Age of the Earth in a Nutshell

Q: In geology class my professor told me that the earth's age is based of off meters how does this work

Thanks for your help

- Theo

A: 

The age of the Earth was initially estimated by scientists by mapping stacks of sedimentary rocks in the UK, then measuring sedimentation rates in similar environments (lakes, rivers, seashore, etc.). In the 19th Century this initially gave startling - even shocking at the time - estimates in the hundreds of millions of years range. In the early 20th Century radioisotopes became available, and these were used to extend the age of the Earth into the billions (billion = thousand million) of years age range*. This physically meant measuring back to the point in time when the mineral hosting the radioisotopes and their daughter-products was last melted. THEN it just became a game of searching all over the Earth for the oldest date-able minerals with uranium and lead in them (for example, a zircon crystal). The oldest rocks found so far are in Greenland and western Australia, and based on these the Earth's age is estimated to be at least 4.55 thousand million years old. This means it is at LEAST that old. 

* Note that in some countries like the US, the word "billion" means a thousand million, while in other countries (e.g., the UK), the word "billion" means a million million.