Another example of yearly layers is the deposition of sediments in lakes, especially the lakes
that are located at the end of glaciers. Rapid melting of the glacier in the summer results in
a thick, sandy deposit of sediment. These thick layers alternate with thin, clay-rich layers
deposited during the winter. The resulting layers, calledvarves, give scientists clues about
past climate conditions. 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.
Age of Earth
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,
geologiststriedtoestimatetheageofEarthwithindirecttechniques. Forexample, geologists
measured how fast streams deposited sediment, in order to try to calculate how long the
stream had been in existence. Not surprisingly, these methods resulted in wildly different
estimates, from a few million years to “quadrillions of years.” Probably the most reliable
of these estimates was produced by the British geologist Charles Lyell, who estimated that
240 million years have passed since the appearance of the first animals with shells. Today
scientists know his estimate was too young; we know that this occurred about 530 million
years ago.
In 1892, William Thomson (later known as Lord Kelvin) calculated the age of Earth in a
systematic fashion (Figure11.24). He assumed that the Earth began as a ball of molten
rock, which has steadily cooled over time. 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.
Thomson’s calculations, however, were soon shown to be flawed when radioactivity was
discovered in 1896. Radioactivityis the tendency of certain atoms to decay into lighter
atoms, emitting energy in the process. 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.
Radioactive Decay
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. To understand how this is done,
it is necessary to review some facts about atoms.
Atoms contain three particles: protons, neutrons, and electrons. Protons and neutrons are
located in the nucleus, while electrons orbit around the nucleus. The number of protons