The Birth of Plate Tectonics Theory

By David Donnenfield and David Howell

The USGS Western Region Headquarters in Menlo Park, CA, may be best known for its surveying, mapmaking, and volcano and earthquake monitoring activities, but in the 1960s a basic research project on the earth's magnetic field by three young USGS scientists, Richard Doell, Allan Cox, and Brent Dalrymple, provided important new insights into some of the earth's secrets. In the end, it laid the foundation for what became a revolution in the earth sciences....a revolution we now call plate tectonics. This fortuitous meeting and blending of three different, but complementary, skill sets resulted in years of intense interest and activity in explaining the dynamic forces responsible for the earth's features.

The 1950's and 1960's were an especially exciting time for the earth sciences, and science in general. The world had emerged from the shadows of World War II and geologists and geophysicists were reaping the benefits of wartime technology, now being put to peaceful uses. Instrumentation that was more sensitive and powerful was extending scientific insight into heretofore hidden processes related to the simmering controversy about continental drift and was providing new information to fire the imaginations of young earth science students.

Richard Doell was one of those students, in the Geology Department at the University of California at Berkeley, who had been fascinated by continental drift while still an undergraduate student. As a graduate student, he was exposed to the idea that paleomagnetism might be instrumental to resolving the matter of continental drift. Paleomagnetism at that time was a relatively new branch of the earth sciences dealing with the ancient magnetism retained in rocks during their formation. Allan Cox came to the University a few years after Doell; he was also attracted to the topic of continental drift and became particularly intrigued by the phenomenon of reversely magnetized rocks. Cox was optimistic that paleomagnetism could solve some of the enigmas of earth science.

So much was awaiting discovery and explanation, with the answers seemingly just beyond scientific reach. Cox and Doell knew this only too well from the extensive survey of paleomagnetism they began while Cox was still a graduate student, and they agreed that if the opportunity arose they would devise a research project to pursue some of those answers together. That opportunity came in 1958 at the USGS, while Cox was still a student and Doell was an assistant professor at MIT. Doell was asked by James Balsley, Chief of the USGS Geophysics Branch, to establish a laboratory for paleomagnetic studies in Menlo Park where there would be little to interfere with the delicate magnetic measurements required for the intricate research. At the time, there weren't many facilities available at the Menlo Park campus to house the lab, and the best candidate turned out to be a tarpaper and wood shack built during World War II to support the Army hospital that was located there during the war. That humble structure was quite well suited to the research work that would be conducted there, being situated away from automobile traffic and other potential magnetic disturbances. The temporary nature of the building lent itself to easy modification to suit their purposes because facilities staff were not terribly concerned about such things as moving walls, rerouting supply lines, and making other modifications that could expedite the advancement of their research work.

The tarpaper shack on the Menlo Park campus where USGS scientists developed the results that would help support the theory of plate tectonics.

Doell and Cox prepared an ambitious project to investigate the issue of polarity reversals in rocks because they suspected that settling the matter of polarity reversals could play an important role in settling the debate over continental drift. In the laboratory, dated volcanic core samples from distant places would first be put through a series of treatments to remove all but the rock's original remnant magnetization....in a sense, what it was born with. The carefully measured orientation of that magnetization in the samples would then be used to determine if it was normal or reversed polarity. If these sample rocks of roughly the same geologic age all showed the same polarity, the researchers would be able to deduce that polarity reversal was due to reversal of the earth's magnetic field, and not some other property of the rock itself. The consistency soon became evident. No matter where the rocks came from, most had a remnant magnetization reversed from what the earth's is at the present time--that is, the magnetic pole when the rock was formed was at the South Pole, rather than near the North Pole as it is today. Incredibly, the logical explanation was that the earth's magnetic field itself was reversed when the rocks were formed.

Precise dating of the rocks was crucial in order to gain further understanding and insight related to patterns of magnetic reversals. In order to calculate the age of a rock, scientists must measure the proportion of specific elements contained in them and use a mass spectrometer to determine the exact amount of argon present. Extracting argon gas from rock samples required elaborate lab equipment and great skill in operating it. Brent Dalrymple was one of the first students at the University of California, Berkeley trained in the use of the Reynolds mass spectrometer. He became acquainted with Doell and Cox while conducting field research in the Sierra Nevada mountain range and, through lengthy campfire discussions, became keenly interested in the dating project. Upon graduation, Dalrymple was asked to join the project to perform argon extraction work and immediately set about the task of developing a radiometric rock dating lab at the USGS. Dalrymple worked day and night with two talented and devoted technicians, Major Lillard and Nat Sherrill, to construct the fragile mass spectrometer equipment, much of it fashioned and built from scratch by hand, and using a glass flight tube from UC Berkeley. Just six months after they began, the USGS team had its own rock dating lab up and running, allowing an accelerated pace of work using the new mass spectrometer equipment. Through the efforts of Dalrymple and others it became quite clear, by early in 1964, that the earth's magnetic field had indeed reversed its polarity about 1 million to 2 1/2 million years ago. Continued field sampling and age-dating led to discoveries that snapped the picture of magnetic reversals for the past 4 million years into sharp focus for the scientific community.

An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis.

While laboratory scientists were chasing the mystery of magnetic reversals, marine geophysicists were pondering some unusual data collected from the seafloor. In 1961, English scientist Drummond Matthews made magnetic measurements over a small patch of seafloor in the Indian Ocean, which revealed a series of alternating magnetic bands of greater and lesser magnitude across the ocean bottom near a submarine ridge. This magnetic banding, similar in pattern to the stripes on a zebra, was completely unique to the seafloor and, in fact, had been detected earlier in other parts of the world's oceans. Various hypotheses had been proposed to account for these peculiar magnetic anomalies, but none had been conclusive or widely accepted throughout the scientific community. However, a doctoral student of Matthews', Fred Vine, put forth a novel explanation which was coincidentally also proposed by Lawrence Morley with the Canadian Geological Survey. It was based on the unproven concept of seafloor spreading proposed some years earlier by Harry Hess of Princeton University. Hess' idea envisioned the seafloor spreading outward on either side of mid-ocean ridges as new seafloor material in the form of molten rock welled up from inside the earth along the ridgelines. Vine and Matthews, together with Morley, deduced that the stripes were magnetically normal and reversed blocks of the seafloor.

A missing link in all of this widespread scientific discovery was provided in late 1965 when Dalrymple presented the findings of the Rock Magnetics Lab at a meeting of the Geological Society of America. Vine was present at that meeting and when Dalrymple unveiled his data, he recognized an exact proportional match between terrestrial paleomagnetic measurements and the mysterious seafloor bands. It was indeed a revelation ... and the start of a revolution in earth science! These unfolding discoveries and revelations generated a burst of new studies, ideas, and interpretations. Together, they created the underpinnings for an entirely new model of how the earth works -- what we now call plate tectonics. The slow, inexorable, and sometimes violent movement of these plates creates earthquakes, volcanoes, and many of the surface features of our planet.

At the time of their investigations, Richard Doell, Allan Cox, and Brent Dalrymple couldn't have anticipated the impact their work would have. To them, polarity reversals of the earth's magnetic field was an overlooked but promising area of scientific inquiry. Who would have guessed that their timescale, developed through age-dating with ingeniously fashioned handmade laboratory equipment, would be the Rosetta Stone that unlocked some of the earth's deepest secrets? ... that it would trigger a revolution in our perceptions of the Earth. But then, revolutions have to begin somewhere. Why not a tarpaper shack?

For more information about plate tectonics, see This Dynamic Earth: The Story of Plate Tectonics, online at http://pubs.usgs.gov/publications/text/dynamic.html

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