How do you know that it was 4.6 billion years ago that the Earth formed? That seems a very specific number to be confident about! (As compared with saying “estimated to be around 5 billion years.”)– David
It’s a question humans have grappled with for centuries – how old is the Earth?
For a long time in the Western world, the history of our planet and species was derived word-for-word from the Christian Bible.
Back in 1642, John Lightfoot, an English clergyman, calculated what he thought to be the exact date of the creation of the universe: September 17, 3928 BC. He arrived at this conclusion by counting backwards through the epic genealogies recorded in the Bible.
According to science, however, the Earth is a much older rock. It’s been labouring around the sun for 4.54 billion years, in fact, which we often round down to 4.5 billion.
Now, it may seem a very specific number, but that age actually has an error margin of around 50 million years each way – no small sum of years, but a pretty tight margin in the grand scheme of things.
So how can scientists peer back into the misty past to actually know how old our planet is? And how have they narrowed the number down so much?
We can credit our increasingly sophisticated dating techniques to the development of radiometric dating. Radiometric dating establishes the age of a material based on the presence of a radioactive isotope within it. Radioactive isotopes are isotopes that decay over time (radioactivity), and radioactive decay is the process by which an unstable atomic nucleus slowly leeches energy over that time period.
The basic logic behind it is that if you compare the presence of a radioactive isotope in an item or material with its known abundance, or with the presence of the stable isotope it’s known to decay into, you can figure out how long that material has been around. This works for isotopes of potassium, uranium, carbon and a few other elements, though carbon-dating might be the one you’ve heard most about.
Carbon dating, however, can only date a material that once belonged to a living creature, such as an animal or a plant, which takes in carbon as part of its life-cycle. In geology, scientists use radioactive isotopes with a much longer half-life than carbon (a half-life being the length of time it takes for half the isotopes in a sample to have decayed). These include potassium-40 (with a half-life of 1.248 billion years), uranium-238 (with a half-life of 4.468 billion years), and rubidium-87 (with a half-life of 47 billion years).
The oldest rocks on our planet and in our cosmic neighbourhood solidified from red-hot metals. These rocks have small amounts of uranium-238. We know that when a rock solidifies, the isotopes within it become locked-in and start to decay. By knowing the half-life of uranium-238, and measuring its abundance in relation to lead, the stable isotope it decays into, we can estimate how long it is since that rock was formed. For example, if the half-life of U-238 is 4.47 billion years, and a rock sample has a 50:50 ratio of U-238 to lead inside it, we could say that rock is 4.47 billion years old.
Armed with this toolkit, scientists have spent decades scouring the planet for its oldest rocks. Some of these are in our own backyard – there are zircon crystals in the Jack Hills of Western Australia that formed 4.4 billion years ago, not long after the formation of Earth itself, and remain the oldest known materials on Earth.
But there’s a problem: we know that rocks are continuously recycling, being formed, reformed and destroyed on an epic geological timescale, thanks to patterns of volcanism and erosion on our dynamic, atmosphere-shrouded planet. So how can we be sure that our oldest rocks really are our oldest rocks? How do we know that an even older rock hasn’t simply ground down to dust somewhere, or been subducted beneath a neighbouring continent?
Well, to get around this problem, scientists have also dated lumps of space rock – specifically meteorites and lunar rocks – believed to have formed from the same disc of material that birthed our own planet, at the same time. The results are strikingly aligned; our neighbourly space debris can be dated to the same 4.5-billion-year-old window.
So, we can date Earth with such a degree of precision because we have multiple lines of evidence pointing to the same window of time.
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There’s no such thing as a stupid science question, but sometimes the answers can be tricky to find.
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