Compare radiometric dating relative dating
Relative dating helps determine what came first and what followed, but doesn't help determine actual age.Radiometric dating, or numeric dating, determines an actual or approximate age of an object by studying the rate of decay of radioactive isotopes, such as uranium, potassium, rubidium and carbon-14 within that object. This rate provides scientists with an accurate measurement system to determine age.
With rubidium-strontium dating, we see that rubidium-87 decays into strontium-87 with a half-life of 50 billion years.
So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive.
When the isotope is halfway to that point, it has reached its half-life.
This provides a built-in cross-check to more accurately determine the age of the sample.
Uranium is not the only isotope that can be used to date rocks; we do see additional methods of radiometric dating based on the decay of different isotopes.
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By anyone's standards, 50 billion years is a long time.