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• Relative Age dating is a method of estimating the age of a
rock or rock layer in relation to other rocks that exist in the
Relative Age Dating
Unit Essential Question:
How does Earth’s past give clues to its present and future?
Presentation Objectives:
§ Review the different Geologic Laws and Principles used for relative age dating.
§ Explain how fossils can be used to determine the period during which a rock formed.
Additional Principles
• There are a few extra geologic principles that will make
relative age dating easier.
• The first is the unconformity, which is a gap in the
rock record due to erosion.
Represented by a wavy line.
• We use the laws of geology to determine who is older and
who is younger.
• Law of Horizontality
• Law of superposition
• Principle of Cross-Cutting
Additional Principles
• Principle of Inclusions
• If a rock layer includes pieces from another layer, the
pieces must be older.
• Fossils are the evidence or remains of once-living organisms.
• Sometimes (although it is very uncommon) the actual remains of
the organism are preserved in their original form.
• Need perfect conditions for this to happen.
• Mummification, freezing, encasement in amber, encasement
in petroleum.
Relative Age Dating With Fossils
• Each organism lived for a specific amount of time in
geologic history.
• Sometimes a layer will contain fossils from multiple species that
lived at different times.
Where they overlap is most likely the age of the rock layer.
• Often the remains are altered.
Mineralization - pore spaces within the fossil are filled with minerals.
• Sometimes an impression or trace is left instead.
Impressions - organism leaves and imprint that is lithified.
• Molds - deep imprints that fill with sediment or minerals.
• Traces - evidence of the organism.
• Footprints, dwellings, tools, etc.