Many independent measurements have established that the Earth and the universe are billions of years old.
Using the known rate of change in radio-active elements (radiometric dating), some Earth rocks have been shown to be billions of years old, while the oldest solar system rocks are dated at 4.6 billion years.
Astronomers use the distance to galaxies and the speed of light to calculate that the light has been traveling for billions of years.
Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time.
So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.
In this way, multiple trees can be used to build a master chronology for a forested region.
European oak trees have been used to build a 12,000-year chronology.
Like the tree rings, this method can be verified by comparison to historical records for weather, as well as to records of volcanic eruptions around the globe that left thin dust layers on the glaciers.
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.
The oldest reliably dated rock formation is in Greenland, where several different isotopes were used to find an age of 3.6 billion years.