Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
Radiometric dating utilizes the decay rates of certain radioactive atoms to date rocks or artifacts.
Uniformitarian geologists consider this form of dating strong evidence that the Earth is billions of years old.
suggested that lead is one of the disintegration products of uranium, in which case the older a uranium-bearing mineral the greater should be its proportional part of lead.
Analyzing specimens whose relative geologic ages were known, Boltwood found that the ratio of lead to uranium did indeed increase with age.
For example, in the The decay constant has dimensions of reciprocal seconds.
In the special case in which parent and daughter atoms are present in equal quantities, the age of the specimen is the half-life of the parent isotope: The first assumption, that the amount of the daughter isotope in the original rock is known, is the weakest assumption.
This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.A commonly used radiometric dating technique relies on the breakdown of potassium (Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized.If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.It takes another 4,460,000,000 years for half of the remaining sample to decay into Pb206 and then another 4,460,000,000 years for half of what’s then left to decay, and so on.The time it takes for half of a sample to decay is called a “half-life.” By measuring radioactive half-lives, by measuring how much parent and daughter are present in any given specimen, and by making certain key assumptions, scientists believe they are able to accurately determine the age of a specimen. The question is what are the underlying key assumptions and how reliable are they?