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Using logs recovered from old buildings and ancient ruins, scientists have been able to compare tree rings to create a continuous record of tree rings over the past 2,000 years.This tree ring record has proven extremely useful in creating a record of climate change, and in finding the age of ancient structures. The thick, light-colored part of each ring represents rapid spring and summer growth.The thin, dark part of each ring represents slow autumn and winter growth.Several other processes result in the accumulation of distinct yearly layers that can be used for dating.Thomson's calculations, however, were soon shown to be flawed when radioactivity was discovered in 1896.Radioactivity is the tendency of certain atoms to decay into lighter atoms, emitting energy in the process.These thick layers alternate with thin, clay-rich layers deposited during the winter.The resulting layers, called varves, give scientists clues about past climate conditions.
The longest cores have helped to form a record of polar climate stretching hundreds of thousands of years back.
For example, layers form within glaciers because there tends to be less snowfall in the summertime, allowing a dark layer of dust to accumulate on top of the winter snow (Figure 11.23).
To study these patterns, scientists drill deep into ice sheets, producing cores hundreds of meters long.
As we learned in the previous lesson, index fossils and superposition are effective methods of determining the relative age of objects.
In other words, you can use superposition to tell you that one rock layer is older than another.
But determining the absolute age of a substance (its age in years) is a much greater challenge.