What evidence turned continental drift into plate tectonics?

Wegener's case (1912)

The German meteorologist Alfred Wegener proposed in 1912 that the continents had once formed a single supercontinent, Pangaea, which later split apart. According to the USGS account, his evidence included:

• the jigsaw-like fit of South America and Africa;
• identical fossil species on coasts now separated by an ocean;
• fossils of Glossopteris and coal of tropical plants in now-polar regions such as Antarctica;
• glacial deposits in places that are now arid, such as parts of Africa.

Most geologists rejected the idea for decades. The decisive objection was the lack of a mechanism: no one could explain what force could move continents through solid ocean floor.

Evidence from the ocean floor

Mapping after the Second World War changed the picture:

• Mid-ocean ridges form a continuous global chain more than 50,000 km long.
• Thin sediment: the Atlantic floor carries far less sediment than expected if the ocean were very old.
• Magnetic stripes: basalt records Earth's magnetic field as it cools. Surveys found alternating bands of normal and reversed polarity running parallel to, and mirrored across, the ridges.
• Rock ages: deep-sea drilling by the Glomar Challenger showed oceanic crust getting progressively older away from ridge crests; USGS calls this the clinching evidence for seafloor spreading.
• Earthquakes cluster along ridges and trenches.

Harry Hess proposed that new crust forms at ridges and old crust is consumed at trenches, which explains why the Earth does not grow and why ocean floor is geologically young. These lines of evidence were unified in the 1960s as plate tectonics: the lithosphere is divided into plates that move relative to one another.

Limits and caveats

Plate tectonics is the consensus framework, but details remain active research, such as the relative importance of slab pull and ridge push, and when plate tectonics began on Earth. The USGS pages summarized here are a popular-science overview written decades ago; they are accurate on the history but not a review of current literature.