The isochron method can determine the age of any rock, but new rocks are formed all the time.

So to figure out the age of the Earth, we have to look somewhere else... Earth has a molten magma layer and plate tectonics, so the "closed system" requirement of these radiometric dating methods is sometimes difficult to satisfy for Earth itself.

You may have heard that the Earth is 4.54 billion years old. If the half-life of a material is 100 years and you have 1 kg of it, 100 years from now you will only have 0.5 kg of it.

This was calculated by taking precise measurements of things in the dirt and in meteorites and using the principles of radioactive decay to determine an age. The rest will have decayed into a different nuclide (called a nuclide).

This nuclide decays to Strontium-87 (Sr87) with a half-life of 48.8 billion years.

Imagine going way back in time and looking at some lava that is cooling to become a rock.

The end result is that the slope of the isochron with Pb207 concentrations graphed against Pb206 (both relative to non-radiogenic Pb204) is equal to: $$m = \frac \frac$$ Data from several meteorites and from a few terrestrial sources are shown in Figure 3.

The data have been replotted from the tables of [2].

Many other methods have been used to date the Earth, with many different sets of radioactive nuclides (and other methods).

They are all consistent with Patterson’s measurement. The mathematical details of the lead-lead isotopic clock are less straightforward than those of the Rb-Sr method.

Meteorites, on the other hand, have been floating around in space since the solar system was formed.

When they come crashing to Earth, analysis of their composition can be geologically analyzed.

Several radioactive nuclides exist in nature with half-lives long enough to be useful for geologic dating.

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