The Geochron

Scientists from the University of Quebec recently announced the discovery of the oldest volcanic rocks ever found. The 3.825 billion year old rocks were discovered near Husdon Bay.

While these rocks may place a limit on the minimum age of the Earth, they can’t accurately date when the Earth itself was formed. Radiometric dating techniques though can be used to arrive at a surprisingly specific figure.

Dating at this time scale is based around the decay of various isotopes of uranium into lead: ²³⁸U to ²⁰⁶Pb, with a half life of 4.47 x 10⁹ years, and ²³⁵U to ²⁰⁷Pb, with a half life of .707 x 10⁹ years.

The decay curves of these isotopes relative to the stable isotope ²⁰⁴Pb can be used to form a linear equation, called The Geochron. The slope of this equation gives an age of the Earth of 4.55 ± 0.07 x 10⁹ years.

These equations are fully developed in this excellent overview by Prof. Stephen A. Nelson of Tulane University.