Rio Tinto | Minerals and Metals for the world

Ever since the world began
Geologists measure time backwards, using natural clocks that started to tick four or five billion years ago. Chris Morrissey climbs into his time machine.

Most people find it tough to relate to the geological time scale. What red blooded male can forget Raquel Welch, shrink wrapped in an animal skin bikini, facing up to man eating dinosaurs in the film One Million Years BC? Set precisely 1,001,966 years ago at the date of its release, that Hollywood epic was probably a fair reflection of how many might see geological time.

The truth, of course, is rather different (see graphic overleaf). Dinosaurs had been extinct for about 65 million years when Homo sapiens first appeared on Earth. A million years ago Raquel Welch would have had to snuggle up in a cave with Homo erectus, who by then was living all over the Old World but never made it across the Atlantic to Hollywood.

Geologists tend to be blasé about periods as short as a million years. For them, time adds up to thousands of millions (billions) of years. Their textbooks differ by as much as 50 million years about key dates in geological time. For individual rocks they use "best fit" dates with margins of error that may run to tens of millions of years.

In everyday life we normally think in terms of short periods like days, months, years and decades. The arrival of a year with three zeros was treated as a Very Big Event. Historians and archaeologists routinely deal in centuries and millennia, but larger units of time are mainly found in astronomy, mythology and religion. The largest unit in any calendar is probably one of about 63 million years in the Mayan Long Count system of measuring time.



Ever since the world began Geologists measure time backwards, using natural clocks that started to tick four or five billion years ago. Chris Morrissey climbs into his time machine. Most people find it tough to relate to the geological time scale. What red blooded male can forget Raquel Welch, shrink wrapped in an animal skin bikini, facing up to man eating dinosaurs in the film One Million Years BC? Set precisely 1,001,966 years ago at the date of its release, that Hollywood epic was probably a fair reflection of how many might see geological time. The truth, of course, is rather different (see graphic overleaf). Dinosaurs had been extinct for about 65 million years when Homo sapiens first appeared on Earth. A million years ago Raquel Welch would have had to snuggle up in a cave with Homo erectus, who by then was living all over the Old World but never made it across the Atlantic to Hollywood. Geologists tend to be blasé about periods as short as a million years. For them, time adds up to thousands of millions (billions) of years. Their textbooks differ by as much as 50 million years about key dates in geological time. For individual rocks they use "best fit" dates with margins of error that may run to tens of millions of years. In everyday life we normally think in terms of short periods like days, months, years and decades. The arrival of a year with three zeros was treated as a Very Big Event. Historians and archaeologists routinely deal in centuries and millennia, but larger units of time are mainly found in astronomy, mythology and religion. The largest unit in any calendar is probably one of about 63 million years in the Mayan Long Count system of measuring time.
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