# Tree Rings, Ice Cores, and Varves

## Scientists can determine absolute ages using tree rings, ice cores and varves.

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Tree Rings, Ice Cores, and Varves

#### How can scientists tell the oldest possible age of this painting?

The Netherlandish paintings, which were painted in the low-lying countries of and near the Netherlands, were painted on solid wood panels, usually oak. The wood was split radially so that tree rings are visible and dates for the paintings date, which are from the 15th and 16th centuries, can be determined. Why does this give the oldest possible age?

### Tree Ring Dating

In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived (Figure below).

Cross-section showing growth rings.

The width of these growth rings varies with the conditions present that year. A summer drought may make the tree grow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in all trees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period.

Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments from old buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is, when it died. The tree-ring record is extremely useful for finding the age of ancient structures.

### Ice Cores

Besides tree rings, other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but in summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure below). Scientists drill deep into ice sheets, producing ice cores hundreds of meters long. The information scientists gather allows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of the ice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest cores allow scientists to create a record of polar climate stretching back hundreds of thousands of years.

Ice core section showing annual layers.

### Varves

Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure below). A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer.

Ancient varve sediments in a rock outcrop.

### Summary

• Where conditions vary seasonally, trees develop distinctive rings, ice contains more or less dust, and 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 going back thousands of years can be used to determine the time they were made.

### Review

1. What is dendrochronology?
2. How do tree rings, ice cores and varves indicate the time at which they were made?
3. How do tree rings, ice cores and varves indicate environmental conditions at the time they formed?

### Explore More

Use the resources below to answer the questions that follow.

1. What do annual tree rings tell scientists?
2. What can be learned from tree rings?
3. What do tree rings indicate about the lost colony of Roanoke, Virginia?
4. What type of trees do scientists look for? Why?
1. What is trapped in the ice cores and when it it from?
2. How long have ice cores been studied?
3. What can be learned from ice cores?
4. What circumstances make the best ice cores? What is a good location to obtain cores like this?
5. How much would sea level rise if the Greenland ice sheet melted?

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### Vocabulary Language: English Spanish

TermDefinition
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.