The oldest reliably dated rock formation is in Greenland, where several different isotopes were used to find an age of 3.6 billion years.To find older rocks that haven’t been eroded, we need to look beyond Earth.If you’ve ever seen a horizontal slice of a tree trunk, you’ve seen how a tree forms a new growth ring each year.In years of drought, the tree grows less quickly so the ring is narrower; in good growing seasons the ring is thicker.
Scientists have drilled ice cores deep into glaciers and found ice that is 123,000 years old in Greenland In your high school science classroom, you may have seen a large poster of the periodic table hanging on the wall.
Some isotopes are unstable, and over time these isotopes “decay” into isotopes of other elements.
For example, Potassium-40 is unstable and decays into Argon-40.
The expansion of the universe gives an age for the universe as a whole: 13.7 billion years old.
Astronomers and geologists have determined that the universe and Earth are billions of years old.