Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
Ionium–thorium dating
The radioactive element uranium-238 is soluble in water. However, when it decays into thorium, the latter element is insoluble and so precipitates out to become part of the sediment. Thorium-232 has a halflife of 14.5 billion years, but thorium-230 has a halflife of only 75,200 years, so the ratio is useful for dating sediments up to 400,000 years old. Conversely, this technique can be used to determine the rate of ocean sedimentation over time.