How do you age a rock?

Chris Smith put this question to Geologist, Owen Weller, who brought in a very old rock into the studio with him.

Owen - Not my favourite rock but it is the world’s oldest rock. This is dated at 4.03 billion years ago and, to put that age into context, as I said before, the Earth is 4.56 billion years ago, so it’s the oldest rock. To date this rock we need to do some form of radiometric dating. This uses the principle that some elements are unstable and they will decay to stable isotopes.

Chris - So radioactive decay?

Owen - Correct. With a known and constant decay rate. So if we can accurately measure the ratio of what they call parent and daughter isotopes in a given sample using a machine such as a mass spectrometer, then we can ascertain the age of the rock.

Chris - So just summarising then… you know that just by chance there are some radioactive atoms in that piece of rock, and we know that radioactive atoms like that break down at a certain rate, which is a constant rate, and they turn into new types of atom. If you count how many of each type are there you get some idea as to how old, or how much time has elapsed because we know how fast that process happens, on average. 

Owen - Yes, that’s correct. There’s actually many different radioactive decay schemes, so many different pairs of elements that you might measure. But the most common one that we use is the decay of radioactive uranium to stable lead, and we most typically measure this in a very special mineral, which is called zircon. Maybe people don’t realise it but there’s zircon present in many rocks, particularly granites, around the world. It has several extremely useful properties. First of all it’s extremely durable so it can last for several billion years. And second of all its structure really likes to take in uranium but doesn’t like lead, so when we find lead in its structure we know that that’s come from the decay of uranium in the structure from when that zircon formed or it was crystallised. So by measuring the ratio of uranium to lead in our zircon, we can say when the zircon, hence the rock crystallised and that’s exactly what was done with this piece of rock that I brought in today to say that it was 4 billion years old.

Chris - Lee?

Lee - I just had a question to follow on because I was thinking the oldest rock on Earth. Are there any fallen objects that are older than that that are on Earth, like meteorites or moon rocks or whatever that are actually older than that that are on planet Earth now? Owen - You’ve caught me. I should have said the oldest terrestrial rock. You’re quite correct. We do have meteorites that do date back to Sun or the Earth so to 4.5/ 6 billion years old.

Chris - Where do they come from - space obviously but were they just rained in from the debris that formed the solar system in the first place at some point in Earth’s history?

Owen - Yes, absolutely. There’s a whole variety of different meteorites that we get and the very oldest ones, they’re a particular kind called chondrite or chondritic meteorites. They’ve been really, really useful for telling us how the Earth formed because we get some slightly younger types which are then either formed of iron or they’re formed of silica minerals. This tells us very on in the Earth’s history we formed the Earth’s iron core and the silica exterior, so meteorites have been extremely useful probing these very, very early Earth planet formation