1.Guide to Rock Dating Geologic Time

2. Guide to Geologic Record How do we understand the Earth and its formation? By studying the past we may unlock the secrets of the formation of the earth AND look at patterns that may affect the Earths future inhabitants How do we examine the Earths past? By examining the evidence left behind 3. Evaluating Earths Past Methods for evaluating Earths past include: Geologic samples Rock records & fossils Tree rings Plant pollen Oxygen isotopes in glacial ice Glacial evidence Plankton and isotopes in ocean sediment 4. Geologic Records Rock formations and fossils have given us valuable information about the past Fossils provide information on organisms that have lived on Earth Their physiology helps to understand the conditions on earth at the time they lived Their physiology also gives clues as to their lifestyle feeding habits & environmental context clues Rock formations provide Clues about the atmospheric and hydrospheric processes occurring 5. Geologic Records Rock Dating How do you date a rock? Sedimentary Very gently they break up easily! Igneous These guys are too cool to be dated! Metamorphic Frequently they love change! 6. But seriously. The age of rocks is important to establish time frames and context for other data Two types of Rock Dating Relative Absolute Relative dating Order of events Absolute Dating The (measured) age in years 7. Principles of Relative Dating Principle of original horizontality Layers started flat Law of Superposition: One layer of rock is older than the one above it and younger than the one below it. Principle of crosscutting relationships A rock MUST be older than a thing that cuts it Principle of faunal succession Species lived in a recognizable order through time and relative ages can be deduced from their fossils Principle of Uniformitarianism The processes of the past are the same as they are today 8. Law of Superposition 9. Hazards to Relative Dating Conformable deposited without interruption Unconformity an interruption in deposition of the rock record. Represent a time gap. Disconformity sedimentary layers parallel to each other. Angular unconformity tilted layers with newer flat material atop Nonconformity sedimentary rocks atop igneous or metamorphic rock. 10. How do unconformities occur? 11. How do Angular Unconformities occur? A) Sedimentary Layers accumulate under the sea B) Over time the sea bed is lifted up C) Wind and water erode the layers of rock above Ocean level D) Eventually the sea rises again and deposits new sediments 12. Intrusion Igneous rock That has forced Its way into existing rock Discontinuity Sedimentary Layers were lifted Up and casued a Rift in the layers 13. What can be used to date layers? Correlation matching up rocks of similar age in different locales Index fossils accurately indicates the age of a rock Key bed a thin, widespread, synchronous sedimentary layer 14. Index Fossil Correlation 15. Absolute Dating Radiometric dating using half-lives to determine absolute ages of rocks. Half-life the time it takes for one-half the atoms of a radioisotope to decompose to another isotope or element Isotopes radioactive varieties of an element Vary by the number of neutrons Carbon -12 and Carbon 14 Uranium 238 and Uranium 235 Decay occurs at a constant rate Half-lives range from thousands to billions of years 16. As the parent Isotope decays its mass decreases and the daughter (product) isotope mass increases. The summation of the two masses will always equal 100% of the starting mass. 17. Dating Ranges Carbon Dating range 100 - 50K years Dating formerly living organisms (~ 5730 yrs) i.e. fossilized bones, shells, wood, plant material Radiometric Dating Mineral Rocks Potassium range 50 K to 4.6 billion years Uranium range 10 million to 4.6 billion years 18. Geologic Time Scale Rock and fossil dating allowed scietists to establish the Geologic Time Scale. The scale is broken into parts based on changes to the organisms that lived during that time frame: Eon broken into four parts Hadean; Archean; Proterozoic and Phanerozoic Era also broken into four parts (note the length of the 1st Era on the timescale) Precambrian, Paleozoic, Mesozoic & Cenozoic Period divides the eras Epoch divides the periods 19. Geologic Time Scale 20. Geologic Time Scale Analogies can help put things in perspective for students. There are numerous analogies out there but here is one that I like to use Eon Years Era Months Period weeks Epoch days .