**Radiometric dating** (or radioactive dating) is any technique used to date organic and also inorganic materials from a process involving **radioactive decay**. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.

The** radioactive decay law** states that the probability per unit time that a nucleus will decay is a constant, independent of time. This constant is called the **decay constant** and is denoted by λ, “lambda.” This constant probability may vary greatly between different types of nuclei, leading to the many different observed decay rates. The radioactive decay of a certain number of atoms (mass) is exponential in time.

**Radioactive decay law: N = N**_{0}**.e**^{-λt}

One of the oldest radiometric dating methods is **uranium-lead dating**. The age of the Earth’s crust can be estimated from the ratio between the amounts of uranium-238 and lead-206 found in geological specimens. The long half-life of the isotope uranium-238 (4.51×10^{9} years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating, including uranium-thorium dating and uranium–uranium dating.

**Uranium-lead dating** is based on the measurement of the **first** and the **last member** of the **uranium series**, which is one of three classical radioactive series beginning with naturally occurring uranium-238. This radioactive decay chain consists of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. In the case of the uranium series, the stable nucleus is lead-206. The assumption made is that all the lead-206 nuclei found in the specimen today were originally uranium-238 nuclei. That means at the crust’s formation, and the specimen contained no lead-206 nuclei. This is a reasonable assumption if no other lead isotopes are found in the specimen. Under this condition, the age of the sample can be calculated by assuming exponential decay of uranium-238. That is:

**The uranium-lead dating** method is usually performed on the **mineral zircon**. Zircons from Jack Hills in Western Australia have yielded U-Pb ages up to 4**.404 billion years**, interpreted to be the age of crystallization, making them the oldest minerals dated on Earth.

## Age of the Earth – Uranium-lead Dating

The **age of the Earth** is about **4.54 billion years**. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.