This is Part 11 of a series of approximately sixty posts that outline evidence, support, and explanations for evolution. Receive updates and notification of all posts from dennismitton.com by selecting the follow button on the sidebar of any page. Thanks!
Copyright Dennis Mitton
Absolute and Relative Dating Methods
In Part 10, I described the basics of radiometric dating where the ratio of parent to daughter isotopes is used to determine the absolute age of a sample. In this post, I will briefly describe non-radiometric techniques used for dating both geologic and archeological artifacts.
It’s not always clearly understood or explained but dating methods fall into two categories. There is some crossover but methods are generally considered absolute or relative. The terms mean just what they imply: absolute methods tell us the age of a thing apart from other references. Radiometric dating is an example. Alternately, relative dating tells us where a thing fits on a number line in relation to other known things. Knowing where it fits on a scale gives an age range but no specific dates. You see this all the time on the television show Antiques Roadshow. Appraisers give a date range for a piece of furniture or art based on its style and where that style fits between all of the different styles. Knowing a range is often equally as important as knowing the date of creation.
Some of these dating methods are simple and self-evident while others are very complex. All of them, though, help researchers create a time line and then fit samples onto that line.
Superposition is the most simple of geologic concepts: in unaltered rock formations, the older rock rests beneath younger rock. Is there any way that this doesn’t make sense? Silt in a lake bed deposited ten years ago will lie beneath silt laid this summer. The method is not without its challenges. We know that plate tectonics shifts, upends, and mangles rock in every way possible so it’s not uncommon to find once horizontal layers in a slant or even turned upside down. Knowing the relative position is key. Once the original layering is determined then geologists can match samples giving insight into local tectonics.
The Geologic Column
The geologic column is an imaginary cut-away of rock that typically spans the whole of geologic time from the formation of the earth through modern times. It is the Law of Superposition writ large providing a useful visual showing how eras, fossils, and ages stack on top of each other. Think of it as a geologic number line that gives hints about where a specific sample fits. Understand that this is not found anywhere and that any one location may have only a few layers of rock. But by correlating these we are able to build a scaffold to which we can hang new findings.
Faults and Cross-cutting
These methods for dating geologic formations are based on the obvious idea that in order for something to break, well, there must be something to break. Thus rocks formed by fault activity must be younger than the rocks in the faulting beds. In the same way igneous rocks formed by intrusion must be younger than the sedimentary rocks they flow through. Natch!
Faunal Deposition and Indexing
Also called biostratigraphy, this method crosses over between relative and absolute dating concepts. Using absolute methods we can determine the age of fossils and of specific rocks. We know that the great textbook dinosaur Tyrannosaurus rex lived in an approximate range of 60-90 million years ago. If we are scraping around a cliff side in Montana and, viola!, find a T. rex fossil, even without dating the rock we have a high level of confidence that the formation is around 60-90 million years old. It doesn’t mean that it can’t be older – maybe we have found the oldest T. rex ever? – and it doesn’t mean that it can’t be younger but it gives us a firm relative date. Indexing is really this easy. Faunal deposition is indexing on a larger scale and argues that older fossils are found in older rocks.
The concept is not difficult to understand. If you are excavating the backyard for a pool and dig up a rusty tail fin from a 1958 Chevy you can be pretty sure that the piece wasn’t buried in the ’30s. You don’t know when it was buried – there just not enough information – but you can be confident that it was buried sometime between 1958 and the day before you moved in to the house.
This interesting chemical relationship was used to take down Piltdown Man, whom I consider to be the greatest science hoax ever pulled. This method determines whether or not bones and teeth are the same age. When these artifacts are buried, two things begin to happen. Organics in the bone are gradually whittled away by bacteria resulting in gradual depletion of nitrogen. Then, as ground water moves through the site, trace amounts of fluorine and other minerals are deposited in the artifacts. Using the predictive power of the scientific method, it is argued that if all the bones and teeth were deposited at the same time they will all have the same ratio of nitrogen to fluorine. If not, though buried at the same site, they were not deposited at the same time. Thus Piltdown Man, an amalgam of different bones, was proved a fake.
This method, similar to fossil indexing, is used to place a sample within a certain era or even location. It can be used as a cross-check for other dating methods and provides increasingly useful information regarding paleoclimatology. The method is based on the fact that, just like seeds, plants produce uniquely individual pollen.
You loved counting tree rings as a kid and these folks get to do it for a living. Scientists use a tree’s growth rings to determine the tree’s age and can statistically compare the rings to a master timeline to determine the absolute age of the artifact. Per Wikipedia (here) there are fully anchored growth ring chronologies in the Northern Hemisphere extending back to almost 14,000 years. The method can also be used in conjunction with carbon dating as an absolute control. Because the growth rings change in size based on climatic conditions, dendrochronology has become a useful tool for climate studies as well.
An interesting application is in art history. The National Portrait Gallery in London purchased a portrait of Mary, Queen of Scots, that was thought to be a later copy of an original. But in 2005, based on dendrochronological analysis of the wood panel, historians determined that the portrait was an original, likely painted in 1552, ten years into her reign as Queen of Scotland. See here at the National Portrait Gallery.
The molecular world never stops. When radiation (and we’re all bathed with the stuff) careens through certain crystalline structures, it collides with electrons and raises their energy state. As time progresses, more radiation equates to more accumulated energy. If we heat this material to high temperatures the stored energy flashes to light and then resets to zero. This light can be measured and is directly proportional to the amount of radiation dose accumulated. Because background radiation is generally constant in any given location, it is easy to calculate the age of the artifact.
Because of the necessity of heat to reset this light emitting clock, the method is used primarily as a technique for dating fired pottery though it can be useful for dating minerals within stones that were heated by fires as well. Per the United States Geological Survey, the method can be used for minerals held with rock for ages up to 500,000 years.
This method, used primarily for cold-weather recent archeology, is based on the consistent growth rate of lichens. It was first developed by measuring lichen growth on cemetery tombstones and correlating the size of the growth with the age of the stone. It is important that the substrate was originally free of growth. The method is especially useful in alpine and cold-weather zones where lichens grow slowly and consistently with little crossover from other lichens or other organisms. Though theoretically useful for dating artifacts thousands of years old it is most useful for recently crafted or exposed items to within around 500 years. Go here for a well written overview.
An excellent overview of dating methods is found here at Nature Education
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