Guide to rock dating chap 4


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HPU NCS2200 earth science for elementary education majors lecture on geologic time and rock dating.

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Guide to rock dating chap 4

  1. 1. Guide to Rock Dating Geologic Time
  2. 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 Earth’s future inhabitants • How do we examine the Earth’s past? – By examining the evidence left behind
  3. 3. Evaluating Earth’s Past • Methods for evaluating Earth’s 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. 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. 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. 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. 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. 8. Law of Superposition
  9. 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. 10. How do unconformities occur?
  11. 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. 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. 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. 14. Index Fossil Correlation
  15. 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. 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. 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. 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. 19. Geologic Time Scale
  20. 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 • .