Relative Age  The age of a rock unit or event relative to the age of another rock unit or event.  In other words which layer or event came first, second, third…  Ex. Mr. Jensen is older than you are or the sandstone was deposited before the fault occurred

Absolute Age  An actual year or a range of years (sometimes expressed as a geologic period)  Can be determined with radiometric dating  Can be determined with the use of Index Fossils and Volcanic Time Markers

Index Fossils  Index fossils (like fads) can help us determine the specific time period that the rock formed  The best index fossils are those that lived for a relatively short period of time and over a large area.

Volcanic Time Markers  Volcanic ash is sent into the upper atmosphere and is carried all over the Earth  Layers of volcanic ash are created all over the Earth.  They can be linked to the eruption and dated

Not every Fossil is an Index Fossil  Some fossils are not useful for determining the age of a rock because they … –Lived for too long a period of time –Too small of an area