Comparison • 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 sequence. 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 • 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. Fossils • 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.