The igneous activity that produced such intrusions...

...calculation was based on the assumption that the substance of the Earth is inert and thus incapable of producing new heat.

Scientists estimate that the Earth is about 4.5 billion years old, based on radioisotope dating techniques. To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.

By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.

The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.

We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.

Often, any one atom has several different forms, called isotopes.

Atoms are made up of electrons, protons, and neutrons, and the number of electrons and protons determines the type of atom.

Other isotopes are unstable because the different number of neutrons interacts with the other atomic components in such a way that, over a period of time, the isotope changes into some other atom.

When these unstable isotopes change to a different atom, they emit radioactivity. An important property of radioactive isotopes is the half-life — the time it takes for half of the atoms to undergo the transition from one atom to the other.

Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.

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