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

Principle of Uniformitarianism What? “The Present is the Key to the Past” What geologists can see going on today are the same processes that occurred in the past.

Oldest at the Bottom; Youngest on Top Law of Superposition The Oldest Layers are deposited first (on the bottom) and subsequent layers are deposited on top. Oldest at the Bottom; Youngest on Top This assumes there has been no overturning

The rock must be there so it can be deformed Crustal Deformation All tilting, faulting, or folding happened after the layers were deposited The rock must be there so it can be deformed This includes contact metamorphism that changes the rock that was “touched” by magma or lava

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