A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.If the solar system formed from a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point.

This value is derived from several different lines of evidence.

Unfortunately, the age cannot be computed directly from material that is solely from the Earth.

There is evidence that energy from the Earth's accumulation caused the surface to be molten.

Further, the processes of erosion and crustal recycling have apparently destroyed all of the earliest surface.

The oldest rocks which have been found so far (on the Earth) date to about 3.8 to 3.9 billion years ago (by several radiometric dating methods).

Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.

Rocks of this age are relatively rare, however rocks that are at least 3.5 billion years in age have been found on North America, Greenland, Australia, Africa, and Asia.

While these values do not compute an age for the Earth, they do establish a lower limit (the Earth must be at least as old as any formation on it).

This lower limit is at least concordant with the independently derived figure of 4.55 billion years for the Earth's actual age.