Just over 200 years ago, the prevailing belief popularized by religious mythology was that the planet earth had been around for just some 10,000 years. Thankfully scientists, like the French naturalist Comte de Buffon, saw the absurdity of the young Earth belief, and in the middle of the 18th century risked excommunication from the church to get his views across.
One of de Buffon’s earliest experiments had him burning his fingers on glowing white-hot spheres, and from that, estimating rates of heat dissipation that could then be used to create a theoretical model of a cooling Earth. Although somewhat primitive in its design, he pioneered a long line of heat dissipation studies that furthered the theory of an old earth, also known as the “deep time” theory. Lord Kelvin, after whom the Kelvin temperature scale is named, made his own heat dissipation calculations and settled on an age of the Earth between 20 and 40 million years, which, although on the right track, fell far short of the now widely accepted estimate of 4.54 billion years.
Missing from his calculations was knowledge of radioactivity, and the fact that certain types of matter decays and gives off energy, slowing the rate of overall cooling. It wasn’t until 1905 when Ernest Rutherford saw the potential of radioactive decay as a method of measuring geological time. It is with this idea that we will begin our look at how we date the age of the earth, geological features, fossils and other evidence that abounds the world over.
To refresh your memory, or in case you missed part one last week, the process of radiometric dating relies on an understanding of the decay of radioactive isotopes and the length of their half-lives. Radioactive isotopes are elements that are unstable and, during the process of decay, change chemically. The rate of decay is constant for specific isotopes, and so, by calculating the ratio of decayed to non-decayed atoms in a sample, an age estimate can be acquired for the material being measured.
The speed at which an isotope decays varies significantly depending on the radioactive isotopes being used. The half-life of fermium-244, for instance, is only 3.3 milliseconds, whereas rubidium-87 comes in with a half-life of a staggering 49 billion years. With all the different radioactive isotopes that exist — 158 among the elements that occur on Earth — it’s important to choose one that produces meaningful results relevant to the objects you want to date. If you want to date an object found near the bottom of the geological column, it wouldn’t do much good to use fermium-244 because it would have long since decayed to a point where no meaningful measurement can be attained. It would be like trying to measure the distance from Winnipeg to Toronto with a centimetre ruler; it’s just not going to work.
So with the understanding of the physical processes involved in radioactive decay firmly at our disposal, it was just a matter of time before reliable estimates for the age of the Earth were proposed. Surprisingly, the first accurate dating of the earth stems from dating objects with an extra-terrestrial origin. Meteorites — remnant pieces from the formation of planets in our solar system — wander the solar system until something gets in their way. In 1953 Clair Cameron Patterson applied radiometric dating to fragments of meteorites that crashed into the Earth and pinned the age of the solar system — and, by association, the earth — to be 4.55 billion years old. This estimate was corroborated with dating of lunar rocks brought back from the Apollo missions. One of the earliest studies conducted by Mitsunobu Tatsumoto and John Rosholt, published in the journal Science in 1970, used the popular methods of Lead-Lead, Uranium-Lead and Thorium-Lead radiometric dating to calculate the age of the moon to be 4.66 billion years. These two extra-terrestrial dates match closely with dates acquired from terrestrial objects.
An article appearing in the journal Nature in 2001 by Simon Wilde and colleagues documented the dating of the oldest terrestrial object — a zircon crystal found in Western Australia — and set the age of the Earth to at least 4.4 billion years. The previous record holder at the time was a Gneiss rock, a type of “metamorphic rock” (existant rocks, changed by the heat and pressure deep inside the earth) found in northwestern Canada. Since then, a more recent (and still contentious) discovery in 2008, in northern Quebec near Hudson Bay, set the record for the oldest known rocks at 4.28 billion years. Although geologists have had luck finding ancient terrestrial objects in Canada, this is not something exclusive to Canada, and all the other continents have found rocks exceeding 3.5 billion years old, using a variety of radiometric dating techniques.
With all of these different radiometric dating methods at our disposal, we are privileged with far more information than merely an understanding of the age of different geological features, or an approximate age of the Earth. We can use these dates in conjunction with fossil finds in parts of the earth which have been dated to different times, allowing us to to develop a timeline of life’s history on Earth. In many instances, fossils are found as a direct result of knowing the ages of different geological layers or strata. The discovery of Tiktaalik roseae, a transitional form between land and sea creatures, was made by Neil Shubin and his team because they sought out a geological area dated to a specific age. Knowing that land animals first evolved around 400 million years ago, they planned their dig around an area of the arctic with exposed rock that dated to 395 million years. With hard work and some luck, they found what they were looking for, and used both the predictive powers of the theory of evolution and the knowledge gained from radiometric dating to bring us closer to understanding life’s journey.
Radiometric dating has provided us with much of our understanding of the history of the Earth, but it’s not the only method at our disposal.
There is a method of dating that enables researchers to literally count years into the past, based on the extrapolation of seasonal and annual cycles determined by the type of sediment deposition in certain lake beds. Counting the layers, termed “varves” — from the Swedish varve, meaning layer — was first recognized as a method of dating by Gerard de Geer in 1884, who first attempted to use varve sequences to establish a timescale for deglaciation in the Stockholm area of Sweden.
With the emergence of technological advancements including digital imaging techniques, more sophisticated approaches to varve sequence tabulation can be employed. These sequences provide valuable palaeoenvironmental information in addition to providing current environmental information on the effects of “acid rain” and heavy metal pollution. As well, varves can be calibrated with a special type of radiometric dating known as radiocarbon dating — which allows for the dating of once-organic material through the decay of the Carbon14 isotope, which ceases being replenished after the organism dies — to produce confirmation of their accuracy.
More commonly known as tree-ring dating, dendrochronology is a method of tabulating the date by counting the sequential order of the tree rings. The diameter of the seasonal and annual rings is dependent on the environment, with optimal weather producing wide rings, and the opposite for droughts and catastrophic events such as volcanoes. By cross-dating distinctive rings, sequences spanning thousands of years back into the past can be achieved. Hypothetically, if we found a continuious sequence of tree rings in various trees, we could determine the exact year a specific tree in a petrified forest had died. But this is unrealistic, and currently dendrochronological sequences rarely date past 12,000 years. This is still very useful however, as it allows us to date major climate shifts, the emergence of civilizations and the date of volcanic eruptions, to name a few things. As with varves, one of the most important functions of dendrochronology is the calibration with radiocarbon timescales to provide a means of conciliating the accuracy of radiocarbon dates.
We have come a long way since Comte de Buffon first challenged the religious myths and superstition of his day. If we want to continue progressing towards a better understanding of ourselves and the universe we inhabit, we must continue that tradition, and challenge any beliefs and ideas brought not from observation and rigorous scientific study, but from dogmatic assertions and ignorance.