The problem with mnemonics is that they can be difficult to remember. But there is one that has always stuck in my mind.
Pregnant Camels Often Sit Down Carefully Perhaps Their Joints Creak.
It is a reminder of the sequence of geological periods:
Pre-Cambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous.
I particularly like the way it distinguishes the three periods beginning with ‘C’ by giving their first three letters. (In North America the Carboniferous is divided into the Mississippian and the Pennsylvanian periods.)
The names for these periods were established during the Victorian era, and their chronological sequence was worked out from the geological strata by assuming that older rocks lie beneath the more recent. Rocks of each period had their own distinctive sets of fossils and these could be matched between rock formations in different locations to produce a world-wide geological ordering. But there was no way to determine absolute ages for the rocks belonging to these periods.
Uranium to Lead
Ernest Rutherford took the first step towards resolving this issue in 1904 when he estimated the age of a rock by measuring the ratio of uranium to helium that it contained, as described in a previous article. The helium atoms in the rock were derived from the alpha particles released in radioactive decay.
During the following academic year (1904-05), Rutherford gave the Silliman Memorial Lectures at Yale University. They were attended by a rather alliteratively named Yale radiochemist Bertram Borden Boltwood (1870-1927) who took the opportunity to discuss his research with Rutherford. Boltwood had been studying uranium-bearing rocks and discovered that every such rock also contained lead. He suggested that lead might be the final stable decay product in the radioactive decay series that starts with uranium. Rutherford saw this as an opportunity for a new method to date rocks. The older the rocks the longer the uranium would have been decaying and so the more lead they should contain. He encouraged Boltwood to investigate whether this observation could be developed into a practical dating technique.
Boltwood found that the lead to uranium ratio did indeed increase with age, as judged by the geological periods of his samples, and in 1907 he published what he considered to be rough provisional data for rocks whose ages ranged from 410 million to 2.2 billion years old. Boltwood’s age estimates are typically about 20% greater than later more accurate dates.
An Undergraduate Project
Arthur Holmes (1890-1965) grew up in Gateshead near Newcastle in the North East of England. At the age of 17 he left for London to study physics at the Royal College of Science, now known as Imperial College. He would switch to geology in his second year. Just as Holmes started his course Robert Strutt, son of the illustrious Lord Rayleigh, arrived from the Cavendish Laboratories in Cambridge to take up the position of professor of physics. Strutt had taken a keen interest in radioactive dating, and had applied Rutherford’s original helium method to date a number of rock samples.
Arthur Holmes (1912).
With Strutt’s encouragement, as a fourth year undergraduate in 1910, Holmes began a project to determine the age of a sample of Devonian rock from Norway. Rather than using the helium method which was known to be problematic, Holmes would follow Boltwood and measure the lead to uranium ratio of the rock.
Holmes gauged the rock’s uranium content from the amount of radon that it contained. He released the radon by pulverizing the rock and grinding it into a fine powder, before chemically processing it with hydrochloric acid. The radon was collected in a flask and he deduced the quantity thus obtained by measuring its radioactivity with an electrometer. A further sequence of laborious chemical processes then enabled him to extract the lead as a white powder.
After several months of arduous work Holmes could assign an age to the rock—370 million years. This is considered to be the first accurate radiometric dating. (It agrees with today’s dates for the Devonian period, which lasted from 419-359 million years ago according to modern analyses.) Not bad for an undergraduate project! Holmes was just twenty years old and would graduate later that year. His project must rank as one of the most remarkable ever completed by an undergraduate.
Holmes then re-analysed Boltwood’s data, improved its accuracy and used the refined dates to assign chronological ages to the geological periods of the rocks. (This is something that Boltwood had not attempted to do.)
The Age of the Earth
Revealing the ancient history of our planet is important work and Holmes was keen to publicize the new radiometric methods for dating rocks as widely as possible. His account of how geologists could now address the most fundamental problem of their science was published in 1913 as The Age of the Earth.
Holmes included the table below summarizing the latest rock-dating results and how they relate to the geological periods. The dates in the column labelled HELIUM RATIO were obtained using Rutherford’s original uranium-helium method, which underestimates the true age of a rock due to the gradual escape of helium over long periods of time.
The column labelled LEAD RATIO gives dates based on the far superior uranium-lead method and includes Holmes’s figure of 370 million years for the Devonian rock analysed in his undergraduate project. These figures are a good match to modern ages for the Carboniferous, Devonian and Silurian/Ordivician periods.
Table matching rock-dating results to their geological periods from the first edition of The Age of the Earth by Arthur Holmes.
It was early days for geochronology and there was still a lot of work to be done, but Holmes had begun the long process of transforming geology into a historical science. Unfortunately, not everyone was convinced. Few geologists had the background in physics and chemistry to fully appreciate Holmes’s results or the long-term potential of radiometric dating. Most still preferred the more familiar, but far less accurate methods based on geological sedimentation.
For many years, Holmes was a lone voice championing the use of radioactive dating. Its broad acceptance by the geological community advanced at an almost glacial pace. But as the decades passed the reliability and accuracy of dating methods gradually improved and geology was slowly transformed. Measuring the uranium and lead concentrations of rocks is laborious work and refining the techniques to make this a precise science took almost half a century, spanning most of Holmes’s career. But it was worth it.
What We Have Learned
Uranium-lead dating remains at the heart of geochronology. It has now been supplemented by techniques based on other radioactive elements such as thorium, rubidium and potassium. The ability to cross-reference ages obtained from a number of radioactive isotopes makes today’s geochronology a reliable and precise science.
It is only because of this work that the evolution of life can be dated so well, as summarized schematically in the chart below. The oldest traces of life were found in a lump of sandstone from Western Australia that formed 3.48 billion years ago. For vast ages the highest form of life was little more than a bacterial blob of slime. The first multi-cellular organisms date back around 900 million years, and complex lifeforms proliferated in the Cambrian explosion, 540 million years ago, when the first easily recognisable fossils such as trilobites emerged.
We know through modern radiometric dating that the first tetrapods, such as ichtyostega, waddled around on land 370 million years ago in the Devonian period, and the first mammals appear in rocks dating to about 180 million years ago in the early Jurassic. Geochronology also tells us that non-avian dinosaurs vanish from the fossil record sixty-six million years ago at the end of the Cretaceous. So we know for sure that Fred Flintstone and his friends definitely didn’t keep dinosaur pets—the last dinosaurs, other than birds, disappeared well over sixty million years before the first humans evolved.
But what did the world of the dinosaurs look like? What was its geography? Arthur Holmes would make a major contribution to explaining the forces that shape the Earth and in the process help to answer these questions.
Further Information
There is more about the end of the dinosaurs here: Hell Creek Apocalypse.
