Dating Methods
An understanding of how objects and events are dated in earth sciences, archaeology and history is important to an understanding of how the earth evolved, how life evolved and how civilization evolved. All of these fields depend on the ability to date, both relatively and absolutely, various events in relation to other events. Some who argue for an absolutely literal reading of the bible criticize history and evolution theories based on a supposed inability to date ancient objects (such as fossilized bones, primitive tools, etc.). What I want to present here are not only the problems with dating methods, but also why these dating methods are reliable within error ranges despite the problems.
There are two types of dating—relative and absolute methods. Relative methods date one thing in relation to another without necessarily placing either event in an absolute time frame. So, for example, when documents from both Egypt under Rameses II and the Hittites under Muwatallis were found that refer to the same events (e.g. the battle of Kadesh) then these two rulers could be dated in relation to eachother, but neither could be dated in any absolute time frame. However, once one of them was dated absolutely, the other could then be dated absolutely since their relative dating was known. This is a simple example of relative dating.
In archaeology and earth sciences, the main relative dating technique is based on stratified layers of earth. Lower layers are older than higher layers, so that objects found in the lower layers are likely to be older than ones found in upper layers. But the only way to date the layers on an absolute scale it find a way to date them to an outside reference.
Relative dating has many problems. One of which is that events that can be dated in relation to eachother tend to occur in clusters, but the clusters cannot always be dated to other clusters. So although there is good documentation that the battles of Salamis and Plataea, where in the ancient Greeks stopped the invasion by Persia, occurred within a year of eachother (some sources would have them occur the same day!), and there is documentation that the fall of Constantinople to the Ottoman Turks preceded the annexation of Serbia and Bosnia by the Turks, how is one to date the Persian invasion in relation to the Turkish Invasion without an absolute time reference?
The best known method of absolute dating is based on radioactive decay. This method is based on the fact that known radioactive isotopes decay with a known half-life to other known isotopes (best known is the decay of Carbon-14 to Carbon-12 with a half life of decay of 5730 years). That way, once an object is formed with a given, initial ratio of one isotope to another, the age of the object can be dated by the change in the ratio of the two isotopes. For example, during life the level of C-14 in an organism remains in equilibrium with the level in the atmosphere or the ocean. Upon an organism's death the C-14 contained in that organism decays at a known rate, and no new carbon from atmospheric carbon dioxide is absorbed. Carbon-14 dating is generally accurate for dating things within the last 50,000 years. Uncertainty in measurement increases with the increasing age of the sample.
Other radioactive dating methods can cover different time periods and can often be used to check eachother since they are based on different physical principles (different methods of absorption into a sample, different methods of radioactive decay, etc.) So taken together, the radioisotopic methods of dating give very reliable dates of an object or layer of sediment in relation to the present. Some also allow for dating back hundreds of thousands of years while others are best for more recent events. Since each isotopic method has its own inherent inaccuracies, it is best to use more than one method of dating where possible. Note that a relative dating between two events can effectively confirm the absolute dating of those events.
A further level of accuracy can be gained if non-isotopic methods can be used to confirm the results of isotopic methods. One argument that I have heard creationists use against evolution and old-earth theories is that the isotopic methods of dating are all based on the same principle (radioactive decay) and so are not effective for confirming eachother. There is a great deal wrong with this objection, but it also can be avoided altogether since there are other dating methods not based on radioactive decay. Thermoluminescence dating relies on the fact that natural ionizing radiation (from cosmic rays and such) induces free electrons in a mineral that can be trapped in defects in the mineral's crystal structure. These trapped electrons escape as thermoluminescence when the sample is appropriately heated. So, by recording the thermoluminescence of a mineral such and assuming a constant natural radiation level, the last drainage of the trapped electrons can be dated. In the case of dating pottery, the specimen is heated until it glows with energy that was trapped in its structure when it was fired in much the same way energy is stored in the crystal structure of a mineral. If such pottery is also dated by a radioactive method as well as relatively (by pottery style, for example) to other events, then a fairly accurate date for that pottery can be arrived at. Other non-isotopic methods are varve and obsidian hydration dating, neither of which do I understand well, but they can also complement isotopic methods.
Dendrochronology, or tree-ring dating, is a method that is easiest to understand and can accurately date events of the past 3000 to 4000 years. This method of dating is based on the well-known fact that trees grow in such a way as to lay down a ring pattern with each ring representing a year in the life of that tree. Since some trees, such as the bristlecone pine of the southwestern US, are thousands of years old, this simple method of counting and measuring tree rings can be extremely helpful. The tree rings are correlated with radiocarbon dating to give very exact measurements.
Thus absolute methods depending on different radioactive decay processes as well as methods based on ionizing radiation, tree rings and other physical properties can be combined to give excellent absolute dates. The dates you read in books about history, archaeology, evolution and earth sciences are all based on these methods. Relative dating based on historical records and documents as well as sedimentation layers can then be used to further confirm and refine absolute dates. The fact that there are some discrepancies in dates read in some of these books can be due to a variety of reasons. Most often it is because one source may be using dates determined before some of the recent advances in dating. For example, radiocarbon dates determined before tree-ring corrections were developed will differ from more recent, tree-ring corrected radio-carbon dates of the same object. Sometimes there will also be disagreement between two dating methods, in which case both methods must be retried or another method applied. This is not always possible to do in a timely fashion, so discrepancies in dates can persist for years. Finally, sometimes a sample is poorly handled when tested, so inaccurate dating will occur. This can only be corrected by more careful handling and dating of other items from the same site. Overall, though, the dating methods used are very accurate and well support the idea of an old earth and of evolution. Consequently, one is forced to abandon any absolutely literal reading of the bible since such a reading would suggest an impossibly young earth. I’d like to note that the Catholic church for one recognizes the validity of these arguments and does not insist on a literal reading of the bible.
SOME RECOMMENDED READING:
Mummies, Dinosaurs, Moon Rocks : How We Know How Old Things Are by James Jespersen, Jane Fitz-Randolph, Bruce Hiscock. (Geared towards young adults) Buy the Book Today!
Radiocarbon Dating (Interpreting the Past, No 1) by Sheridan Bowman. Buy the Book Today!
Art, Artifacts, and Chronology in Classical Archaeology (Approaching the Ancient World) by William R. Biers. Buy the Book Today!
See a Time Line of Evolution
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Email: michad03@mcrcr.med.nyu.edu
