The maths beneath the mantle

Last edited: 29 May 2013

The phenomena of the spreading of the sea-floor, magnetic anomalies and the role of earthquakes were finally joined up in the 1960s by one of two men credited with the 'final' breakthrough on tectonic plate movement. BP Magazine meets one of them - Professor Dan McKenzie

A University of Cambridge PhD student at the time, McKenzie published a renowned paper in 1967, explaining the mathematical model behind convection in the Earth’s mantle and thereby the movement of its crust. Within the sphere of Earth science, his findings had an impact that was comparable to that of understanding the structure of DNA on biology, and changed our view of the Earth forever. 

Professor McKenzie has worked closely with BP in a consultative capacity for many years and, while he will continue to teach, retired from the Chair towards the end of 2012. Amidst much looking at maps and globes and some nifty paper-sliding techniques, BP Magazine interviewed him about the impact his discoveries had in the 1960s.

Is tectonic plate movement as complex as non-geologists fear?

Tectonics is the easiest part of geology to understand; it is all geometric, and the most astonishing thing about it is that it is taught in primary schools. It is an extremely simple notion, once people understand that this is about motion on a sphere, not on a plane. The Earth is spherical, so the plates move on the spherical earth. When we first started teaching it, we used National Geographic globes to show the undergraduates the rotation of the plates and get them to reconstruct it.

Your 1967 paper described this for the first time. What did people previously believe?

That’s the question that every undergraduate asks and it’s easy to answer – we had no understanding of what was going on whatsoever. It wasn’t that we didn’t know how the plates moved, because we didn’t even know there were plates. The idea that the South Atlantic was formed by separating Africa from South America goes back years, but the difficulty was that it was a rather woolly notion and no one could actually do anything with it. The early idea up to the 1960s was that the continents ploughed through the oceans like icebergs. Once we started looking at the oceans, we could see that was nonsense because the sediments coming up to the continents were absolutely flat and it made no sense. 

What was the initial reaction to your paper, of both academia and the oil industry?

I did this when I was 25, and I was quite clear that I was right and everybody else was wrong. I probably came across as extremely arrogant and made a few enemies. Particularly in the States, these ideas were really objected to, to the degree that if you believed in the mobility of the continents, you couldn’t actually get an academic job. There was just a very small number of outfits, so this department and departments at Princeton and Toronto, where people did believe this. The oil industry, including BP and Shell, absolutely dismissed the whole show, saying; “No, this does not happen.” 

How did those reactions change?

The theory has not budged since 1967, but it did take probably another 10 years for this to become a part of every undergraduate course around the world. Even here at Cambridge, I spent a lot of committee meetings in the 1970s struggling to get plate tectonics talked about to the undergraduates. I finally got two lectures a week in 1977, but when I went on sabbatical, they removed them! It happened very, very suddenly when the next generation of graduates went through and joined oil companies and educated their bosses that this was absolutely central to geology. 
"The one thing you can absolutely guarantee about scientific research is that it will go off on its tangents, and you never know what those tangents are going to be."
Professor Dan McKenzie

Was it the last piece of the geological jigsaw, or a whole new piece of understanding?

Not really either, but it made the whole thing clean. By the 1960s, we understood earthquakes, we understood volcanism, we understood plates, and that magnetic anomalies had reconstructed the oceans. We still did not understand exactly what caused the plates to move, but there was a sudden, universal acceptance that they did move in the way I was describing, and that there were geological rules about their movement.

Did you still encounter outright scepticism?

As a scientist, once I know I’ve got something right, I lose interest! With plate tectonics, I was not in any way interested in trying to convince people that this was right, I just felt they should go away and read what had been written. Without it, I felt their whole understanding and ability to understand where to find oil was up the creek. In the late 1970s, I was based in the US, and the most influential geologist at Shell at the time asked me to come to Houston and deliver a talk on my ideas about sedimentary basins. “But,” he warned me, “nobody in your audience believes in plate tectonics, so be a little bit diplomatic.”

How has tectonic science affected the oil sector?

One of the really puzzling things about sedimentary basins is that the North Sea, for example, has about five kilometres (three miles) of sediment, and the oil is down at the bottom. All of this sediment was deposited in shallower water, so why has it gone on sinking? Working in Greece and Turkey, looking at the present-day motions that are causing earthquakes, it suddenly dawned on me that the whole of the Aegean was being stretched, and if you stretch a rigid plate, it gets thin. What then happens is that the hot mantle of the Earth comes up and fills the hole, then it cools and shrinks and that is how sedimentary basins are formed. This work told us two major things; how fast the basin goes down, and the temperature. Being able to understand how the source rock matures and produces oil was absolutely fundamental to the oil business, and it became the woodwork of the whole sector. I am absolutely delighted about the economic use that has been made of my ideas. The use that companies make of scientific ideas seems to me to be the reason we can all live in the comfort that we do and I am extremely pleased to have been a part of that.

There is still a lot we don’t understand about the Earth. If you could only fathom out one more puzzle, what would it be?

The one thing you can absolutely guarantee about scientific research is that it will go off on its tangents, and you never know what those tangents are going to be. There are things we can do now that we could not have conceived of. I would love to understand how the West Siberian basin was formed, and we don’t have the slightest idea what went on there. West Siberia is missing the geological faults that other basins have that show us where the break was, and that is the big conundrum. You can never say never with science, because you just do not know where things are going to go

Related content