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Age of rocks from radioactivity, Earth internal structure

A. Explain how the age of rocks can be determined by radioactivity. Give an example.
b. How can scientists determine the internal structure of the Earth, if the depth of the deepest well is only a tiny fraction of the earth's diameter?

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(a) You may like to see some texts on the internet about the age of rocks.
Here is a link I have found that may be convenient:

There is a table of radioactive isotopes useful for geological dating you can use for your examples.

I shall try to explain the principle in very few words in the hope to make it easier to understand.

You obviously know that there are unstable nuclei which from time to time explode and turn into one or more nuclei of another type.

The process of decay can be described by a "Decay Time" T or by a "Decay Constant" which is just 1/T; and there are other ways to describe the same.
The definition of the decay time is that if at time t = 0 ...