Each interval is different length of time. Principle of relative dating. A geologic feature that cuts across another geologic feature must be younger. Canyon cutting came after deposition. A. Basis for radiometric dating.
Many isotopes are unstable and they are said to be radioactive.
The rate of decay is very regular but different for every radioactive element. Determine the rate of decay for each element in the lab. We can analyze parent amount and compare daughter amount. Because we know the rate of decay of parent to daughter, we can then get at the time since the mineral crystallized. Half-life is expressed in units of time.