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Sometimes human observation can be maintained long enough to measure present rates of change, but it is not at all certain on a priori grounds whether such rates are representative of the past.
This is where radioactive methods frequently supply information that may serve to calibrate nonradioactive processes so that they become useful chronometers.
The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope.
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*.Based on a direct proportion between thickness and time, the three interglacial intervals were determined to be longer than postglacial time by factors of 3, 6, and 8.To convert these relative factors into absolute ages required an estimate in years of the length of postglacial time.Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.
Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.