The Essentials - The Original Dating Game

Radiometric dating is relatively new to other methods for determining the age of old things.  There are two others that are far more low tech but yield reliable results and also insight into the question of Earth's age.  These two methods are based on the regular accumulation of strata, or layers in the earth and in a group of living organisms- trees.

Dendrochronology is the science of aging (that is, determining the age of)trees.  In its simplest form this discipline counts the rings revealed in the cross section of a tree.  One ring is formed per year of the trees life so then the total number of rings equals the total number of years the tree has been around.  If you have ever counted the rings of a tree I recommend you take a moment right now to say, "Hey, I'm an amateur dendrochronologist." If you'd like, and you're feeling crafty, you can make yourself a badge to wear the next time you go tree ring counting.  

There is a bit more to the science of tree ring counting but at its heart it really is as simple as
1 ring = 1 year of growth so the total number of rings = the age of the tree. 

Dendrochronologists can use this simple relationship to gain insights beyond the life span of a tree.  The size of the rings reveals information about the climate of the year that it was formed. If there was plenty of water and suitable temperatures the tree will grow rapidly and a wide ring will result.  Similarly a narrow ring reveals more harsh growing conditions such as drought or perhaps colder than normal temperatures. 

This discipline can also reach back beyond the life of a tree.  By comparing tree rings of overlapping age, chronologies have been created that stretch back tens of thousands of years.  The usefulness of this is best understood in an example. 

Say a wood tool is found in an archeological dig site.  The archeologists want to know when the people who used that tool populated the site.  By comparing the ring structure revealed on the wood tool to the known tree ring record the tool, and the people who used it, can be placed in their proper historical time.


The smaller samples have common sections that are alligned to assemble a lengthy chronology. The dig sample is compared to the known chronology to determine its position in history.

Tree ring chronologies can also be used to validate radio carbon dating by comparing the estimated age, based on the ratio of C14 to N14, to the known age based on the tree ring chronology.
Geological strata (layers in the Earth) can be used like tree rings for but can go much farther back than tree chronologies.