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How are free rings used to date historical events?

Tree fragments of unknown age can be compared with the rings of living trees in order to establish the date when the fragment was part of a living tree. This information helps to establish the year in which historical events took place. For instance, by determining the age of a piece of wood within the structure of a medieval cathedral, you can learn the year in which the cathedral was constructed. And fragments of trees that were toppled in an earthquake or volcanic eruption can be dated to learn the year in which the event occurred.

Trees add new cells to the center of their trunk each year. This forces the previous years' growth outward, forming concentric rings (increasingly larger rings with a common center point) with the oldest ring on the outer edge. A tree's annual ring is composed of one wide, light ring and one narrow, dark ring. During...

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