What year did Alfred Wegener make the discovery of continental drifting?
Since Wegener’s day, scientists have mapped and explored the great system of oceanic ridges, the sites of frequent earthquakes, where molten rock rises from below the crust and hardens into new crust. We now know that the farther away you travel from a ridge, the older the crust is, and the older the sediments on top of the crust are. The clear implication is that the ridges are the sites where plates are moving apart (click on the picture at the left to see a map of the age of the ocean crust). Where plates collide, great mountain ranges may be pushed up, such as the Himalayas; or if one plate sinks below another, deep oceanic trenches and chains of volcanoes are formed. Earthquakes are by far most common along plate boundaries and rift zones: plotting the location of earthquakes allows seismologists to map plate boundaries and depths (click on the picture at the right to view a map of quake epicenters). Paleomagnetic data have allowed us to map past plate movements much more precisel
