A related article on the age of the earth and geologic ages presented the current best known values for these dates: Ages.The figures shown in that article are based on radiometric dating.Con All radioactive dating except Carbon 14 are based on atoms found in igneous rocks. So radioactive dating can be used to directly date fossils. However Carbon 14 has a relatively short half life so it cannot be used on fossils much older than 50,000 years which makes it useful for anthropology but not geo history.Also most fossils no longer contain Carbon they have been turned to stone.The corresponding dates obtained from these isochrons (based on the slopes of the lines), together with statistical standard deviations, are: 4.396 ± 0.18, 3.673 ± 0.014, 2.991 ± 0.15, and 4.478 ± 0.034 (each figure is in billions of years). But with the advent of mass spectrometry beginning in the 1970s, even very small samples can now be accurately dated.For example, the "SHRIMP" ion microprobe now in use in numerous laboratories around the world can reliably measure U-Pb and Pb-Pb ages from spots only 0.02 mm (i.e., 20 micrometers) in size within a zircon crystal [Dalrymple2004, pg. It should be emphasized, though, that even relatively unsophisticated equipment can perform radiometric measurements of dates fairly well.By some simple algebraic manipulation of the basic radioactivity formula above, one can show that the following formula must hold at any time t: (Sr87/Sr86) is the ratio of these two isotopes at time t.

Uranium-thorium dating, for instance, can be used to date specimens up to about 500,000 years old (since the half-life of the U-Th decay is 75,000 years), but Rubidium-Strontium dating can be used to date specimens billions of years old (since the half-life of the Rb-Sr decay is 48.8 billion years).

As mentioned above, the isochron dating method boils down to plotting multiple data points, after some calculation, on a graph, which, if the measurements and calculations are done properly, should lie on a straight line, or very nearly on a straight line.

The slope of this line, after another simple calculation, then gives the age.

The following is a brief technical description of how scientists determine dates with radiometric schemes.

This section may be omitted if readers do not wish to follow the math (although the math used here is nothing beyond what is typically taught in a good high-school math analysis class).