How We Got Plate Tectonics

 

Naomi Oreskes, editor, Plate Tectonics: An Insider’s History of the Modern Theory of the Earth

 

2003, Westview Press, 452 pp.

This book, comprising an introduction by Oreskes and seventeen papers, provides multiple first-person accounts of the mid-20^th century revolution in earth science.  Together, they provide an overview of plate tectonics, an engaging view of the excitement of a quickly developing field, and some reflections on the nature of scientific discovery.

[Looking back over this, I see I got rather carried away while writing this up.  It was hard not to, because I liked the book so much.  If you don’t want the rather long description of the book’s contents, just skip down and read the last couple paragraphs.]

“[C]onsider the history of the concept of continental drift,” writes Jack Oliver in his contribution to this volume.  “It was first proposed in A.D. 1596 by Abraham Ortelius, the famous Flemish cartographer of German ancestry, probably because he was first, or among the first, to see the key observational data, namely reasonably accurate maps of the Atlantic coasts of Africa and South America.  That great idea was quickly forgotten or ignored in the absence of additional observational data to test it.  Others later proposed the same idea independently, and were also pretty much ignored, basically for the same reason.  It was not until the early 20th century, when Alfred Wegener, a German geophysicist / meteorologist, had the idea of drifting continents, backed it up with sound observation, and communicated it well, that it began to catch hold with some scientists.  By then, the geology of the land areas had become reasonably well-observed, and so there was considerable support from geology for Wegener’s model.  However, Wegener’s idea faltered and sputtered for a long while, many decades.”

Continental drift stalled because nobody could conceive of how continents could plow through the ocean floor.  And besides, what difference did it make if the continents had moved?  It didn’t seem to have any bearing on geology.

Credit: National Geographic Society.  https://images.app.goo.gl/z3TRYQBLurHrTXDU9

Had maps like this been available, scientists would quickly have tumbled to the idea that the continents were moving away from the mid-ocean ridges.  But the ocean floor was terra incognita, the zipper-like ridges completely unknown.

In the years following World War II, a new breed of scientists, mostly physicists, turned their attention to studying the earth.  These “geophysicists” applied new tools, sonar and sensitive magnetometers developed during the war, to study the earth’s oceans.  Cold war imperatives drove the US Navy to undertake an extensive bathymetry program (basically, developing a topographic map of the ocean floor).  The Limited Nuclear Test Ban treaty led to the development of the WWSSN (World Wide Standard Seismograph Network) to locate nuclear tests and verify compliance with the treaty.

All this led to a flood of data.

First up was the discovery of magnetic stripes off the west coast of North America.  Stripes aligned with earth’s magnetic field, a few tens of miles wide, alternated with stripes that were reversely magnetized.  The magnetic fields had been frozen into the rocks when they solidified.  Land geologists were just figuring out that earth’s magnetic field reverses roughly every million years or so.  But how could that cause a set of parallel stripes on the sea floor?

In 1965, Tuzo Wilson published a key paper positing “sea-floor spreading” to explain the ridges, magnetic stripes, and heat flow data, and linked them to continental drift. Rock was flowing up at the mid-ocean ridges, cooling, and moving away in both directions, carrying with it a signature of earth’s magnetic field at the time it solidified.  Every time the field reversed, a new magnetic stripe would begin to form.  The new seafloor widened the gap between the continents on either side.

“A major question was then obvious,” Jack Oliver writes. “If new sea floor was created at the ridges, then what?  Was Earth expanding to accommodate the new surface area?  Or was old surface area being lost as it descended somewhere else?  Some said the former, some said the latter, and if crust did descend there was disagreement about where….  At Lamont we had the data to provide the answer.”  It came from seismographic studies around Fiji and Tonga: the depth pattern of the earthquakes showed the seafloor sinking into the mantle at the deep Marianas trench. 

Earth, it seemed, was covered by thin, rigid plates.  The material in the plates originates at the mid-ocean ridges and moves across the plate until it collides with another plate, at which point one plate dives back into the mantle.  There it circulates in a giant, slow convection current, eventually reappearing at a ridge.

The continents, geophysicists gradually realized, weren’t plowing through the ocean floor.  They were just along for the ride, lightweight material too buoyant to sink.  Peter Molnar quotes an unknown Brit out of context, “The upper crust is just a bunch of crumbs sticking together.” 

Confirming evidence came in.  WWSSN data showed that earthquakes and volcanoes are mostly located on plate margins.  Sample cores from a 1968 cruise of the Glomar Challenger showed the age of the seafloor increased with distance from the mid-Atlantic ridge at a rate consistent with the magnetic stripe data.

Although plate tectonics was a great theory of the seafloor, what use was this geophysical data to land geologists? Tanya Atwater showed the way at a key 1969 conference.  “[W]e were all blown away by one of Bill Menard’s graduate students from Scripps, a modest young woman named Tanya Atwater,” writes John F Dewey.  “She showed the geological world, for the first time, how quantitative relative plate motion could be translated into geological predictions that could be tested against the known geology of western North America.  The results were spectacular.  Tanya showed quietly, with detail and precision, how the migration of two plate triple junctions and the growth of the San Andreas Fault explained the timing of continental margin volcanism in western North America.”

Geologists came to realize that a whole host of poorly understood processes could be explained by tectonics.  For example, geologists had long understood that the Appalachians had folded as they were somehow compressed, and the Basin and Range district of the American west somehow had been stretched out, but had no idea why.  Now there was a theory.

And here lay the true revolution.  “It seems to me that plate tectonics accelerated a transition in the earth sciences from a 19^th-century natural science that treated the history of the earth as an end in itself, to a 20^th-century physical science focusing on a quantitative understanding of the processes that have shaped the earth,” writes Peter Molnar.

As time went on, more evidence accumulated.  Radio astronomy and then GPS were able to actually measure the rate at which continents are moving relative to each other – a couple of inches a year.  Radar altimeter measurements from space of the sea surface show that it dips over the ocean trenches and rises over the mid-ocean ridges, exactly as plate tectonics would predict.

But by then the revolution was over.  “By 1969,” writes Dan McKenzie, “the present theory was essentially complete….  The development of the theory stopped very suddenly: in the 1960s continental drift became sea floor spreading, then plate tectonics, as the theory became more precise and as its scope increased.  Then, equally quickly, the changes stopped: the theory was complete and rapidly became accepted.”

It had been accepted so quickly, as John G. Sclater writes, “because one simple concept could explain so many different sets of observations, which previously had no theory to explain them.”

In addition to describing the development of plate tectonics, Oreskes and a couple of the contributors discuss the nature of scientific discovery.  Since the publication of The Structure of Scientific Revolutions in 1962, Thomas Kuhn’s view of scientific discovery has dominated.  Kuhn posits that science is theory-driven, and that belief in the theory causes scientists to disregard disconfirming evidence until it builds up to the point that it precipitates a crisis, followed by the development of a new theory.

The problem with applying this paradigm to the earth sciences revolution is that, as Oreskes puts it, geologists never realized there was a crisis!  Instead of being driven by theory, the development of plate tectonics was driven by data, huge masses of it, that demanded explanation.  Geophysicists weren’t held back by adherence to an outmoded theory because there really wasn’t one.

Multiple authors noted the importance of people of different backgrounds working together.  Geologists with their history of synthesizing multiple sources of data combined with physicists able to take difficult measurements and their history of forming quantitative theories and testing them.  It is probably not a coincidence that the theory developed at four institutions that had a strong history of interdisciplinary work.  Scientific openness was also key.  Oreskes writes, “A striking feature of the stories in this volume is how many of the players moved back and forth among Cambridge, Lamont, Princeton, and Scripps, and how data-sharing facilitated the rapid development of ideas, and idea-sharing facilitated the effective interpretation of data….  Research thrives where smart people can work together and share data and ideas.”

The accessibility of the papers in this volume varies.  Although all are in the form of professional memoir, some are mostly technical, to the point where I won’t pretend to have completely understood them.  Others are clear and easy to understand.  The authors were evidently told to include some personal details, as all do.  Some tell just a little, others tell engaging and often self-deprecating stories.  Some, particularly Atwater, communicate their love for their work.  (It’s worth borrowing the book from the library just to read Tanya Atwater’s paper.  If she’s half as effervescent in person as on the page, she would be a great dinner companion.)

The reader will have to put some work into this book, because it is not told as a coherent narrative by a single, all-knowing author.  Many events are told multiple times from different points of view.  But it’s worth it for the insight into the development of the theory, the process of doing science, and the excitement of discovery.  I recommend this work to anyone interested in the history of modern earth science.