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The Age of the Earth

Asteroid belt

Q: I was watching 'How the Earth Was Made' and they explained that  the age of the Earth was determined by finding the age of meteorites found at Meteor Crater in Arizona. How do geologists know that the earth and these meteorites are roughly the same age? Couldn't the meteorites be older than the Earth?

From Mona Chaiti (July 2011)

Reply by Dr Ted Nield

That is quite right – the baseline age for the planets is taken as 4567 million years, as this is the oldest radiometric age obtainable from any matter in the Solar System. The date is obtained from meteorites, which are, as you suggest, much older than any rock that you can now find on Earth. Most meteorites come from the Asteroid Belt and are the remnants of the Protoplanetary Nebula – the swirling mass of gas and dust that coalesced to form the Sun and the planets. Having failed to make it yet to any planet, they have been mostly unaffected by any kind of geological process and are therefore samples of “primitive” Solar System matter.

The Earth is a very active planet geologically, recycling the rocks on its surface continuously, so it is very rare to find rocks even 4000 million years old. Most of the Earth surface is much younger. No ocean floor, for example, is much older than about 200 million years. This recycling means that the chemistry of the surface of the Earth is also different from that of the original Nebula, undifferentiated meteorites, and the Sun (into which 99% of all matter in the Nebula eventually fell). This is because the Earth has differentiated into a metallic core made of nickel and iron, and a silicate shell consisting of crust and mantle. So those chemical elements that are more soluble in molten metal have mostly disappeared from the crust and now reside in the core.

This is how we were able to tell that there was a huge meteorite strike at the end of the Cretaceous period 65 million years ago,. The meteorite was still rich in the element iridium, which has become rare on the Earth’s surface because it is more soluble in metal phases and has fled to the core long ago. The arrival of a massive meteorite therefore created a layer anomalously rich in iridium that was detected in the late 1970s and published in 1980 by Alvarez, Alvarez, Asaro and Michel.