Rob Butler, Professor of Tectonics at the University of Aberdeen, reflects on 50 years of plate tectonic theory, and announces our new online project, Plate Tectonic Stories- Plate Tectonics: The Dan McKenzie archive
- BBC: Plate Tectonics – When we discovered how the Earth really works
- BBC World Service ‘Science in Action’ 50 years of plate tectonics
- BBC Radio 4 ‘Inside Science’ on 50 years of plate tectonics
“Over a few years in the mid-1960s the world changed. Plate tectonics was born. Up until then, studying geology was essentially an exercise in collecting (apparently disparate) facts and memorizing information.
Geological textbooks reflected this – with an emphasis on stratigraphy, geological time and how gradual changes in environment had influenced the deposition of sedimentary rocks and the records of life they contained. The sites where these rocks accumulated were thought of as sags – “geosynclines” – formed by mysterious processes. Structural geology was concerned with descriptions and understanding deformation for its own sake. Geochemistry was simply what you did to add information to descriptions of a myriad of different rock types, chiefly igneous. Universities had distinct departments of mineralogy, geophysics and geology. Nothing was joined up.
Original figure from McKenzie, D & R L Parker, “The North Pacific: an example of tectonics on a sphere”, ‘Nature’ vol 216 (1967), pp1276-1280.
c. The Geological Society, McKenzie archive
Plate tectonics is about the skin of the Earth being in a relentless state of constant, albeit slow, relative motion. The old world view had de-emphasized or ignored horizontal motions. Of course there were exceptions – such as W.Q. Kennedy’s recognition of lateral offset, of more than 100km, along the Great Glen Fault. And there were estimates of displacements on thrust faults, not only in NW Scotland but also in Norway, the Swiss Alps and the Appalachians. The idea that continents might have moved was hardly accepted. But all this changed once sea-floor spreading and the destruction of ocean floor by subduction were demonstrated. And it wasn’t haphazard but geometrically organized. Moving plates allowed new reconstructions of ancient fits between the continents that were encased within them.
Podcast: Listen to Dan McKenzie, co-author of the 1967 paper ‘The North Pacific: An Example of Tectonics on a Sphere’, on how the plate tectonics revolution came about
Disconnected geology joined up. The distribution of species, the variations in fauna and flora between continents, found explanations. No more did mythical, transient “land bridges” need to be invoked to explain how land animals appeared able to jump oceans. Life was a passenger on moving plates. The cycling of material between the Earth’s surface and the mantle explained volcanic processes and the composition of their rock products. These in turn could explain the development of ore-forming fluid circulation. Long-term global climate could be related to models of oceanic circulation that changed as the shape and connections between oceans changed. The same concepts meant that periods of stratification of seawater columns could be correlated – and used to predict the timing and distribution of hydrocarbon source rocks. Those thrusts long-recognised in mountain belts were shown not simply to reflect the collapse, like giant land-slips, of uplifting blocks of crust. Rather they reflected the substantial horizontal motions associated with the formation of mountain ranges as continents collided and drove into one another.
Podcast: Listen to Jean Hewitt, who used computing to create the mantle convection model at Cambridge – one of the first such uses of computing resources
Podcast: Listen to Xavier Le Pichon, one of the pioneers of plate tectonic theory, explaining what it was like to be at the centre of a new science
Overnight, geology ceased to be about the rocks themselves and became more about the Earth processes they record. In tectonics, everything becomes valuable as possible evidence to challenge hypotheses. True understanding depends on all aspects of earth science. Textbooks on stratigraphy embraced plate tectonics, drawing comparisons between reconstructions of parts of Britain and Ireland back in geological time with other parts of the world in the present day.
Podcast: Listen to Cynthia Ebinger, Chair in Geology at Tulane University, on the significance of plate tectonic theory to geologists now
Podcast: Listen to James Jackson of the University of Cambridge on what it was like to study geology in the wake of plate tectonic theory
The Geological Society is marking 50 years of plate tectonics by telling a range of Plate Tectonic Stories through twenty sites from around Britain and Ireland. The oldest rocks, in North West Scotland, tell of the formation of continental crust, and pose a fundamental question – were the processes operating three billion years ago the same as those of today? The youngest features are raised beaches from just 15 thousand years ago (the blink of an eye in geological time) which chart the changes in sea level brought about by variations in the volume of ice stored, principally, on Antarctica. Antarctica is especially cold because plate tectonics has broken the link between South America and the Antarctica peninsula. This allows unimpeded circulation around the Southern Ocean – isolating Antarctica from the warming currents that might otherwise reach it from equatorial regions.
Discover more Plate Tectonic Stories by exploring the online resources the Geological Society has developed to celebrate 50 years of plate tectonics – or even by visiting the sites themselves!”
The Geological Society is holding an exhibition of materials from the Dan McKenzie archive, ’50 Years of Plate Tectonics’, until the end of December. If you have a smartphone you can also listen to excerpts from interviews with McKenzie about various aspects of his life and career.