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Of Magnetism and Time |
Scientists had known since the 1920s that rock from different geological times could show opposite magnetic polarities. Sometimes the orientation was "normal," north-pointing, as it is today; at other times the geomagnetic field was reversed. In 1963, Allan Cox, Richard Doell, and Brent Dalrymple of the U.S. Geological Survey and Ian McDougall at Australian National University began to establish a quantitative timescale of reversals, by measuring the magnetic directions in lava flows on land and determining their ages by radioactive methods. It was a painstaking process, but by 1966 researchers had charted the reversal timescale for the past 3.5 million years.
 At sea, meanwhile, researchers were finding an intriguing pattern. Magnetic surveying of the seafloor had been developed during World War II as an outgrowth, like echo sounding, of the effort to improve detection of submarines. In 1961, Arthur Raff and Ronald Mason of Scripps Institution of Oceanography noted magnetic anomalies in the pattern of stripes on the ocean floor off the coast of Washington state. One year later geophysicist Drummond Matthews of Cambridge University, who had gathered magnetic surveys of an undersea ridge in the Indian Ocean, also noticed a distinct and curious pattern of magnetic stripes--stronger and weaker magnetic signals in parallel bands on either side of the ridge crest. Back in England he talked over his findings with Fred Vine, a graduate student at Cambridge University who was focusing on marine geophysics. The two of them hypothesized that the seafloor had recorded Earth's magnetic field orientation at the time the new molten rock oozed up from the mantle. If spreading of the ocean floor occurs as Harry Hess suggested, these blocks of alternately normal and reversely magnetized material would be carried away parallel to either side of the ridge.
The Vine-Matthews hypothesis, published in the fall of 1963, garnered little support in the geophysical community, partly because the magnetic reversal timescale was not yet complete, so the seafloor anomaly data matched their hypothesis only poorly. But two years later, in 1965, Fred Vine found himself in the company of Harry Hess, who had arrived at Cambridge on sabbatical leave, and J. Tuzo Wilson, there from the University of Toronto, continuing some of his own research on midocean ridges.
Wilson was examining Raff and Mason's maps of the seafloor area off the coast of Vancouver Island and south to California, and he suggested that the maps showed a seafloor spreading ridge. Vine and Wilson published a paper in October 1965 proposing a model for seafloor spreading in the northeastern Pacific, using as evidence bands of reversed magnetism that marched out from either side of the ridge. Shortly thereafter, a slight discrepancy between the seafloor reversal bands and the timing of known field reversals on land was smoothed out by a new land-based field reversal discovered by Doell and Dalrymple. With this addition, the two sets of data matched astonishingly well.
The confirmation of seafloor spreading was supported by other observations made in 1965 and 1966. Key among them were ocean sediment samples analyzed by Lamont's Neil Opdyke. The samples were from vertical cores, 16 to 40 feet long, taken from the ocean floor in the South Pacific. The timing and pattern of magnetic reversals in Opdyke's core samples matched those determined from lava flows on land and from seafloor magnetic stripes. |
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