1. Geologic Time Marble demo Some Index Fossils Coin Toss Sheet Color Copies of HW last page Expected Values for X 2 Time Scale.doc

2. Determining geological ages Determining geological ages Relative age dates – placing rocks and geologic events in their proper sequence Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)

3. First principle of relative dating First principle of relative dating Law of superposition Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top

4. Superposition illustrated by strata in the Grand Canyon

5. 2nd principle of relative dating 2nd principle of relative dating Principle of original horizontality Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

6. 3rd principle of relative dating Principle of cross-cutting relationships

7. 3rd principle of relative dating Principle of cross-cutting relationships

8. Unconformities (loss of rock record) An unconformity is a break in the rock record produced by erosion and/or nondeposition Types of unconformities –Angular unconformity – tilted rocks overlain by flat-lying rocks –Disconformity – strata on either side of the unconformity are parallel (but time is lost) –Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost

9. 8_9 (a) (b) (c) Layered sedimentary rocks Nonconformity Metamorphic rock Igneous intrusive rock Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks Disconformity Brachiopod (290 million years old) Trilobite (490 million years old)

10. Horizontal younger sediments over tilted older sediments Cambrian Tapeats sandstone over Precambrian Unkar Group What type of unconformity is this? Grand Canyon in Arizona

11. Formation of an angular unconformity Formation of an angular unconformity

12. Development of a Nonconformity Pennsylvanian sandstone over Precambrian granite is a nonconformity

13. Nonconformity in the Grand Canyon - Sediments deposited over Schist

14. Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist

15. Correlation of rock layers Matching strata of similar ages in different regions is called correlation http://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg

16. Correlation of strata in southwestern United States Sections are incomplete Match with fossils and lithology

17. Correlation of rock layers with fossils Correlation often relies upon fossils Principle of fossil succession (Wm. Smith) –fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it Index Fossils - useful for correlation –Existed for a relatively brief time –Were widespread and common http://www.csun.ed u/~psk17793/ES9 CP/ES9%20fossils.htm Most fossils are just impressions. A few may have small amounts of some original tissue

18. 8_10 Rock broken to reveal external mold of shell Rock broken to reveal fossil cast Shells buried in sediment Mold, or cavity, forms when original shell material is dissolved Cast forms when mold is filled in with mineral water Shells settle on ocean floor How impression fossils form (the most common type)

19. Determining the ages of rocks using overlap of fossils

20. Index Fossils Fossils can be preserved in several rock types. Note the use of overlapping fossil ranges in two distant outcrops (one is turned on its side for this illustration), even though the sediment facies are different. Two Measured Sections

21. Correlation in spite of Disconformities

22. Geologic time scale The geologic time scale – a “calendar” of Earth history Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale Eon, Era, Period, Epoch

23. Geologic Timescale Divisions based on fossils Eon, Era, Period, Epoch Homework Learn Timescale.doc less Epochs

24. Origin of Period Names

25. Geologic time scale Structure of the geologic time scale Names of the eons –Phanerozoic (“visible life”) – the most recent eon, began about 545 million years ago – PreCambrian (Cryptozoic) PreCambrian subdivisions: Proterozoic – begins 2.5 billion years ago Archean – begins 3.8 bya Hadean – the oldest eon begins 4.6 bya Read from bottom to top – Oldest to Youngest

26. Geologic time scale Precambrian time Nearly 4 billion years prior to the Cambrian period Long time units because the events of Precambrian history are not know in detail – few fossils, most rock modified Immense space of time (Earth is ~ 4.6 Ga) PreCambrian spans about 88% of Earth’s history

27. Geologic time scale Structure of the geologic time scale Era – subdivision of an eon Eras of the Phanerozoic eon –Cenozoic (“recent life”) begins ~ 65 mya –Mesozoic (“middle life”) begins ~ 248 mya –Paleozoic (“ancient life”) begins ~ 540 mya Eras are subdivided into periods Periods are subdivided into epochs

28. Using radioactivity in dating Importance of radiometric dating Allows us to calibrate geologic timescale Determines geologic history Confirms idea that geologic time is immense

29. The oldest known minerals ever found on Earth are included in some sediment from NW Australia. The rock contains detrital grains of the mineral zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons. Claims of older zircons Similar dates are known from Yellow Knife Lake, NWT, Canada Radiometric Age Determinations of the Earth

30. Radiometric Age Determinations of the Earth However, the age of the Earth is thought to be about 4.5 - 4.6 billion years Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time.

31. Recall Isotopes The number of protons in an atom's nucleus is called its atomic number –defines “element” Protons + neutrons called atomic weight The number of neutrons can vary Atoms of the same element with different numbers of neutrons are called isotopes. Some are radioactive

32. 8_19 p p p Atomic mass not changed much; atomic number increases by 1 because Neutron becomes proton (b) Beta decay Beta particle Radioactive parent nucleus Decay processDaughter nucleus Atomic mass unchanged; atomic number decreases by 1 (c) Electron capture Beta particle Atomic mass decreases by 4; atomic number decreases by 2 (a) Alpha decay Alpha particle Proton Neutron p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p Emission of 2 protons and 2 neutrons (alpha particle) An electron (beta particle) is ejected from the nucleus electron combines with a proton to form a neutron

33. Using radioactivity in dating Using radioactivity in dating Parent – an unstable radioactive isotope Daughter product – stable isotopes resulting from decay of parent Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product

34. A radioactive decay curve A radioactive decay curve 1/2 = 50% parent: 1 half-life has passed 1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed 1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed

36. How do we actually “date” a rock? 1.Collect sample 2.Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids 3.Measure parent/daughter ratio of target isotopes - mass spectrometer

37. Dating sediments without fossils Radiometric Dating with Igneous Rocks Or Bracket between fossiliferous layers Morrison Fm older than 160 my Wasatch Fm. younger than 66 my Mancos Shale and Mesa Verde Fm. older than 66 my

38. Basalt Lava flow 2 200 mya Lava flow 1 209 mya We can bracket this limestone’s age between 209 and 200 mya Even better: we get lucky. A layer we need to date is between two datable beds So we have and upper and lower bound on the age of this limestone:

39. Mineral crystal Mineral crystal formed in igneous rock Parent atoms Daughter atoms 1 Igneous rock buried beneath younger rocks; daughter atoms formed by normal decay 2 Dating a crystal (3) We calculate age based on half-life

40. 8_22b Deep burial and metamorphism during mountain building causes daughter atoms to escape from crystal 3 After mountain building ends, accumulation of daughter atoms in crystal resumes 4 Heat Resets the clock But IF: Easily recognized, useful in studying metamorphism Rock looks as if it just formed: it looks young Age found dates from metamorphic event

41. Dating with carbon-14 (Carbon Dating) Half-life only 5730 years Used to date very young rocks Carbon-14 is produced in the upper atmosphere Useful tool for geologists who study very recent Earth history

42. 8_24 Atoms split into smaller particles, among them neutrons Neutrons strike nitrogen atoms Nitrogen atoms lose a proton and becomes carbon-14 C-14 mixes with atmospheric oxygen to produce CO 2 CO 2 taken up by plants C-14 absorbed by living organisms CO 2 dissolved in water C-14 intake ceases when organism dies; C-14 concentration decreases Cosmic rays bombard atmospheric atoms Carbon-14

43. 8_27 Sediment layers with tree logs to be collected for dendrochronology Annual-ring similarities show correlation Current year Years of age 50 100 150 200 400 500 Buried tree logs Tree growth rings A A B B C C D D Tree Rings

44. 8_28 Turbid water Heavy runoff into lake Very little or no runoff Summer layer (coarse, thick, and light-colored) Clear water Winter layer (fine, thin, and dark-colored) Summer Winter Ice Dating with Lake Varves Modern Lakes, just count back from present. Fossil pollen track climate. Southern lakes track glaciation

46. http://bcornet.tripod.com/Cornet94/Cornet94.htm

47. http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html Hanneke Bos

48. End of Geologic Time Lecture