How to tell the time

Everyone knows how to tell the time don’t they?

Well, it gets a bit more complicated when we get on geological time. It is really easy to say something is so many millions of years old, but how do we know?

Relative geological time

One way of telling how old a rock is, is by looking at the fossils. Different plants and animals lived a different times, because there were different environmental conditions at different times. Therefore, by looking at the fossils we can say which time period they came from.

Absolute geological time

The other way of telling time is by looking at the chemistry of the rocks. Elements called isotopes break down in to other elements at a constant rate. If you work out how many times this reaction has taken place you can tell how old the rock is in millions of years.

Two of the best geological timescales I have found are at the British Geological Survey website and the International Commission on Stratigraphy who are the group set up to help define geological time periods.

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