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.
In fact, organic samples from every portion of the Phanerozoic record (spanning the last 500 million years on OE dating) show detectable amounts of C-14.
The implication is that this organic material was either contaminated by new C-14, or it was buried much more recently and OE dating methods are suspect.
Ice cores from Greenland and Antarctica show annual layers (varves) and can be traced up to about 40,000 years before the layers become too thin due to compaction.
Similarly, annual lake sediments can be used to estimate relative age and conventional interpretation for the Green River varves suggests they have been formed over some 20 million years.
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 assumed the ocean was initially pure water and that it's salinity was derived from continental erosion.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.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.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.These estimates give 4.4-4.5 billion years for moon rock, and 4.54 billion years for iron metreorites.These techniques utilize the physical parameters of the earth, such as ice cores, annual lake sediments, and astronomical cycles.Most people accept the current old-earth (OE) age estimate of around 4.6 billion years.This age is obtained from radiometric dating and is assumed by evolutionists to provide a sufficiently long time-frame for Darwinian evolution.This implies the earth is at least 20 million years old.Astronomical cycles can also be used to measure relative age.In some cases these astronomical cycles in rock appear to have been laid down over some 25 million years (and radiometric dating puts the absolute age of the rock at some 200 million years).