FOSSILS & RELATIVE DATING 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 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) Radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during) 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 RELATIVE DATING & AGE Relative Dating: putting rocks and geological events in correct chronological order Relative Age: how old something is in comparison to something else HOW? Use of sedimentary rocks Use of fossils Study of strata INDEX FOSSIL Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Short-lived (found only in a few layers of rock worldwide) Wide distribution (geographic range) Ex/ INDEX FOSSIL: AMMONITE Ammonite fossils are found worldwide, but they existed for only a very specific period of time this means ammonites are found in very specific layers of rock when an index fossil is found, the age of the rocks it is preserved can be determined Two Conceptions of Earth History: Catastrophism Assumption: Great Effects Require Great Causes Earth History Dominated by Violent Events Uniformitarianism Assumption: We Can Use Cause And Effect to Determine Causes of Past Events Finding: Earth History Dominated by Small- scale Events Typical of the Present. Catastrophes Do Happen But Are Uncommon Uniformitarianism Continuity of Cause and Effect Apply Cause and Effect to Future - Prediction Apply Cause and Effect to Present - Technology Apply Cause and Effect to Past – Uniformitarianism The Present is the Key to the Past Ripple Marks, Bay Beach Fossil Ripple Marks, Baraboo Range Modern Mud Cracks Fossil Mud Cracks, Virginia LAW OF HORIZONTALITY Sediments are originally deposited in horizontal layers Folds or inclines: layers must have been deformed after they were deposited. LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest) Principle of Crosscutting geologic feature which cuts another is the younger of the two features. What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know? LAW OF SUCCESSION Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A LAW OF INCLUSIONS If a rock body (Rock B) contained fragments of another rock body (Rock A), then Rock B must be younger than the fragments of rock it contained 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 Pseudofossils Look Like Fossils But Aren't Dendrites Concretions Pseudofossils Natural or Sculpture?
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