Moving story of the sea floor

Nick Petford is swept away by a discovery that revolutionised geology.

What was the greatest observational discovery of 20th-century science? Anyone pondering such a question is spoilt for choice: the Hubble expansion, the atomic structure of DNA, the cosmic microwave background radiation? But how about magnetic anomalies on the sea floor? Not at the top of everyone's list, I suspect, but a discovery nevertheless potent enough to revolutionise an entire branch of the natural sciences. I am, of course, talking about the theory of plate tectonics.

The idea that the outermost part of the earth is not one fixed shell but made up of about 12 mostly rigid blocks in relative motion (and that continental drift is a geological reality) is the most widely unifying concept in geology since Lyell's principle of uniformitarianism. What were once regarded as disparate solid-earth phenomena - earthquakes, volcanoes, and sea-floor spreading - are united in a single theory. Plate tectonics has helped to elevate geology from a primarily observational science to a predictive one. If Alfred Nobel had included geology in his list of six prize categories, then several laureates would be among the contributors to this book. A bold claim perhaps, but in bringing together the key individuals responsible for the development of plate tectonics, from the first tantalising geomagnetic observations to fruition as a mature theory, Naomi Oreskes reminds us how important and wide ranging the discovery was.

Many of the contributors have kept company previously, with their work published as papers on plate tectonics in earlier thematic sets. What makes this book unique is that for the first time we are offered an insight, sometimes in a highly personalised and anecdotal form, into the thought processes and research methods that led to the final confirmation of the theory.

It is easy to forget that this is possible mainly because plate tectonics is just 30 years old. Many of the authors were graduate students or just starting their scientific careers in the years up to 1968 when the theory finally came together, and are still alive today. Imagine being able to ring up and talk to the principal scientists involved in, say, the 19th-century controversy between the Neptunists and Plutonists and ask them to write down their recollections as they unravelled one of the greatest geological puzzles of their time?

This also shows us that although past scientific revolutions are named mostly after prominent personalities, they may be the work of many individuals. Whether, in due time, plate tectonics becomes known as the Hessian/Wilsonian/McKenzian revolution (like the Newtonian and Darwinian revolutions) is doubtful. Whatever the outcome, this book will provide an important record of events for future historians of science.

This is Oreskes's second book about plate tectonics, following her scholarly account ( The Rejection of Continental Drift: Theory and Method in American Earth Science ) of the history of the debate that raged in the first part of the 20th century between American and European geologists over the scientific status of continental drift.

The idea that continents were not static on the surface of the earth but have moved over it throughout geological time was first proposed in 1912 by the German meteorologist Alfred Wegener. His theory can be traced back through a series of earlier ideas of the contracting earth models of James Dana and Eduard Seuss and even to Francis Bacon.

These ideas are reviewed by Oreskes in the first part of the book that deals with the historical background. As many of the present contributors are themselves North Americans, this is especially relevant. The following chapters are grouped into sections on the early work on palaeomagnetism and sea-floor spreading, heat flow and seismology, the plate model itself and the contribution made by geologists working on the continents. An epilogue adds the extraterrestrial dimension.

The first part of the book on the early exploration of the sea floor using magnetic methods is most revealing. It conjures up a vision of a small group of hardy seafaring scientists, pitting their wits against nature as they surveyed the earth's magnetic field beneath the ocean. The chapter by Ron Mason captures this spirit, and exposes in part the debt that the theory of plate tectonics would owe to the second world war and events proceeding it.

A number of contributors served in the British and US navies, gaining valuable technical skills. Flux-gate magnetometers, designed to be carried by aircraft for the detection of enemy submarines, were modified so they could be towed in the water behind scientific ships. Postwar funding by the US Navy enabled the survey work that led to the first discovery on the sea floor of magnetic reversals - the fact that the earth's magnetic field had reversed direction many times in its long history. Advances in seismology, aimed primarily at detecting nuclear weapons tests, also led to more accurate determination of slip directions on geological faults, showing that the plates were indeed moving as the theory predicted.

Serendipity abounds. For example, Mason recounts that an early ship-borne survey was delayed because the US Hydrographic Office feared that towing the magnetometer would slow down the vessel. Had the survey been conducted on a north-south track (parallel to many oceanic ridges) as opposed to an east-west one (at right-angles to the ridges), which maximised the chances of crossing the anomalies, their discovery might have been delayed considerably. Of course, no one knew this at the time.

The 1961 map of sea-floor magnetic anomalies produced by Art Raff and Mason, which summarises the marine surveys around the Mendocino Fault off California, is reproduced several times by other authors. What is still so striking about this and subsequent images is the high degree of symmetry in the magnetic anomalies, or stripes, which led some to suggest that the data were too "perfect" to be explained by geological processes. Ground-breaking papers, especially those by Harry Hess and Tuzo Wilson on transform faults, and the 1963 Nature paper by Fred Vine and Drummond Matthews, "Magnetic anomalies over oceanic ridges", pointed the way. If the ocean basins were spreading apart by the addition of new molten material at the mid-ocean ridges, the sea floor must eventually sink back into the mantle. Plotting patterns of seismicity, Jack Oliver, Brian Isacks, William Dickinson and others discovered these sites (known as subduction zones) and showed that the focal-mechanism solutions for mid ocean-ridge earthquakes accorded with the transform-fault hypothesis of Wilson.

The fact that the observed magnetic anomalies on the sea floor pointed to processes taking place in the core - still the most enigmatic region of the earth's interior - was problematic. But Keith Runcorn and co-workers at Cambridge were finding evidence from palaeomagnetic studies of continental rocks in support of continental drift. Still the requirement that the earth must periodically reverse its magnetic field was simply too outlandish for most geologists, who ironically used their scepticism to challenge the validity of the palaeomagnetic data. The issue was compounded by poor geochronology (errors in dating methods were often greater than individual reversal events). Dismissed as the lunatic fringe and referred to as "drifters" by most geologists, their work continued unperturbed but was largely ignored.

It was not until 1967 that Walter Pitman and others in the Lamont group finally correlated the observed magnetic reversals in the ocean basins independently with terrestrial lavas and marine sediments. Within 12 months, plate tectonics was transformed from myth into reality. As Dan McKenzie points out in a later chapter, the failure of the early geomagnetists to convince others of the correctness and significance of their results remains something of a mystery.

The chapters on sea-floor spreading also raise a number of important points concerning the role of observation and induction in science. For example, sociological theories claim that hypothesis testing lies at the heart of the scientific method. However, as pointed out by Mason, and discussed later by John Sclater and McKenzie, the discovery of sea-floor magnetic stripes was not the result of any hypothesis-testing methodology - nor could it have been, since no one knew the stripes existed. Yet their discovery played a critical role in the subsequent development of plate tectonic theory. Heat-flow data from mid-ocean ridges tell a similar story of observation-led discovery.

Also important is that the book confirms the existence of simultaneous discovery in science. Lawrence Morley in Canada and Vine and Matthews in the UK independently proposed that magnetic anomalies could prove the existence of sea-floor spreading. Again, in 1967-68 three scientists, McKenzie and Robert Parker at Cambridge University and the Scripps Institution of Oceanography, and Jason Morgan at Princeton University, also working independently, synthesised the sea-floor data into a quantitative theory using Euler's theorem of rigid-body rotation over a sphere. Furthermore, it is noteworthy that although it is considered a geological theory, plate tectonics was devised mainly by scientists whose formal training was in experimental physics. It was not until the early 1970s that the first field geologists, led by John Dewey, and Peter Molnar with more quantitative modelling, began to use the theory to explain tectonic features such as mountain belts and more widely distributed forms of deformation found on the continents.

So does all this mean that plate tectonics offers a complete understanding of the structure of the earth? Well, not quite. Like any theory, it is still provisional and questions remain. Plate tectonics, as McKenzie points out, is a kinematic theory of geometry not a dynamic one of forces. How fluid dynamic processes in the mantle lead to the present-day plate configuration, or even the observed number of plates, is still unclear. Perhaps a greater understanding will come in future from studies of other planets.

Oreskes's book generates a sense of overwhelming intellectual activity at a time when no one could see that they were working collectively towards a monumental achievement. It is hard to conceive of a breakthrough of similar magnitude occurring in the solid earth sciences any time soon.

Nick Petford is reader in geology, Kingston University.