, but his bold insight that the scattered continents were once one became one of the greatest foundations of modern Earth science.
Common misconceptions
As soon as continental drift was announced, every scientist accepted it right away thanks to perfect evidence.
When it was announced, it couldn't explain the specific force that pushes heavy continents, so the scientific community rejected it for decades. Only after evidence piled up, such as sea-floor spreading and the magnetic record in sea-floor rocks, did it develop into plate tectonics and win acceptance.
Where you meet it
Put the east coast of South America next to the west coast of Africa, and they fit together like a puzzle.
The same plant (Glossopteris) and reptile fossils turn up in Paleozoic rock layers on two continents separated by the Atlantic Ocean.
In one sentence
The supercontinent Pangaea, once a single landmass, split apart and moved, and this idea became the foundation of modern plate tectonics.