- Describe four methods of absolute dating.
- Explain what radioactivity is and give examples of radioactive decay.
- Explain how the decay of radioactive materials helps to establish the age of an object.
- Estimate the age of an object, given the half-life and the amounts of radioactive and
daughter materials. - Give four examples of radioactive materials that are used to date objects, and explain
how each is used.
Introduction
As we learned in the previous lesson, index fossils and superposition are effective methods
of determining therelative ageof objects. In other words, you can use superposition to
tell you that one rock layer is older than another. But determining theabsolute ageof a
substance (its age in years) is a much greater challenge. To accomplish this, scientists use a
variety of evidence, from tree rings to the amounts of radioactive materials in a rock.
Tree Rings
In regions outside the tropics, trees grow more quickly during the warm summer months than
during the cooler winter. This pattern of growth results in alternating bands of light-colored,
low density “early wood” and dark, high density “late wood.” Each dark band represents a
winter; by counting rings it is possible to find the age of the tree (Figure11.22). The width
of a series of growth rings can give clues to past climates and various disruptions such as
forest fires. Droughts and other variations in the climate make the tree grow slower or faster
than normal, which shows up in the widths of the tree rings. These tree ring variations will
appear in all trees growing in a certain region, so scientists can match up the growth rings of
living and dead trees. 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.
Ice Cores and Varves
Several other processes result in the accumulation of distinct yearly layers that can be used
for dating. 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
(Figure11.23). To study these patterns, scientists drill deep into ice sheets, producing cores
hundreds of meters long. Scientists analyze these ice cores to determine how the climate has
changed over time, as well as to measure concentrations of atmospheric gases. The longest
cores have helped to form a record of polar climate stretching hundreds of thousands of years
back.