How do we know the age of the planet?

Chris Smith asked John Underhill about how we know the age of our little world...

John - The answer I gave before was very much rooted in rocks on Earth and where we find them and how we age date them. But of course, there are bits of debris that come in from space, and if we could date some of that debris we can get an age for other parts of the solar system and so that's been done as well. We're using the same techniques with the radioactive decay and that gives us older ages for the solar system of the order of 4.6/4.7 billion years, and even older than that.

Chris - What was here in the cosmic neighbourhood before the Earth formed?

John - Well mainly a lot of debris that was spinning around and starting to amalgamate,and the Earth didn't really get into the state that we know it to be until about 4.5 billion years ago.

Chris - It's amazing to think that actually you can put a date on when we formed in this patch of what was previously empty space with a bunch of rubbish floating around.

John - Absolutely. And if you go to those stable interior parts of the tectonic plates, you can get those age dates from Australia, from the Baltic Shield, from the Canadian Shield.

Chris - Thank you very much, John.