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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.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.While tree rings and other annual layers are useful for dating relatively recent events, they are not of much use on the vast scale of geologic time.During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques.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.

Rapid melting of the glacier in the summer results in a thick, sandy deposit of sediment.From these assumptions, he calculated that the Earth was 100 million years old.This estimate was a blow to geologists and supporters of Charles Darwin's theory of evolution, which required an older Earth to provide time for evolution to take place.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.These thick layers alternate with thin, clay-rich layers deposited during the winter.