Radiometric dating instruments
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This conclusion is not based on just one measurement or one calculation, but on many types of evidence.Here we will describe just two types of evidence for an old Earth and two types of evidence for an old universe; more types can be found under further reading.
For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years.
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.
Meteorites are rocks from the solar system that have fallen to Earth recently and haven’t suffered much erosion.
Their pristine interiors give an age that dates back to their formation at the beginning of the solar system.
As time passes, a rock will have more and more Argon-40 and less and less Potassium-40.
Radiometric dating is possible because this decay occurs at a known rate, called the “half-life” of the radioactive element.
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.
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.
The annual ice layers in glaciers provide a similar method that goes back much further in history.