Lesson 5 Activity 1: Dating a Volcanic Eruption

• 45-minute work session
• Students learn how to "read" tree rings to determine the date of a volcanic eruption and the effects an eruption has on plant growth.

Key teaching points

• If you look at the top of a tree stump, you will see a series of concentric rings in the cross section of its trunk. Because a single tree ring is usually formed each year, tree rings can be used to date the tree.

• Tree ring boundaries are distinguished by a change in appearance between the small thick-walled cells produced at the end of a growth season and the large thin-walled cells produced at the beginning of the next growth season. The wood between these boundaries is formed during one growth season and constitutes one growth ring. (fig. 2). Once the ring has formed, it remains unchanged during the life of the tree.

• Scientists use tree ring data to help them establish the date of volcanic eruptions: the width of each year's growth records evidence of natural events, such as floods, drought, fires, and volcanic eruptions, that increased or decreased the width of that year's ring growth (fig. 2).

  This 120-year old tree records the 1912 eruption of Mount Katmai, Alaska.

• Tree rings record both the negative and positive effects of volcanic eruptions. After the 1980 eruption of Mount St. Helens, ash that blanketed forests to the northeast of the eruption retarded growth for about 2 years. As rain washed away the ash, the rate of tree growth recovered and actually increased. The effect of the ash was equivalent to putting mulch on garden plants—the mulch reduced competition for water and nutrients by plants in the understory (in a garden mulch helps retain moisture and keep weeds from competing with other plants.)

Materials

• Activity Sheets 5.1 a - b
• Log with visible tree rings (optional)

Procedures

1. Using either a piece of cut log or a diagram drawn on the chalkboard of a cross section of a tree, explain what tree rings are and what evidence they record about natural events that occurred during a tree's life.

2. Discuss the impact of the May 18, 1980, eruption of Mount St. Helens on the animal and plant life in the area around the volcano. Use poster fig. 19 to point out that a massive number of trees were killed during the eruption by the lateral blast, by forest fires started by lightning, and by avalanches and mudflows. Trees that survived, however, were charred or had their growth affected by the ash that covered the ground.

3. Distribute Activity Sheets 5.1a-b. Explain to students that they will use tree ring data to determine the date of an eruption of Mount Katmai in Alaska and the effect that the eruption had on the growth of a tree. (The tree's growth decreased for 3 years following the eruption but then increased for 12 years.) As a library assignment, they will make a time line and record on it important events that occurred during the life span of the Mount Katmai tree.

Extension