### The Geochron

**S**cientists 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: ^{238}U to
^{206}Pb, with a half life of 4.47 x 10^{9} years, and ^{235}U to ^{207}Pb, with
a half life of .707 x 10^{9} years.

The decay curves of these isotopes relative to the stable isotope ^{204}Pb 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 10^{9} years.

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