Modern evolutionary synthesis
Alfred Wegener's idea of continental drift became accepted around 1960.  The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates. These plates slowly move on the underlying asthenosphere.  This discovery links phenomena such as volcanos, earthquakes, orogeny, and provides data for many palaeogeographical questions. One major question is still unclear: when did plate tectonics begin?