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13-Jun-2017 03:38

Rocks relate to events in Earth's history, and we can use them to put together a timeline that shows us both the order of events as well as when and for how long they occurred.Our understanding of the shape and pattern of the history of life depends on the accuracy of fossils and dating methods.

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The methods are all based on radioactive decay: The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old.

Every few years, new geologic time scales are published, providing the latest dates for major time lines.

Older dates may change by a few million years up and down, but younger dates are stable.

Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.

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A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age.For example, it has been known since the 1960s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.