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.
Radiometric dating is used to tell the age of rocks asian ladies online dating ukrainian
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.
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.
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.
Radioactive materials in Earth's interior provide a steady source of heat.
Calculations of Earth's age using radioactive decay showed that Earth is actually much older than Thomson calculated.
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.
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).