Question

How long ago was Earth formed and how does anyone know that for sure?

Answer 1

At least `4.404` billion years, probably `4.54` billion years.

Explanation:

Date estimates are obtained by radiometrically dating very old rocks.

OK so far, but:

• How can you find a very old rock when rocks are typically recycled by tectonic processes?

• How do you know that a rock is actually part of the Earth and not part of a meteorite?

• What is this radiometric dating anyway and how do we know that it is accurate?

Firstly, though rocks are subsumed in the mantle and new rock formed from volcanic eruptions, there are some samples of very old zircon found in Austrailia that managed not to be recycled.

Meteorites have specific characteristics that allow us to differentiate them from other rocks. They may have fusion crust - though that does tend to break off after time. They may differ in density from terrestrial rocks, or exhibit particular kinds of fractures, etc.

Radiometric dating measures the relative proportion of different isotopes. For example, over time uranium turns into lead. So by measuring the relative proportions of lead and uranium in a sample you can date it. That's a very crude description of what is a precise and carefully executed technique.

By means of radiometric dating we have measured the age of the old zircon in Australia at `4.404` billion years and meteorite material at `4.54+-0.05` billion years.

So what does the age of meteorites tell us about the age of the Earth? According to our understanding, they are kind of left over material from the formation of the solar system. Unlike rocks on Earth they have not been subject to volcanic activity, but were formed at about the same time as the Earth. So it is generally reckoned that their age is the same.

How sure are we?

Radiometric dating techniques are well corroborated. Uranium based dating has the advantage of the ability to cross check between `""^235U -> ""^207Pb` and `""^238U -> ""^206Pb` decay processes for extra confidence.

So I would say we are pretty sure.

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Footnote

Beware of some internet sites which claim the radiometric dating is faulty. They often have unscientific agendas.