So I'm here in front of the Rutherford Building on the campus of the University of Manchester. Why I'm here will become apparent soon. What I want to talk about in this lecture is the age of the Earth. How do we know that it's four and a half billion years old and over time how has that number changed? Our story starts in 1650 with James Usher who was an archbishop in Ireland who's also the vice chancellor of Cambridge University. He took the events in the Bible and added up the length of time during which each of these events occurred and reconstructed, in a scientific way, the age of the Earth. All the way back to when God created it. He determined that that age was October 23, 4004 BC. What we recognize about Usher is that he was working within the framework of scientific thought, that he was taking a document and then, using it as a historical document to reconstruct the age of the Earth from that. So perhaps, nowadays he's viewed in a little bit harsh light. And he wasn't the only person that tried this at the time. The next part of the story starts in 1862 with Lord Kelvin. He took a very different approach to calculating the age of the Earth, a very physical approach. He started with the assumption that the Earth must have been molten at its formation, and that it reached its present temperature now. Using these assumptions he could then calculate the age of the Earth. How long did it take to cool from a molten state to its current temperature. When he did this he ended up with an age of the Earth of somewhere between 20 and 40 million years old. We know that this is way too short. We know he missed a crucial element to the story. And that is radioactivity, an extra source of heat that would've prolonged the cooling of the Earth. And that story of radioactivity is one of the reasons why we're here at the Rutherford Building. The next part of the story starts in 1899 with John Joly, an Irish physicist. He took a different approach to calculating the age of the Earth. He looked at the present salinity of the ocean and said well, let's go back to the beginning when the oceans were formed and we can see the rate in which the salts are being added to the ocean and we can make these calculations to determine the age of the Earth. John Joly made three assumptions. First, that the Earth's oceans had no dissolved salts initially. That the addition of new salts was relatively constant over time, due to the rivers. And third, that the volume of the oceans has not changed with time. Using these three assumptions he could calculate then, the age of the Earth. Or at least, the age of the oceans at the current level of salt input to them. In doing this, he arrived at a figure between 90 and 100 million years old. We know this value is much too short. Again, here, he did not account for the fact that there are processes that take the salts out of the ocean, and we'll see some of that later in the course. But, the story with Joly doesn't end here. Radioactivity had just been recently discovered, and Ernest Rutherford had recently moved to the University of Manchester. Already a Nobel Prize winning physicist, he still had one of his greatest discoveries left to make which was that the nucleus was a relatively small part of the volume of the atom. In 1903, Joly first hypothesized that these radioactive elements could be used to date the Earth. And so, by 1913, working with Ernest Rutherford here in this building on campus. He made the first calculations of a rock layer based on its radioactive decay. In 1913 they used the Leinster Granite from Ireland, to calculate the base of the Devonian period, when that granite was formed. Their calculation of 400 million years is remarkably close to the modern date for that granite of 404 million years ago. First in 1913, and then later in 1927, Arthur Holmes, working at Durham University determined that the age of some very old Archean rocks was first 1.6 billion years. And then extended that age back to 3 billion years ago. In the 1940s, by looking at the abundances of uranium isotopes in very old rocks, he was able to determine the age of the Earth at 4.5 billion years ago.
