Geol162 geologic time


Published on

For Geologists

Published in: Education
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Geol162 geologic time

  1. 1. eological Time - really, really, really long! Motion pictures are generally projected at 32 frames per second. Therefore, each frame (image) is on the screen for only split second- let each frame represent 100 years. Start movie at present and go back in time. •The Declaration of Independence would show up 1/16 of a second into the movie. •The Christian era (BC-AD boundary) would be 3/4 of a second into the movie. •The most recent Ice Age would be 7 seconds into it. •The movie would run about 6 hours before we got to the end of the Mesozoic era (extinction of the dinosaurs). •Wed have to watch the movie for about 2 days to see the beginning of the Paleozoic era (macroscopic life). •The whole movie (to the beginning of geologic time on
  2. 2. QuickTime™ and aTIFF (Uncompressed) decompressor are needed to see this picture.
  3. 3. Geologic Time• Two ways to relate time in geology: > Relative: Placing events in a sequence based on their positions in the geologic record.> Chronologic : Placing a specific number of years on an event or rock sample.
  4. 4. Geologic Time Scale• a combination of the two types of age determinations > a relative sequence of lithologic units - established using logical principles > measured against a framework of chronologic dates.
  5. 5. Geologic Time and the "geologic column" • Developed usingusing logical rules relative Developed logical rules to establish to establish sequences of events relative sequences of events - superposition - cross-cutting relationships - original horizontality - lateral continuity • Added to as new information is obtained and refined is refined data - Use of fossils for correlation and age determination • Numerical Dates attached to strata after the - development of Radiometric techniques Still being refined as more information becomes available
  6. 6. The Geologic Time Scale (1:2)
  7. 7. The Geologic Time Scale (2:2)
  8. 8. Relative Dating Methods• determines the relative sequence of events. > which came first, which came last. > no numeric age assigned• 6 Relative age principles: > Superposition > Original Horizontality, > Lateral continuity > Cross-cutting Relationships > Inclusions > Fossil succession. Those in yellow are most useful
  9. 9. History of Historical Geology • Niels Stensen (Nicolaus Steno) - Fundamental Principles of Relative Time > Principle of Superposition- see below > Principle of Original Horizontality- see below > Principle of Original Lateral Continuity- see below
  10. 10. Law of Superposition• In undisturbed strata, the layer on the bottom is In undisturbed strata, the layer on the bottom is oldest, those above are younger.
  11. 11. Original Horizontality• Sediments are generally deposited as horizontal layers. Lateral Continuity• Sediment layers extend laterally in all direction until they thin & pinch out as they meet the edge of the depositional basin.
  12. 12. Charles Lyell • 1st Principles of Geology text - included description and use of > principles of cross-cutting relationships > principles of inclusions • relative time tools
  13. 13. Cross-cutting RelationshipsThat which cuts through is younger than theObject that is cut dike cuts through granite is cut
  14. 14. Relative Ages of Lava Flows and Sills
  15. 15. Principle of Inclusions• Inclusions (one rock type contained in another rock type) are older than the rock they are embedded in. That is, the younger rock contains the inclusions
  16. 16. Principle of Inclusions
  17. 17. Faunal/Floral Succession•• Fossil assemblages (groupings of fossils) succeed one another through time.
  18. 18. • Correlation- relating rocks in one location to those in another using relative age stratigraphic principles- Faunal Succession- Superposition-- Lateral Continuity-- Cross-cutting-
  19. 19. Unconformities• surfaces represent a long time. a time when rocks were not deposited or a time when rocks were erodedHiatus the gap in time represented in the rocks by an uncon- formity 3 kinds Angular Unconformity Nonconformity Disconformity
  20. 20. Disconformities A surface of erosion or non-deposition betweenParallel sedimentary rock beds of differing ages.
  21. 21. Angular Unconformities• An angular unconformity is an erosional surface on tilted or folded strata, over which younger strata have been deposited.
  22. 22. NonconformitiesA nonconformity is an erosional surface on igneous ormetamorphic rocks which are overlain by sedimentary rocks.
  23. 23. Breakout in to groups and discuss the sequenceobserved here
  24. 24. Age Estimates of EarthCounting lifetimes in the BibleComparing cooling rates of iron pellets.Determine sedimentation rates & compareEstimate age based on salinity of the ocean.all age estimates were off by billions of yearssome were more off than others!
  25. 25. Absolute Dating Methods Radioactive Decay sequences acts as an atomic clock we see the clock at the end of its cycle analogous to starting a stopwatch allows assignment of numerical dates to rocks.> Radioactive isotopes change (decay) into daughter isotopes at known rates. rates vary with the isotope + + 235 40 14 e.g., U , K, C, etc.
  26. 26. Decay unstable nuclei in parent isotope emits subatomic particles and transform into another isotopic element (daughter). does so at a known rate, measured in the lab• Half-life The amount of time needed for one-half of a radioactive parent to decay into daughter isotope. Assumptions?-you bet Cross-checks ensure validity of method.
  27. 27. Rate of Decay All atoms are parent isotope or somet 0 known ratio of parent to daughter 1 half-life period has elapsed, half of thet 1 material has changed to a daughter isotope (6 parent: 6 daughter) 2 half-lives elapsed, half of the parentt 2 remaining is transformed into a daughter isotope (3 parent: 9 daughter) 3 half-lives elapsed, half of the parent remaining is transformed into a daughtert 3 isotope (1.5 parent: 10.5 daughter) We would see the rock at this point.
  28. 28. Radioactive Isotopes • analogous to sand in an hour glass - we measure how much sand there is > represents the mass of elements - we measure the ratio of sand in the bottom to sand in the top - at the end (present) > daughter (b) and parent (t) - we know at what rate the sand falls into the bottom > the half life of the radioactive element - how long would it take to get the amount sand in the observed ratio starting with all of it in the top? 100 % parent remaining Parent 50 Daughter 25 13 time----------->
  29. 29. Five Radioactive Isotope Pairs EffectiveDating Range Minerals and Isotopes Half-Life of Parent (Years) Rocks That Can Parent Daughter (Years) Be DatedUranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion UraniniteUranium 235 Lead 207 704 million MuscoviteThorium 232 Lead 208 14 billion 48.8 billion Biotite Potassium feldsparRubidium 87 Strontium 87 4.6 billion 10 million to Whole metamorphic 4.6 billion or igneous rockPotassium 40Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock
  30. 30. Radiocarbon and Tree- Ring Dating Methods•• Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample. > Valid only for samples less than 70,000 > Valid only for samples less than 70,000 years old. years old. > Living things take in both isotopes of > Living things take in both isotopes of carbon. carbon. > When the organism dies, the "clock" starts. > When the organism dies, the "clock" starts.Method can be validated by cross-checking with treerings
  31. 31. Carbon 14 Cycle
  32. 32. Recognizing Patterns of changeWalthers Law • The vertical sequence is repeated by the horizontal sequence- walking from A to B to C to the Coast you would encounter the rocks that would be encountered by drilling a core into the earth at any point (A, B, or C)
  33. 33. Facies Diagram • distribution of lithofacies (rock-types) - these are associated with their respective EOD • biofacies are similar but refer to fossils rather than rock types
  34. 34. Eustasy, relative sea-level, and relative positionof lithofacies • Eustasy= changes in volume of water in ocean • lithofacies depend on - sea-level - land level - geometry of coast - sediment supplyVail Curve • an attempt at global • correlation of lithologies - for better production - of petroleum resources
  35. 35. Rock designations • Rock units called Lithostratigraphic units - described in terms of Group, Formation, & Member > each term has specific meanings in geological parlance • Formation - a mappable lithostratigraphic unit > has a location for identifying the type-section > has a rock designation describing the lithology - sometimes not all the same lithology > in which case the term "Formation" takes the place of lithologic type • Groups are composed of several formations • Members are distinctive units within a formation - group is largest and contains formations and members - formations are next and contain members
  36. 36. Fundamental lithological units Formation- a rock layer with distinctivecharacteristics that is mappable over a large are at“typical” map scales 1:62,500 or more commonly 1:24,000Formations have Members smaller layers that are unique that are not mappableover larger areas and won’t show up at typical map scalesGroups have formations; formations have members