How could you tell the age of this ruin?
Mesa Verde in Southwestern Colorado is the site of beautiful cliff dwellings. The people who inhabited these dwellings are long gone. If you go there you will hear the history of the area, complete with dates. How do archeologists know when the people lived here? How do they know the ages of the structures there? Tree ring dating is extremely useful for finding the age of ancient structures.
Tree Ring Dating
Cut into a tree and you can see its rings. In some locations, the summer growth band is light-colored and the winter band is dark. Each light-dark band represents one year. You can tell how long the tree lived by counting the number of tree rings (Figure below).
Cross-section showing growth rings.
You can tell other things from tree rings too. In a good year a tree will produce a wide ring. A summer drought will produce a smaller ring. These variations will appear in all trees in a region. The same pattern can be found in all the trees in the area. This pattern helps scientist to identify a particular time period.
Scientists have records of tree rings going back over the past 2,000 years. By matching up patterns they can tell when a tree (or a piece of wood from one) lived. Wood fragments from old buildings and ancient ruins can be age dated. The outermost ring gives the date when the tree died.
An example of how tree-ring dating is used to date houses in the United Kingdom is found in this article: http://www.periodproperty.co.uk/ppuk_discovering_article_013.shtml.
Other processes create yearly layers that can be used for dating. On a glacier, snow falls in winter. In summer, dust accumulates. This leads to a snow-dust annual pattern. This pattern can be seen down into the ice (Figure below). Scientists drill deep into ice sheets. Ice cores can be hundreds of meters long. The ice cores show how the environment has changed. Gas bubbles in the ice can be analyzed to show how atmospheric gases have changed. This can yield clues about climate. Long ice cores have allowed scientists to create a record of polar climate going back hundreds of thousands of years.
Ice core section showing annual layers.
Lake sediments also have an annual pattern. This is easy to see in lakes that are located at the end of glaciers. The glacier melts rapidly in the summer. This produces a thick deposit of sediment in the lake. Thin, clay-rich layers are deposited in the winter. The resulting layers are called varves. Varves give scientists clues about past climate conditions (Figure below). A warm summer might result in a very thick sediment layer. A cooler summer might yield a thinner layer. Like tree rings, these patterns can be used to identify time periods.
Ancient varve sediments in a rock outcrop.
ice core: Cylinder of ice extracted from a glacier or ice sheet.
tree ring: Rings of wood equaling one year of tree growth in a tree trunk.
varve: Paired deposit of light-colored, coarser sediments and darker, fine-grained sediments deposited in a glacial lake. This represents an annual cycle.
- Where conditions vary seasonally, trees develop distinctive rings. Ice contains more or less dust. Lake sediments show more or less clay.
- Tree rings, ice cores and varves indicate the environmental conditions at the time they were made.
- The distinctive patterns of tree rings, ice cores and varves go back thousands of years. They can be used to determine the time they were made.
Use these resources to answer the questions that follow.
Science Nation - Lord of the Tree Rings at http://www.youtube.com/watch?&v=FAOYkx8E-Gc (2:29)
- What do tree rings tell scientists?
- What can be learned from tree rings?
- How are tree rings being used to help current climate change?
- What type of trees do scientists look for? Why?
Science Nation - Ice Core Secrets Could Reveal Answers to Global Warming at http://www.youtube.com/watch?v=NENZ6TSc1fo (5:00)
- What is trapped in the ice cores?
- How long have ice cores been studied?
- What can be learned from ice cores?
- Where are ice cores collected?
- Why are these three dating methods considered absolute age dating?
- Describe how tree rings indicate time. Do the same for ice cores and varves.
- Describe how tree rings, ice cores and varves indicate what was going on in the environment when they formed.