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FOSSILS & RELATIVE DATING GEOLOGIC TIME SCALE a series of time intervals that divides Earth’s history • Each layer of rock represents specific interval of time • Index fossils help determine specific period • Time periods divided by specific events like mass extinctions ROCKS TELL A STORY Rocks can tell where they were made and when Sedimentary rocks can have fossils in them Rocks can tell when mass extinctions happened LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top Supai is oldest – WHY? PALEONTOLOGY the study of fossils remains of ancient life Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like footprints Scientific dating Absolute dating (gives age in years) uses radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during) uses observation of rock layers FOSSILS Traces and preserved remains of ancient life found within rock layers Fossils show: Biodiversity How species have changed over time Correlation between rock layers from around the world Relative ages to particular strata Evidence for the geological time scale SCIENTIFIC DATING Absolute Dating: numerical dating to give rocks an actual date or date range, in number of years Relative Dating: compare how old something is in comparison to something else; used to put rocks and geological events in correct chronological order HOW? Use sedimentary rocks Use fossils Study strata INDEX FOSSIL Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Exists over a short geologic time range (found only in a few layers of rock) Wide distribution (geographic range is world-wide) Ex/ INDEX FOSSIL: AMMONITE Ammonite fossils are found worldwide, existed for only a very specific period of time This means ammonites are found in very specific layers of rock; Once we know the ammonites, then we can determine the age of any fossil found next an ammonite fossil. Quiz yourself! What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know? • Radiometric dating uses decay of unstable isotopes. – Isotopes are atoms of an element that differ in their number of neutrons. – A half-life is the amount of time it takes for half of the isotope to decay.