What determines isotopes for carbon dating
Each dark band represents a winter; by counting rings it is possible to find the age of the tree (Figure 11.22).
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".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.But determining the absolute age of 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.In 1892, William Thomson (later known as Lord Kelvin) calculated the age of Earth in a systematic fashion (Figure 11.24).
He assumed that the Earth began as a ball of molten rock, which has steadily cooled over time.
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.
For example, an especially warm summer might result in a very thick layer of sediment deposited from the melting glacier.
Thinner varves can indicate colder summers, because the glacier doesn’t melt as much and carry as much sediment into the lake.
Scientists analyze these ice cores to determine how the climate has changed over time, as well as to measure concentrations of atmospheric gases.