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Các nhà địa chất tìm kiếm một thử nghiệm quyết định đối với đáy biển lây lan stumbled khi đảo ngược từ dưới đáy đại dương. Công nhận sự đảo chiều của trường địa từ bắt đầu vào đầu những năm 1950. | MARINE GEOLOgy MAGNETIC SURVEYS Geologists looking for a decisive test for seafloor spreading stumbled upon magnetic reversals on the ocean floor. Recognition of the reversal of the geomagnetic field began in the early 1950s. In 1963 the British geologists Fred Vine and Drummond Mathews thought that magnetic reversal would be a decisive test for seafloor spreading. Experiments using sensitive magnetic recording instruments called magnetometers towed behind ships over the midocean ridges Fig. 40 revealed magnetic patterns locked in the volcanic rocks on the seafloor. These patterns alternated from north to south and were mirror images of each other on both sides of the ridge crest. The magnetic fields captured in the rocks also showed the past position of the magnetic poles as well as their polarities. As the iron-rich basalts of the midocean ridges cool the magnetic fields of their iron molecules line up in the direction of Earth s magnetic field at the time of their deposition. As the ocean floor spreads out on both sides of the ridge the basalts solidify. They establish a record of the geomagnetic field at each successive reversal somewhat like a magnetic tape recording of the his- Figure 40 A crew member lowers a magnetometer over the stern of the oceanographic research ship USNS Hayes. Photo courtesy U.S. Navy 54 MARINE EXPLORATION Midocean ridge Figure 41 Magnetic stripes on the ocean floor are mirror images of each other and indicate that the ocean crust is spreading apart. ------- ---------------------- tory of the geomagnetic field. Normal polarities in the rocks are reinforced by the present magnetic field while reversed polarities are weakened by it. This process produced parallel bands of magnetic rocks of varying width and magnitude on both sides of the ridge crest Fig. 41 . Here at last was clinching proof for seafloor spreading. In order for the magnetic stripes to form in such a manner the ocean floor had to be pulling apart. Two or three times every
