The time required for half the original number of parent atoms to decay is called the half life.
Some half-lives are listed below: It follows that uranium-lead, potassium-argon (K-Ar), and Rubidium-Strontium (Rb-Sr) decay can be used for very long time periods, whilst radiocarbon dating can only be used up to about 70,000 years. This uses a simple exponential decay formula linking the original number, Po, of parent atoms in rocks and minerals to the P atoms now present, thereby enabling an estimate of geological age.
The technique gave 90 million years, but took no account of the non-constant erosion rate, or the loss and recycling of salt, or the fact that salt is obtained from other sources as well as continents.
More recently, work has been done on ocean sediments [S. This suggests that, given the current annual rates of erosion (some 27.5 billion tons), all earth's continents would be delivered into the oceans in just 14 million years.
Some claim Genesis in particular, and the Bible in general looks mythical from this standpoint.
A full discussion of the topic must therefore include the current scientific challenge to the OE concept.
For instance, measurable amounts of C-14 have been found in fossil material, such as coal (traditionally Carboniferous period c300 mya).The earth precesses (wobbles like a spinning top) around the sun in a series of cycles.These cycles affect sunlight and hence long-term can form layers in rock.One problem with earth dating is that the original earth surface is assumed to have eroded long ago.But assuming the earth was formed at the time of the rest of our solar system, then recovered moon rock and meteorites can also be used to estimate the age of the earth.And OE Christians (theistic evolutionists) see no problem with this dating whilst still accepting biblical creation, see Radiometric Dating - A Christian Perspective.This is the crucial point: it is claimed by some that an old earth supports evolutionary theory and by implication removes the need for biblical creation.This challenge is mainly headed by Creationism which teaches a young-earth (YE) theory.A young earth is considered to be typically just 6,000 years old since this fits the creation account and some dating deductions from Genesis.This implies the earth is at least 20 million years old.Astronomical cycles can also be used to measure relative age.