Uranium 238 dating rocks
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
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.
Second, zircon has a high trapping temperature of 900°C.
Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.
In many cases the radiation is in the form of extremely short-wavelength gamma radiation, and also there are often particles, alpha (helium nuclei) or beta (electrons), that can have considerable energies.
As the nuclei break apart, the resulting radiation is absorbed by matter to produce heat.
Consider the concordia: as zircons age, they move outward along the curve.
But now imagine that some geologic event disturbs things to make the lead escape.
The oldest zircon yet found dates from 4.4 billion years ago.
With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's "Earliest Piece of the Earth" page, including the 2001 paper in Nature that announced the record-setting date.