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1. How old is the Earth?
The chronological figures related to genealogies in Scripture add up to approximately 6,000 years since the creation of described in Genesis. Many creationists consider these figures to be relatively complete, and thus the Earth is considered to be about 6,000 to perhaps as much as 10,000 years old. Biblical scholars do not agree on whether the Bible indicates that the planet Earth was created at the beginning of creation week or if it was already present as a lifeless, wet and dark planet here prior to creation week.  Thus, the question of the age of the Earth could refer to the time since the Genesis creation week or to the time since the planet was first created. The Bible does not give an age for the Earth, nor is any theological point drawn from the age of the Earth, so it may not be as important as some of the other issues. Most scientists believe the Earth is about 4.5 billion (4,500,000,000) years old. This figure is based on radiometric dating. Some creationists have attempted to reconcile this figure with the creation in Genesis by proposing that the rocky, lifeless planet was created long ago (perhaps 4.5 billion years), and God created life on this planet later (perhaps 6,000 to 10,000 years ago), when it suited His purposes to do so. Other creationists hold that the planet itself was created during creation week, perhaps early in the first day of creation. In this case, the entire planet would be about 6,000 to 10,000 years old.
2. What is the basis of radiometric dating?
The minerals of the earth often contain atoms that are unstable and decay into a different kind of atom. Such unstable atoms are said to be radioactive. The rate of radioactive decay can be described by a mathematical equation that compares the ratio of “parent” radioactive atoms to “daughter” atoms resulting from radioactive decay. Using this equation, scientists can calculate how long it would take for radioactive decay to produce the observed ratio of parent and daughter material. Solving the equation gives the age of the sample. Several different types of radioactive atoms are used to calculate radiometric dates, such as uranium-lead, rubidium-strontium, etc.  The most popular method is probably the potassium-argon method. Potassium atoms vary in their numbers of neutrons, resulting in atoms with different masses (called isotopes). One isotope, potassium-40, is radioactive, and decays into argon-40, an inert gas. Potassium-argon dates are calculated from the ratio of daughter to parent material. The amounts of parent and daughter materials can be measured very precisely, but the accuracy of the date depends on the reliability of three major assumptions: constant decay rate; closed system; and initial concentration. The constant rate of decay hypothesis seems valid; there is little observational evidence against it, although it has been questioned.  The closed system hypothesis is carefully considered (the method is not applied to rocks that obviously have been chemically altered), but there is need for caution here. The initial concentration hypothesis may be the weakest part of the process of calculating radiometric dates. Attempts are made to estimate the initial concentrations as reasonably as possible, but there is no way to be certain the estimates are correct. One cannot go back in time and examine the rock sample when it was first formed. Creationists suspect there are problems with the assumptions of the method, although alternative models to explain radiometric ages have not been fully developed.
3. What is the meaning of “half-life?”
Radioactive atoms in a sample decay at a mathematical rate such that half the parent atoms decay in a fixed period of time known as the “half-life.” The time of the “half-life varies with different kinds of radioactive atoms. For potassium-40, the half-life has been determined to be about 1.3 billion years. This means that if one started with 1000 atoms of potassium-40, 500 of them would change into argon-40 in 1.3 billion years. After another 1.3 billion years, only 250 atoms would remain, while there would be 750 atoms of argon-40. A third half-life would reduce the potassium-40 to 125 atoms, with 875 atoms of argon-40. At this point, the ratio of one part potassium-40 to 7 parts of argon-40 would indicate an age of about 3.9 billion years, which is close to the age of the "oldest" known rocks on Earth. Eventually, the amount of parent material becomes too small to detect, so that particular kind of atom can no longer be used to calculate the age of a rock. A useful estimate is that after ten half-lives, the amount of parent material remaining is too small to be useful in calculating radiometric dates.
4. How can creationists explain radiometric dates of many millions of years?
Creationists suspect there is some kind of systematic error in the methodology of radiometric dating. Some possibilities have been proposed,  but they are not compelling because they often often invoke supernatural activity or unknown mechanisms. Another approach to explaining ages of millions of years is to propose that the rocks of the earth are very old because the planet was created long before life was placed on it. This theory proposes that Genesis refers only to the creation of life on the planet, and not to the creation of the planet itself. This idea is sometimes called the two-stage creation hypothesis. This theory does not adequately explain why fossils are often found in association with rocks that have old radioactive ages. Another possibility is that God created a mature planet, with mature trees, mature animals, and mature humans. Therefore, it is reasonable that He would create rocks that appear mature also. There may even be some functional reason that existing levels of radioactivity are more appropriate for life than other levels would be. This explanation can be called the mature-earth creation hypothesis. In summary, the age of the earth as calculated by radiometric dating methods remains one of the more problematic issues for creationists. Perhaps some future discovery will shed light on this issue.
5. What unsolved questions about the age of the Earth are of greatest interest?
The most difficult question is probably the apparent sequence of radiometric dates, giving older dates for lower layers in the geologic column and younger dates for upper layers. Other questions include: why radiometric dating systematically gives ages that are much older than suggested by the biblical record; why different dating methods often give similar ages; an explanation for the cooling of the magma composing the ocean floor;  and an explanation for the long series of layers in ice cores.
 See: (a) Geyh, MA, and H Schleicher (Translated by Newcomb RC). 1990. Absolute Age Determination. Berlin and NY: Springer-Verlag; (b) Faure G. And Mensing, TM. 2004. Isotopes: Principles and Applications. NY: John Wiley and Sons.
 Vardiman, L, A Snelling and EF Chaffin (eds). Radiosiotopes and the Age of the Earth: Creationist Research, Institute for Creationist Research, (2000); Snelling, AA, EF Chaffin, L Vardiman (eds). Radioisotopes and the Age of the Earth, Vol 2. Institute for Creation Research (2005).
 See: (a) Brown RH. 1983. How solid is a radioisotope age of a rock? Origins 10:93-95; (b) Brown RH. 1977. Radiometric age and the traditional Hebrew-Christian view of time. Origins 4:68-75; (c) Giem PAL. 1997. Scientific Theology. Riverside, CA: La Sierra University Press, p 116-136; (d) Brown RH. 1996. Radioisotope age, Part 1. Geoscience Reports No. 20; (e) Webster CL. 1996. Radioisotope age, Part 2. Geoscience Reports No 21; (f) Clausen BL. 1997. Radioisotope age, Part 3. Geoscience Reports No 22. Loma Linda, CA: Geoscience Research Institute