The 1950s was a busy time for marine geologists. Not only were Bruce Heezen and Marie Tharp creating the first maps of the seafloor, but other scientists were collecting data on the remnant magnetism of the rocks that were found on the bottom of the ocean. It was during this time that Arthur Raff and Ronald Mason made a fascinating discovery. The two men were conducting a magnetic survey of the Pacific Ocean seafloor off the coast of Oregon. As they sailed back and forth across the mid-ocean ridge, they found that the ocean bottom showed some very unusual magnetic properties. In some places, the magnetic field was stronger than normal, while other locations showed it to be weaker than normal. Called magnetic anomalies, these features took the form of long, narrow strips that ran parallel to the mid-ocean ridge across the seafloor.
At first, Raff and Mason did not know what to make of these "magnetic stripes," but they published their findings in 1961 anyway. The paper caught the attention of two geologists working in England. Drummond Matthews was a geophysicist working at Cambridge University who, along with a graduate student named Frederick Vine, had been studying events called magnetic reversals. During these periods, it appeared that Earth's magnetic field had totally
Figure 4.5 At right is an illustration of the magnetic "striping" in the seafloor. As the magnetic orientation of the Earth switches over millions of years, the magnetic particles in the ocean point toward whichever pole is the magnetic north at the time that the new crust is formed.
Raff and Mason. They thought that the two sets of data had to be linked some way.
In both cases, the magnetic stripes were centered on the mid-ocean ridges. They also noted that the stripes were not of even thicknesses. Some were fairly narrow and some were quite thick, so when they were viewed as a group, the stripes made a very distinctive pattern, almost like the bar code that we use on packages today. When Vine and Matthews looked at the stripe pattern on one side of the mid-ocean ridge, they found that it was almost identical to the pattern on the other side, except reversed. It was like the two sets of stripes were mirror images of each other.
After studying the problem, Vine and Matthews came up with a radical idea to explain the origins of the magnetic stripes. They asked a simple question: What if the magnetic anomalies were really caused by past magnetic reversals that were preserved in the ocean rocks? In places where the anomaly was stronger, the rocks matched the present-day magnetic field. In places where the anomaly was weaker, the rocks were formed when Earth 's magnetic field was reversed. They then explained that the reason the patterns of magnetic stripes formed a mirror image across the mid-ocean ridge was due to the fact that this is where new seafloor was being produced. As fresh lava flowed out of the ridge, it would push the old seafloor out in opposite directions.
Vine and Matthews published their theory in 1963, right after Hess published his theory about seafloor spreading. The two theories fit together perfectly. At first, many geologists were skeptical about this bold idea because the amount of data to support it was limited. Also, there were still many questions about the accuracy of the magnetic anomalies. As new data came in, it started to look like they were on to something.
The real clincher came in the late 1960s, when geologists began getting radioactive age dates for the rocks at the bottom of the ocean. Drilling into the seafloor produced samples that clearly showed that the rocks near the ridge were very young. As they brought up more samples from the ridge going out in either direction, the scientists found that the seafloor got older, with the oldest rocks being found farthest from the ridge. In 1965, Vine published another paper with Canadian geologist J. Tuzo Wilson.
Their data all but confirmed that Hess's theory of seafloor spreading was correct.
By the mid-1960s, the evidence to support the idea of continental drift was overwhelming. Magnetic stripes and age dates from the ocean coupled with the polar wandering data from the land left little doubt that the continents had moved. Even though most geologists were convinced that Earth's surface was shifting, they were still at a loss to explain how it all worked. What was needed was a better understanding of what was happening deep inside the planet. All the information was there waiting to be discovered. What was needed was a way to see it.
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