Relative AgeRelative Age

The age of a rock unit or event The age of a rock unit or event relativerelative to the age of another rock to the age of another rock unit or event.unit or event.

In other words which layer or event In other words which layer or event came came firstfirst, , secondsecond, , thirdthird……

Ex. Ex. Mr. Jensen is older than you are Mr. Jensen is older than you are oror the sandstone was deposited the sandstone was deposited before the fault occurredbefore the fault occurred

Absolute AgeAbsolute Age An actual year or a range of years An actual year or a range of years

(sometimes expressed as a (sometimes expressed as a geologicgeologic periodperiod))

Can be determined with Can be determined with radiometricradiometric datingdating

Can be determined with the use of Can be determined with the use of Index Index FossilsFossils and and Volcanic Time MarkersVolcanic Time Markers

Index FossilsIndex Fossils

Index fossils Index fossils (like fads)(like fads) can help us can help us determine the determine the specific time periodspecific time period that the rock formedthat the rock formed

The best index fossils are those that The best index fossils are those that lived for a relatively lived for a relatively shortshort period of period of time and over a time and over a largelarge area.area.

Volcanic Time MarkersVolcanic Time Markers

Volcanic ash is sent into the Volcanic ash is sent into the upper upper atmosphereatmosphere and is carried all over and is carried all over the Earththe Earth

LayersLayers of volcanic ash are created all of volcanic ash are created all over the Earth.over the Earth.

They can be linked to the They can be linked to the eruptioneruption and and dateddated

                                    

                                    

Not every Fossil is an Index Not every Fossil is an Index FossilFossil

Some fossils are not useful for Some fossils are not useful for determining the age of a rock determining the age of a rock because they …because they …– Lived for Lived for too longtoo long a period of time a period of time– Too Too smallsmall of an area of an area