Dating the earth
A tree’s age can be found by simply counting its rings.
By comparing the pattern of thick and thin rings to weather records, scientists can verify that the method is accurate.
The annual ice layers in glaciers provide a similar method that goes back much further in history.
Each year, snowfall varies throughout the seasons and an annual layer is formed.
Geologists have found annual layers in ice that are easily counted to multiple tens of thousands of years, and when combined with radio isotope dating, we find hundreds of thousands of years of ice layers.
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.
The half-life is the time that it takes for half the radioactive sample to change from one element into the other.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years.