By the early twentieth century, geology had strong evidence for ancient seas, mountain building and large vertical changes in Earth’s crust, but most geologists still treated continents and ocean basins as fundamentally fixed in their horizontal positions.
Alfred Wegener challenged that framework in 1912. The fit of continental margins, matching fossils and geological structures across oceans suggested that continents had once been joined and later separated. Many geologists rejected continental drift, however, because Wegener lacked a convincing mechanism and his continents appeared to require ploughing through resistant oceanic crust.
The decisive evidence came from the ocean floor. Sonar mapping revealed mid-ocean ridges; seafloor-spreading models proposed that new crust formed there; magnetic surveys found symmetrical stripes recording geomagnetic reversals on either side of the ridges; and oceanic crust became progressively older away from them. Earthquake and volcano belts outlined the boundaries of moving lithospheric plates.
By the 1960s, plate tectonics unified these observations. Continents move because they are embedded within larger plates, not because they independently push through stationary seafloor.
Plate motion can now be measured directly. Research continues into the early history and detailed driving forces of plate tectonics, but the fixed-Earth framework it replaced is no longer viable.
