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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.
Each dark band represents a winter; by counting rings it is possible to find the age of the tree (Figure 11.22).
The width of a series of growth rings can give clues to past climates and various disruptions such as forest fires.
For example, an especially warm summer might result in a very thick layer of sediment deposited from the melting glacier.
Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.The discovery of radioactive materials did more than disprove Thomson's estimate of Earth's age.It provided a way to find the absolute age of a rock.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.