The orbital and rotational motions of the Earth and Moon define our familiar units of time. In these units, geological time is immense, involving hundreds of millions, even billions of years. It is subdivided into practical units in terms of the rock and fossil records. These records were used in the infancy of geology to estimate the Earth’s age, returning false values because of inherently wrong assumptions. We will see how radiometric dating using the decay schemes of various radioactive isotopes gives consistent, reliable ages, and shows that the Earth is around 4.5 billion years old. The workhorse of modern analyses is the mass spectrometer, which can determine the age of a tiny sample, even of a single grain. Radiometric dating can be used in environmental situations, for example to date great ice-sheets and even groundwater.
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