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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.
In other words, half of the radioactive isotope in a sample would have decayed to Nitrogen-14 (N-14) in just 5,730 years.
C-14 dating of carbon-bearing materials is therefore limited to roughly 50,000 years.
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.Radioactive parent (P) atoms decay to stable daughter (D) atoms e.g.the carbon isotope C-14 decays to nitrogen-14 and the uranium isotope U-235 decays to the lead isotope Pb-207.One early approach was based upon ocean salinity [John Joly, 1800's].This assumed the ocean was initially pure water and that it's salinity was derived from continental erosion.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.This implies the earth is at least 20 million years old.Astronomical cycles can also be used to measure relative age.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.