Mr Calaski

Home » Relative Dating

Relative Dating


Relative Dating:  a method of figuring out the relative ages (younger/older) of rocks and events.

Objective:  I can use the principles of superposition, faunal/floral succession, and crosscutting relationships to determine the relative of the rock layers.


Principle of Superposition

The principle of superposition states that older layers of rock lie underneath younger layers.

Sedimentary rocks are formed particle by particle and bed by bed, and the layers are piled one on top of the other. Thus, in any sequence of layered rocks, a given bed must be older than any bed on top of it. This Principle of Superposition is fundamental to the interpretation of Earth history because at any one location it indicates the relative ages of rock layers and the fossils in them.

super positiongrand canyon superposition

rock layers


Principle of Faunal/Floral Succession

The principle of faunal/floral succession states that older fossils of animals/plants will be found in older rock layers, underneath younger rock layers and fossils.

In other words, the age of fossils have a predictable pattern (get older) as you dig deeper into the Earth.

Screen shot 2014-01-15 at 10.56.13 AM

Faunal succession is useful because it can be used to correlate rock layers from rock columns that are miles apart:



Principle of Crosscutting Relationships

The principle of crosscutting relationships states that a fault or intrusion is always younger than the rock layers it cuts through or into.


In the above diagram, layers 1, 2, and 3 were deposited before the intrusion (4).  Then the intrusion cut through the first three layers.  Finally, layer 5 was deposited on top.

Screen shot 2014-01-15 at 11.21.47 AM



Index Fossils

A fossil found in rock layers of only one geologic age and is used to determine the relative age of rock layers.

Criteria to be considered an index fossil:
1.  Only lived for a specific period of time
2.  Species was widespread geographically
3.  Abundant population



%d bloggers like this: