In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
Why are unstable isotopes useful for dating rocks
The thin, dark part of each ring represents slow autumn and winter growth.
Several other processes result in the accumulation of distinct yearly layers that can be used for dating.
For example, layers form within glaciers because there tends to be less snowfall in the summertime, allowing a dark layer of dust to accumulate on top of the winter snow (Figure 11.23).
As we learned in the previous lesson, index fossils and superposition are effective methods of determining the relative age of objects.
In other words, you can use superposition to tell you that one rock layer is older than another.
But determining the absolute age of a substance (its age in years) is a much greater challenge.
To accomplish this, scientists use a variety of evidence, from tree rings to the amounts of radioactive materials in a rock.
Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.
These tree ring variations will appear in all trees growing in a certain region, so scientists can match up the growth rings of living and dead trees.
Using logs recovered from old buildings and ancient ruins, scientists have been able to compare tree rings to create a continuous record of tree rings over the past 2,000 years.
This tree ring record has proven extremely useful in creating a record of climate change, and in finding the age of ancient structures. The thick, light-colored part of each ring represents rapid spring and summer growth.