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. The wood between these boundaries is formed during one growth season and constitutes one growth ring. Once the ring has formed, it remains unchanged during the life of the tree. 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. 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. 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.