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TIME ITS MEASUREMENT

TIME ITS MEASUREMENT. THE TWO TYPES OF TIME. Relative time—two events; known is their relation to each other but not the time between Absolute time—two events; known (in some time units) is the time between them EXAMPLES. EARLY USE OF TIME IN GEOLOGY. Relative—the geologic time column

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TIME ITS MEASUREMENT

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  1. TIMEITS MEASUREMENT

  2. THE TWO TYPES OF TIME • Relative time—two events; known is their relation to each other but not the time between • Absolute time—two events; known (in some time units) is the time between them • EXAMPLES

  3. EARLY USE OF TIME IN GEOLOGY • Relative—the geologic time column • A great deal can be (and has been) done • Based on understanding how rocks are formed and… • Superposition • Cross-cutting relationships • Derived fragments

  4. What’s up?? • Rocks and rock layers may be • twisted, • tilted, • folded, • turned upside down • Features in rocks show ‘up’

  5. Superposition • Sedimentary (or in some cases, lava) rock • Need other evidence of ‘up’ • Younger on top

  6. Cross-cutting relations • Any rock • Feature which cross-cuts is younger

  7. Cross-cutting relations • An intrusive rock (an igneous dike) • Also (by some) called the ‘law of intrusion’—any rock may be intruded • The intrusion is younger

  8. Cross-cutting relations • An unconformity • Marks a time of loss of record—also an errosional surface • By two features, it is younger

  9. Derived fragments • Sedimentary (can be applied to some igneous) • Also ‘law of inclusion’ • Rock containing the derived fragments as inclusions is younger

  10. Derived fragments • These sedimentary layers are upside down!

  11. Early Geologic Column--simple • Quaternary* • Tertiary* • Secondary • Primary

  12. Modern Geologic Time Column • Modern column has absolute dates as well—ignore for the moment

  13. Geologic time column

  14. ABSOLUTE AGEDATES • The problem in geology • Need a clock that operates over looong times • That is accurate even over looong times • One that keeps a record of the passage of time • And is a part of the rocks and may be preserved

  15. Absolute time • The solution was not available until approximately 1950; needed • An understanding of isotopes and radioactivity • Accurate ways of measuring the ratios of isotopes present in a sample • An accurate determination of half-lives and decay processes

  16. Absolute time • These became available following the research into atomic energy during and after WWII; and the availability of that information • Now, isotopic determinations, using a mass spectrograph, are routine

  17. Radioactive age dating • Presently usable on • igneous and metamorphic rocks (give date of solidification and of metamorphism) • Carbon bearing materials that were once living and are less than about 60,000 years old (gives date of death) • There are specific procedures and problems for each set of isotopes and type of rock

  18. Radioactive age datingan example—K40 • Decay – K40+e-  Ar40; ½-life = 1.3 by • Magma – K common, Ar is rare; K fits in many minerals, Ar (a noble gas) doesn’t • Let K represent a K40 atom, A represent an Ar40 atom (daughter) derived from a K40, and ‘+’ represent a K39 atom • As far as a mineral is concerned, all isotopes of K are chemically the same; and Ar is not a fit, but it is physically trapped in the crystal lattice as a decay product (daughter atom)

  19. Crystallization of a K mineralonly a tiny part of lattice shown • ++++K+++++++K++K+++++++ K++++++K++++++++++K++++ ++K++++++++++++K++++++++ K+++++K++++++++++++++K++ +++++++++++K+++++++++++ +++K++++K++++++K+K++++++

  20. After one half-life; Ar:K40 = 1or after 1.3 billion years • ++++A+++++++A++K+++++++ K++++++K++++++++++A++++ ++K++++++++++++K++++++++ K+++++A++++++++++++++A++ +++++++++++A+++++++++++ +++K++++A++++++A+K++++++

  21. After 2 half-lives; Ar:K40= 3or after 2.6 billion years • ++++A+++++++A++K+++++++ K++++++A++++++++++A++++ ++A++++++++++++A++++++++ K+++++A++++++++++++++A++ +++++++++++A+++++++++++ +++A++++A++++++A+K++++++

  22. Other ratios • A graph or a math formula can be determined and is used for other ratios of Ar to K-40 (including fractional ratios)

  23. FOSSILS • There are two aspects to fossils • As remnants of life forms and how they are formed, preserved, and interpreted • As a way of doing another type of relative age dating

  24. Fossil = remnant of life form • Defined – remnant or evidence of a life form, preserved in the geologic past • Remnants are usually hard parts—bone, teeth, shell, scales, claws, seeds (rare), pollen; these don’t rot or are not eaten (or are passed undigested) • Evidence—tracks, footprints, trails, imprints, casts, carbon outlines, etc. • Geologic past—if it smells, it belongs to biology

  25. Fossil tracks • Probably Jurassic reptile tracks • Note the hammer at top-right for scale • 1966, Hartford, Connecticut (now a park)

  26. Fossil dinosaurs • Top – Triceratops, Cretaceous • Bottom – Stegosaurus, Jurassic • Both reconstructed and at the Amer. Museum of Nat. History

  27. Fossil ‘bird’ • Probably one of the best known of all fossils • Archaeopteryx, a toothed, earliest bird, Jurassic, Bavaria • Amer. Museum of Nat. History

  28. Fossil trilobites • Trilobites, Ordovician, ?

  29. Fossil invertebrates • In order – clam, clam, clams, horn (solitary) coral • Mid continent U. S., Devonian

  30. ‘trapping’ and preservation of fossils • #1-quick burial • #2-hard parts • By far the most common—marine creatures—widespread seas with abundant life and burial by sediments • Rarest—hominids, jelly fish, forest birds—land creatures are rarely trapped and buried and jelly fish have no hard parts

  31. ‘trapping’ and preservation of fossils—Rancho La Brea—Hancock Park Once in a while things work exactly right—in L.A., pits containing oil seeps were commonly water holes for the land animals for about the last 50,000 years; many stepped or got pushed into the sticky tar and trapped—the tar is also an excellent preservative, preserving seeds, skin, feathers, hide, fur, small and large animal bones

  32. La Brea Tar Pit drawing

  33. ‘trapping’ and preservation of fossils—generalized • In the seas past and present—moderately common—maybe 1 in 10,000 • On the land—maybe 1 in 10 million • Alpine forests and deserts—maybe 1 in 100 million • Then preserving for a looong time, finding and recognizing

  34. Fossils for relative dating • After many of the major sedimentary rock units were dated relatively, it was discovered that many forms of life in the seas succeeded one another in an consistent manner • This came to be a commonly used and useful way to do relative dating referred to as ‘using faunal succession’; there are probably more than 5000 references detailing examples of faunal succession

  35. Fossil foraminifera • These are drawings of one of the more important fossils used in relative age dating • Actual size 0.1 – 1 mm • Widely used in the petroleum industry • Small, common, highly varied in shape over time, easily recoverable

  36. Look again at the Geologic time column • All not to be memorized • Major units and ~times

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