(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)
For previous posts in this series on the age of the Earth, see here.
The discovery of radioactivity and the associated concept of half-lives of elements opened up the possibility of determining the absolute age of rocks. This argument was developed by Bertram Boltwood in the very early 1900s and is the process now referred to as radiometry (Jackson, p. 237). The argument goes like this. Suppose a sample of rock is found to contain 100 grams of a parent radioactive material P and 300 grams of the stable final daughter element D in its radioactive series, and also suppose that the half-life of this decay has been measured to be 10 years. If we assume that the rock initially had only the parent element P and no D, then we can assume that rock sample initially had 400 grams of P, then after 10 years, it had 200 grams of P and 200 grams of D, then after another 10 years, it would have 100 grams of P and 300 grams of D, and so on. Hence the rock must have been formed 20 years ago.
The more general formula for calculating the age of a rock is age=(half-life)xlog2(1+Dnow/Pnow), where Pnow is the measured amount of the parent radioactive element in the sample at the current time and Dnow is the amount of the final stable daughter element.
Of course, nothing is that simple in real life. Apart from all the difficulties in finding and measuring the properties of rocks, an obvious complicating factor is that some of the parent and daughter elements may have escaped or entered the sample, which is a real possibility during the time that the rock was molten before it solidified. The effect of this can be corrected for by the method of isochron dating, which I will not get into here but the link gives a very clear discussion. In addition, if there are many different radioactive series with multiple elements and multiple rocks involved, and their results converge around a single age, that lends further confidence to the result.
Now that the absolute ages of rocks could be determined, that meant that scientists could determine the absolute ages of the various geological layers in the Earth’s crust (that were formed by sedimentation) in addition to their relative ages. They did this by measuring the ages of the igneous rocks that were formed by volcanic activity that created molten rocks that cooled and are now buried in those layers. This provides an additional measure of the ages of the fossils that are found in those layers.
Apart from all this valuable information about the absolute ages of geological layers and fossils that were now possible to determine, the race was also on to find the oldest rocks on the planet, because those would set a lower bound on the age of the Earth.
Lord Rayleigh in 1905 found rocks that were 141 million years old and shark’s teeth that were 77 million years old. In 1907, Boltwood published the ages of mineral specimens that ranged from 400 million to 2.2 billion years (Jackson, p. 237). Such results would have been unthinkable just ten years earlier. The fact that these figures were now not summarily rejected shows how rapidly views had changed.
These much larger times were not uncritically accepted, however. Oddly, some critics were from the field of geology itself. Those who had got used to developing their theories to accommodate a 100 million year old Earth and had resisted efforts to lower it to 20 million, were now disconcerted by the much larger times that were now being suggested. Critics charged that we could not know that the half-lives of radioactive elements had remained the same through all time. Maybe they used to decay faster in early days, misleading us as to their ages.
But as more research was done, these objections began to disappear, and soon other methods (such as the presence of what are known a pleochroic halos in some materials) led to estimates of ages of rocks that were also in the hundreds of millions of years.
Geologist Arthur Holmes (1890-1965) produced a time scale in 1911 based on the rocks found in various geological strata and produced dates for the Carboniferous (340 million years), Devonian (370 million years) and Silurian (430 million years) eras, values that hold up remarkably well when compared with current values of 360, 416, and 444 million years respectively (Jackson, p. 245). Interestingly, the record for the oldest rocks at that time produced by Holmes were from my country of origin Sri Lanka (then called Ceylon), which were 1.64 billion years old.
It is often pointed out, correctly, that scientists can dogmatically cling on to their theories even in the face of contrary evidence. As Max Planck said, “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it” (Burchfield, p. 165). The deep reluctance to accept the new paradigm may be restricted to only those scientists who, like Kelvin and the age of the Earth, have invested a lot of their own time and prestige in working with the old paradigm. It is often the case that the scientific community as a whole changes its views faster than individual scientists.
The age of the Earth is a case in point. A little over a decade after the discovery of radioactivity, and just four years after Kelvin died unconvinced that the Earth was more than tens of millions of years old, there was no doubt anymore that the Earth was really and truly old, in the billions of years. It was a momentous change in thinking occurring over a remarkably short time, showing how science goes where the evidence leads even if individual scientists resist the changes.
(Main sources for this series of posts are The Chronologers’ Quest: The Search for the Age of the Earth (2006) by Patrick Wyse Jackson and Lord Kelvin and the age of the Earth by Joe D. Burchfield (1975).)
POST SCRIPT: The grand old traditions
It’s time once again to declare war on Christmas.
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