In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
This chain eventually ends with the formation of a stable, nonradioactive daughter nuclide.
Each step in such a chain is characterized by a distinct half-life.
A favorite tactic of Young-Earthers involves citing studies which show trace amounts of Indeed, this results from a unique decay mode known as "cluster decay" where a given isotope emits a particle heavier than an alpha particle (radium-226 is an example.) This fact is extremely inconvenient and creationist literature, accordingly, usually does not mention it.
The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.
Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.
If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.