How Geological Time is Estimated

> 1800 most of the western world agreed that Earth was about 6000 years old

> this date arrived at by interpretation of the Hebrew bible by Archbishop James Ussher                        
                                               Relative Age Dating

> Hutton declares the theory of Uniformitarianism ..... the same processes at work today are the same as in the past only rate intensity and scale have varied over time
> Original horizontality = sedimentary rocks are laid down in a horizontal manner and the oldest rocks are those in lowest layers which is an application of superposition
> if a particular layer of sedimentary rock's age has been determined then by inference a similar layer in a distant but similar sequence can dated relatively with reasonable confidence
                                                  Absolute - Age Dating

> this method requires the use of  the known half life of radioactive isotopes
> isotopes decay into daughter products at very predictable rates over time and if the isotope and daughter product are trapped in rock this makes for the possibility of dating the rock through precise analysis

Isotopes used to determine the Absolute-Age of Rocks