During the Second World War, linear bands of positive and negative magnetic anomolies were found in the ocean floor, stretching for hundreds of miles, with an almost perfect symmetry either side of mid-ocean ridges. It was realised that these anomalies were evidence of periodic reversals of the earth's magnetic field. Molten basalt had been magnetised in the direction of the field and then cooled to 'fossilise' that direction. Each time the earth's magnetic field reversed, a stripe was added to the 'bar-code'.
By dating onshore lava flows where magnetic reversals had occurred, a timescale of magnetic reversals was built up. It then became clear that, in the undersea anomalies, the youngest rocks were near the ridge, while the oldest were farther away and nearest the continents. Either side of the ridge, stripes of exactly the same age could be matched with each other.
There was already evidence of continental drift, but this new evidence helped to explain how the earth's crust is moving and how the sea floor is spreading.