As my wife will attest, anytime I hear about an interesting archaeological discovery, my first question is always, “how was it dated?” This is not an idle question. Archaeologists have many ways of dating past events, and each one has its own advantages and disadvantages. Knowing how a date was arrived at helps me to better understand the significance of the find.
Dates obtained by archaeologists can help to resolve controversies about how to best understand past events, including many of the events described in the Bible. They can also create controversy. No dating method is prefect, however, and any date could potentially be shown to be wrong. The professional literature is full of debates over the dating of one or another ancient site. And yet, archaeologists almost universally accept that the overall pattern of dates they have obtained is correct. There is a reason for this, but it’s not one that I have often seen in explanations aimed at the general public. This article is an attempt to rectify that.
Archaeological Dating
The first thing to understand about archaeological dating is that there are two basic categories of dating, with a number of different methods in each category. These two categories are relative dating and absolute dating, and they mean just what you’d think. Relative dating tells us the order in which events occurred, but not exactly when. If, for example, I find that two distinct types of pottery are both found at the same site, but one of them was recovered from a deeper level than the other, I can infer that the more deeply buried type is the older of the two. This inference is strengthened if I find the same situation occurs with those same two pottery types at other sites as well. That’s relative dating.
Absolute dating, on the other hand, tells us how much time has passed since some particular event occurred. Apart from using written records, there was no practical way to do this until the early twentieth century, when an astronomer named Andrew Ellicott Douglass figured out how to use tree rings to date the construction of ancient wooden structures. Many other methods of absolute dating have since been developed, beginning in the mid twentieth century. About forty methods of absolute dating are in common use today. In this short article I can’t explain in detail how they all work, but I can give a quick overview of the most common dating methods, before I go on to explain why archaeologists accept the results as valid.
You may have noticed that I said “event” not object. That’s because archaeological dating always relates to some specific event that occurred in the past, whether it’s firing a pot, building, or destroying a house, cooking a meal, etc. Whenever you see an archaeologist talk about dating an object, you should understand that what is usually meant is either the creation of that object, or the time that it was lost, buried, or otherwise put into the ground for an archaeologist to find.
Relative Dating Methods
The most basic form of relative dating uses the law of superposition, also sometimes known as Steno’s Law, after Danish scientist Nicolas Steno. Simply stated, this law says that, when you have layers of sediment, lower layers are older than higher ones. In other words, gravity works, and nobody is lifting up existing layers of earth in order to spread new layers underneath them. While this law is basically a duh! we need to keep in mind that it applies to layers of soil or other sediment, not to individual artifacts. There are many processes that can move artifacts up or down from one layer to another. The layers themselves, however, remain in the same order, unless some geological process overturns the entire set of layers at a particular location.
Seriation, another important method of relative dating, relies on change in styles over time. Just as photos can often be roughly dated by clothing or cars, archaeological sites can sometimes be dated by stylistic changes in things like pottery or stone projectile points. This requires first finding a site where different styles are present at different levels. Usually we have to put together information from many sites in order to get the full sequence. Once that is done, however, any site that has even one of the identified styles present can be fitted into the sequence. So, for example, if I’ve identified that pottery type A is older than type B, and B is older than type C, I can reasonably assume that all the sites that have only type A pottery are older than the ones that have only type B. As I said, this is a relative dating method; we can figure out the sequence, but to actually put dates on the different styles requires another method.
Absolute Dating Methods
Absolute dating requires us to find something that can tie the event we’re investigating to our own calendar. One obvious way to do this is to use historical documents, but there are some problems with those records. The first is, obviously, that they aren’t always available. Many past cultures did not have writing at all, and even those that had it didn’t write down everything, and what they did write didn’t always include a date. Further, not all of what was written in the past has survived. Nevertheless, where they are available, historical records can be a valuable means of dating past events, along with the objects and sites associated with those events.
Some of the best documents for this purpose are royal records, especially for kingdoms in the ancient Near East. The use of this kind of record is not new; the authors of the Biblical books of Kings and Chronicles clearly used royal records as the basis of their accounts. Because such records often document wars and treaties, a careful comparison between records can tie two or more kingdoms together into an overall chronology. The problem of connecting these records to our own calendar remains, however, since the BC & AD system we use wasn’t invented until centuries later. One important way to link ancient and modern calendars is with ancient accounts of eclipses. Because astronomers can calculate the precise date and time of past astronomical events, we can use records of eclipses to tie the ancient accounts to our modern calendar. Once that is done, dates can be calculated for all the other events that were recorded.
Dendrochronology
Dendrochronology uses tree rings to date past events. In the early 20th century it was realized that, because tree rings vary in width based on the rainfall and temperature the tree was exposed to, trees close to each other will have the same pattern of wide and narrow rings. This fact allows us to fit trees whose lives overlapped into a sequence that is longer than the life of any one tree. In addition to providing dates, it also gives us valuable information about environmental conditions in the past. With a broad enough sample, it’s even possible to separate out regional climate conditions from local variation, as well as tie all the samples together into a single, very long, chronology. The use of many samples also enables us to recognize when a particular tree has occasionally produced no ring during a bad year, or two rings during an exceptionally good one. Through the use of many thousands of samples, many of them from trees that died millennia ago, the tree ring chronology in the northern hemisphere currently reaches back to 13,900 years before the present, and will likely be extended even further as sampling continues. (The “present” in archaeological dating is defined as AD 1950, so it’s not constantly changing.)
Radiocarbon Dating
When cosmic rays strike the upper atmosphere, they cause some of the nitrogen atoms there to transform into radioactive carbon-14 (sometimes written as 14C). Unlike regular carbon (carbon-12), which is stable, carbon-14 decays back into nitrogen. Half of any given sample of carbon-14 decays within a period of 5,730, plus or minus 30, years. This period of time is called the half-life. Decay is progressive, so only a quarter of the sample will remain after two half-lives, an eighth after three, etc. Since all living things take in carbon from their environment, and a small percentage of the atmosphere at any given time is made up of carbon-14, we can measure the amount of carbon-14 still present in something that is no longer alive and calculate how long it has been dead. Because it can be used on almost any organic material, even charcoal, carbon-14 has become the most common method of absolute dating used in archaeology.
There are a number of complicating factors with radiocarbon dating, however, the most important of which is that the amount of carbon-14 in the atmosphere has not remained constant over time. We compensate for this variation with a calibration curve, a mathematical formula that allows the calendar date to be calculated from the radiocarbon date. Calibration curves are produced by using carbon-14 to date a large number of objects whose ages are already known through other means. As with dendrochronology, this has required dating many thousands of samples. This, by the way, is an important point to understand about every method of archaeological dating; they were not developed or tested with just a few samples, but with many thousands of samples, taken from sites all over the world.
Luminescence Dating
In some types of minerals, microscopic defects in the crystals can trap electrons. When the crystal is exposed to radiation, including the very low level background radiation in soil, these electron traps gradually fill up. If the crystal is then heated to a temperature of several hundred degrees, the traps release their electrons in a burst of light that can be measured. If the level of background radiation is also measured, this allows us to calculate how much time has passed since the object was last heated. This method of dating, called thermoluminescence, or TL, is useful for objects that are known to have been heated at some point in the past, such as fired pottery.
In other minerals, exposure to light, rather than heat, frees the trapped electrons. With these minerals, releasing the electrons and measuring the emitted light lets us calculate how much time has passed since a buried object was last exposed on the surface. This method of dating is called optically stimulated luminescence, or OSL, and it is often applied to grains of sand, in order to date a particular layer of sediment.
Other Methods of Scientific Dating
As I indicated earlier, there are approximately forty different methods of obtaining absolute dates that are currently in use. The majority of them involve naturally produced radiation in one way or another, either by measuring radioactive decay, or by measuring the changes that occur when materials are exposed to radiation. However, the specific substances that are examined and the assumptions that lay behind the dating methods vary widely. In many cases the object that is being dated is not the object that the archaeologist is actually interested in (for example, OSL can date grains of sand from a layer in which an artifact was found). In this case, establishing the connection between the date and the object of interest requires careful work.
Why Do Archaeologists Trust These Dates?
It’s universally recognized by archaeologists that any date can be wrong. Accidents happen. Samples get contaminated, people make mistakes, instruments malfunction, objects that are thought to be closely associated may not actually be associated at all, and occasionally people are simply dishonest. Not only can any one of the dates be wrong, in principle they could all be. And yet, almost all archaeologists trust the overall story of the past that those dates reveal.
The reason for this trust is consistency. Think about clocks for a moment. If I go around the city and look at a hundred clock, and I see that ninety of them agree with each other, I would conclude that those ninety are right. Any clock could be wrong, but we would not expect that ninety different clocks from different places all over the city will all show the same wrong time.
The same logic applies in archaeological dating, and this is the point that I don’t see mentioned very often in popular explanations. We don’t trust any one date, but we trust the overall dating of the past because many thousands of samples, collected by different people and tested by different labs using different methods that rely on different assumptions, give results that agree with one another.
I want to be clear on this point: archaeologists do not accept dates because they agree with some particular theory. That would be arguing in a circle, and no archaeologist would be able to get away with that. (Because there are always multiple, competing models, put forth by different people, we have plenty of incentive to check each other’s work.) We accept dates because they agree with each other, and the absolute dates agree with the sequences established through relative dating.
How Should Christians Think About Archaeological Dates?
I know somebody is going to be asking this, but I really don’t like telling people what they should think. So instead I’ll just tell you how I think about archaeological dates.
God created all things; rocks, trees, sand, even radiation. The processes of nature are all his handiwork, and he guides them so that nothing happens outside of his control. The world God created is as truly his revelation as is the Bible. (Psalm 19:1-4). Further, we have discovered that God designed a universe in which nature puts date stamps on many different kinds of objects, and archaeologists have begun learning to read those date stamps.
Sometimes conflicts arise between what we think the archaeological evidence is telling us and what we think the Bible is telling us. Until we reach perfect knowledge, that’s always going to happen, and it’s not something that should undermine anyone’s faith.
Looking back at history: Psalm 96:10 tells us, “Say among the nations, ‘The Lord reigns.’ The world is firmly established, it cannot be moved; he will judge the peoples with equity.” Psalm 104:5 says, “He set the earth on its foundations; it can never be moved.” 1 Chronicles 16:30 reads, “Tremble before him, all the earth! The world is firmly established; it cannot be moved.” We understand that the Biblical authors are speaking metaphorically in all these passages, but the reason we understand this is that scientists in the 16th and 17th centuries discovered that the earth revolves around the sun. It is not literally true that the earth is stationary. The Biblical texts themselves could legitimately be interpreted either way, however.
The findings of astronomy did not contradict God’s word, even though some people at the time thought that they did. What they actually did was improve our understanding of God’s word, by showing that one possible way of interpreting these particular passages, rather than the other, was correct. God’s revelation in nature helped us understand his revelation in Scripture.
Because I know God, I trust that what he tells me in Scripture does not, ultimately, contradict what he tells me in nature. If they appear to, I am probably misreading one, or the other, or both. My experience has been that most of the apparent conflicts I see disappear when I take the time to find out what actual Bible scholars and experts in Biblical languages say about the passages in question, instead of relying only on my own understanding. When they don’t, I can trust that God will make the solution clear at the proper time. As I wrote in a previous article, humility is always required. And remember, always, that our faith and our hope are not based on some event in the Bible having occurred at one date rather than another, but in the fact of Christ’s resurrection from the dead.