How The Discovery Of Geologic Time Changed Our View Of The World

Imagine trying to understand history without any dates. You know, for example, that the First World War came before the Second World War, but how long before? Was it tens, hundreds or even thousands of years before? In certain situations, before radiometric dating, there was no way of knowing.

By the end of the 19th century, many geologists still believed the age of the Earth to be a few thousand years old, as indicated by the Bible, while others considered it to be around 100 million years old, in line with calculations made by Lord Kelvin, the most prestigious physicist of his day.

Dr Cherry Lewis, University of Bristol, UK, said: "The age of the Earth was hugely important for people like Darwin who needed enormous amounts of time in which evolution could occur. As Thomas Huxley, Darwin's chief advocate said: 'Biology takes its time from Geology'."

In 1898 Marie Curie discovered the phenomenon of radioactivity and by 1904 Ernest Rutherford, a physicist working in Britain, realised that the process of radioactive decay could be harnessed to date rocks.

It was against this background of dramatic and exciting scientific discoveries that a young Arthur Holmes (1890-1964) completed his schooling and won a scholarship to study physics at the Royal College of Science in London. There he developed the technique of dating rocks using the uranium-lead method and from the age of his oldest rock discovered that the Earth was at least 1.6 billion years old (1,600 million).

But geologists were not as happy with the new results as, perhaps, they should have been. As Holmes, writing in Nature in 1913, put it: "the geologist who ten years ago was embarrassed by the shortness of time allowed to him for the evolution of the Earth's crust, is still more embarrassed with the superabundance with which he is now confronted." It continued to be hotly debated for decades.

Cherry Lewis commented, "In the 1920s, as the age of the Earth crept up towards 3 billion years, this took it beyond the age of the Universe, then calculated to be only 1.8 billion years old. It was not until the 1950s that the age of the Universe was finally revised and put safely beyond the age of the Earth, which had at last reached its true age of 4.56 billion years. Physicists suddenly gained a new respect for geologists!"

In the 1920s the new theory of continental drift became the great scientific conundrum, and most geologists were unable to accept the concept due to the lack of a mechanism for driving the continents around the globe.

In 1928 Arthur Holmes showed how convection currents in the substratum (now called the mantle) underlying the continents could be this mechanism. This proved to be correct but it was another 40 years before his theories were accepted and the theory of plate tectonics became a reality.

The theory of plate tectonics has proved to be as important as the theory of evolution and the discovery of the structure of the atom, but without the discovery of how to quantify geologic time, confirmation of plate tectonics would not have been possible.

Today, few discussions in geology can occur without reference to geologic time and plate tectonics. They are both integral to our way of thinking about the world. Holmes died in 1964 having lived just long enough to see sea floor spreading confirm his ideas of continental drift.