The last piece of the continental drift puzzle

by Paul Curzon, Queen Mary University of London

Image by Gerd Altmann from Pixabay 

A computer helped provide the final piece in the puzzle of how the continents formed and moved around. It gave a convincing demonstration that the Americas, Europe and Africa had once been one giant continent, Pangea, the pieces of which had drifted apart.

Plate tectonics is the science behind how the different continents are both moving apart and crashing together in different parts of the world driven by the motion of molten rock below the Earths crust. It created the continents and mountain ranges, is causing oceans to expand and to sink, and leads to earthquakes in places like California. The earth’s hard outer shell is made up of a series of plates that sit above hotter molten rock and those plates slowly move around (up to 10cm a year) as, for example, rock pushes up between the gaps and solidifies. or pushes down and down under an adjacent plate. The continents as we see them are sitting on top of these plates.

The idea of continental drift had existed in different forms since the early 19th century. The idea was partly driven by an observation that on maps, South America and Africa seemed almost like two jigsaw pieces that fit together. On its own an observation like this isn’t enough as it could just be a coincidence, not least because the fit is not exact. Good science needs to combine theory with observation, predictions that prove correct with data that provides the evidence, but also clear mechanisms that explain what is going on. All of this came together to show that continental drift and ultimately plate tectonics describe what is really going on.

Very many people gathered the evidence, made the predictions and built the theories over many decades. For example, different people came up with a variety of models of what was happening but in the 19th and early 20th centuries there just wasn’t enough data available to test them. One theory was that the continents themselves were floating through the layer of rock below a bit like ice bergs floating in the ocean. Eventually evidence was gathered and this and other suggestions for how continents were moving did not stand up to the data collected. It wasn’t until the 1960s that the full story was tied down. The main reason that it took so long was that it needed new developments in both science and technology, most notably understanding of radioactivity, magnetism and not least ways to survey the ocean beds as developed during World War II to hunt for submarines. Science is a team game, always building on the advances of others, despite the way individuals are singled out.

By the early 1960s there was lots of strong evidence, but sometimes it is not just a mass of evidence that is needed to persuade scientists en-masse to agree a theory is correct, but compelling evidence that is hard to ignore. It turned out that was ultimately provided by a computer program.

Geophysicist, Edward Bullard, and his team in Cambridge were responsible for this last step. He had previously filled in early pieces of the puzzle working at the National Physical Laboratory on how the magnetism in the Earth’s core worked like a dynamo. He used their computer (one of the earliest) to do simulations to demonstrate this. This understanding led to studies of the magnetism in rock. This showed there were stripes where the magnetism in rock was in opposite directions. This was a result of rock solidifying either in different places or at different times and freezing the magnetic direction of the Earth at that time and place. Mapping of this “fossil” magnetism could be used to explore the ideas of continental drift. One such prediction suggested the patterns should be identical on either side of undersea ridges where new rock was being formed and pushing the plates apart. When checked they were exactly symmetrical as predicted.

	Jacques Kornprobst (redesigned after Bullard, E., Everett, J.E. and Smith, A.G., 1965. The fit of the continents around the Atlantic. Phil. Trans. Royal Soc., A 258, 1088, 41-51)

Image reconstruction of Bullard’s map by Jacques Kornprobst
from Wikipedia  CC BY-SA 4.0

In the 1960s, Bullard organised a meeting at the Royal Society to review all the evidence about continental drift. There was plenty of evidence to see that continental drift was fact. However, he unveiled a special map at the meeting showing how the continents on either side of the Atlantic really did fit together. It turned out to be the clincher.

The early suggestion that Africa and South America fit together has a flaw in that they are similar shapes, but do not fit exactly. With the advent of undersea mapping it was realised the coastline as shown on maps is not the right thing to be looking at. Those shapes depend on the current level of the sea which rises and falls. As it does so the apparent shape of the continents changes. In terms of geophysics, the real edge of the continents is much lower. That is where the continental shelf ends and the sea floor plummets. Bullard therefore based the shape of the continents on a line about a kilometre below sea level which was now known accurately because of that undersea mapping.

Maps like this had been created before but they hadn’t been quite as convincing. After all a human just drawing shapes as matching because they thought they did could introduce bias. More objective evidence was needed.

We see the Earth as flat on maps, but it is of course a sphere, and maps distort shapes to make things fit on the flat surface. What matters for continents is whether the shapes fit when placed and then moved around on the surface of a sphere, not on a flat piece of paper. This was done using some 18th century maths by Leonhard Euler. At school we learn Euclidean Geometry – the geometry of lines and shapes on a flat surface. The maths is different on a sphere though leading to what is called Spherical Geometry. For example, on a flat surface a straight line disappears in both directions to infinity. On a sphere a straight line disappearing in one direction can of course meet itself in the other. Similarly, we are taught that the angles of a triangle on a flat surface add up to 180 degrees, but the angles of a triangle drawn on a sphere add up to more than 180 degrees… Euler, usefully for Bullard’s team, had worked out theorems for how to move shapes around on a sphere.

This maths of spherical geometry and specifically Euler’s theorems form the basis of an algorithm that the team coded as a program. The program then created a plot following the maths. It showed the continents moved together in a picture (see above). As it was computer created, based on solid maths, it had a much greater claim to be objective, but on top of that it did also just look so convincing. The shapes of the continents based on that submerged continental line fit near perfectly all the way from the tip of South America to the northern-most point of North America. The plot became known as the ‘Bullard Fit’ and went down in history as the evidence that sealed the case.

The story of continental drift is an early example of how computers have helped change the way science is done. Computer models and simulations can provide more objective ways to test ideas, and computers can also visualise data in ways that help see patterns and stories emerge in ways that are both easy to understand and very convincing. Now computer modelling is a standard approach used to test theories. Back then the use of computers was much more novel, but science provided a key early use. Bullard and his team deserve credit not just for helping seal the idea of continental drift as fact, but also providing a new piece to the puzzle of how to use computers to do convincing science.

More on …

  • Read the book: Science: a history by John Gribbin for one of the best books on the full history of Science including plate techtonics.

Related Magazines …

EPSRC supports this blog through research grant EP/W033615/1. 

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