How Was Seafloor Spreading Discovered? Stripes on a Magnetic Map

Alfred Wegener assembled a substantial case from 1912 onward: the fit of the continental coastlines, matching rock formations and mountain belts across oceans, identical fossil species on continents now separated by thousands of kilometres of deep water, and glacial deposits in places now tropical. The evidence was strong and the mechanism was not. Wegener proposed that continents ploughed through oceanic crust, and physicists demonstrated that the forces he invoked were far too weak and that the crust was far too rigid, which was a correct objection to an incorrect mechanism. The geological establishment, particularly in North America, rejected the whole idea, and the standard alternative explanations invoked land bridges that had since sunk, which required them to disappear without trace. Wegener died on an expedition in Greenland in 1930 with his hypothesis in disgrace, and it remained marginal until the evidence arrived from a direction nobody had been looking.

Submarine warfare made the seabed strategically important, and the postwar surveys funded for that reason produced discoveries that had nothing to do with submarines:

• •A continuous mountain range running down the middle of the Atlantic and continuing around the world, mapped in detail by Marie Tharp and Bruce Heezen, whose physiographic maps made the structure visible to everyone
• •A deep rift valley running along the crest of that ridge, which Tharp identified from the soundings and which Heezen initially dismissed as girl talk before the evidence forced the point
• •Earthquake epicentres falling in a narrow band along the ridge and along ocean trenches, which plotted onto the same lines
• •Ocean sediment far thinner than expected for a seabed assumed to be as old as the earth, and thinnest at the ridge
• •No ocean floor anywhere older than about two hundred million years, against continental rocks billions of years old
• •Heat flow measurements showing far more heat escaping at the ridges than elsewhere

The decisive evidence came from magnetism. Volcanic rock records the direction of the earth's magnetic field as it cools, and it had been established from lava flows on land that the field reverses polarity at irregular intervals of hundreds of thousands to millions of years. Magnetic surveys towed behind ships across the ocean floor found a pattern of alternating stronger and weaker magnetisation, arranged in long stripes parallel to the mid-ocean ridge. In 1963 Fred Vine and Drummond Matthews, with Lawrence Morley reaching the same conclusion independently and having his paper rejected, proposed the explanation: if new crust is continuously created at the ridge and spreads outward, each piece records the field polarity at the moment it solidified, so the seabed is a tape recording of magnetic reversals. The prediction that follows is specific and testable, that the stripes must be symmetrical either side of the ridge and that their widths must match the known sequence of reversals dated on land. They are, and they do.

Harry Hess had proposed seafloor spreading in a paper circulated from 1960, describing it candidly as an essay in geopoetry because he could not prove it, and the magnetic stripes supplied the proof. The remaining pieces fell rapidly. Deep-sea drilling confirmed that sediment thickens and seafloor age increases with distance from the ridge exactly as predicted. Tuzo Wilson identified transform faults, a type of boundary that only makes sense if the seafloor is spreading, and the direction of motion on them was confirmed by earthquake analysis. Trenches were recognised as places where old crust descends, resolving the question of where the new crust goes. By 1968 the pieces had been assembled into plate tectonics, a theory describing a rigid outer shell broken into plates moving over a weaker layer beneath, which explains mountain building, earthquakes, volcanic distribution and the drift Wegener described, and which was accepted across the discipline within a few years, an unusually rapid and complete change of view.

Continental drift was rejected for fifty years because Wegener's proposed mechanism was physically impossible, not because his evidence was weak. Postwar naval surveying revealed a global mid-ocean ridge with a rift along its crest, thin sediment and no seafloor older than about two hundred million years. Magnetic surveys found stripes parallel to the ridge, which Vine and Matthews explained in 1963 as a record of field reversals in crust created at the ridge and spreading outward, symmetrical on both sides.