1. Geologic Time

3. 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)

4. 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

5. Superposition illustrated by strata in the Grand Canyon

6. 2nd, 3rd principles of relative dating Principle of original horizontality Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting) Principle of cross-cutting relationships Younger features cut across older ones

7. Cross Cutting Relationships in strata

8. Question: How many geologic “events” mustA) 4 have occurred in this region to produceB) 6 the contacts shown here? “Events” can C) 8 Include: 1) periods of deposition; 2) uplift andD) 10 erosion; 3) burial and metamorphism orE) 12 4) igneous intrusion?

9. Grand Canyon younger strata cutting across older ones Cambrian Tapeats sandstone over Precambrian Unkar Group

10. Grand Canyon younger strata cutting across older ones Cambrian Tapeats sandstone over Precambrian Unkar Group

11. 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 sedimentary rocks overlain by flat-lying sedimentary or volcanic 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)

12. Formation of an angular unconformity

13. An angular unconformity at Siccar Point, England

14. Development of a Nonconformity The basement-cover contact near Boulder (Pennsylvanian sandstone over Precambrian granite) is a nonconformity (visible on Flagstaff Road near bouldering area)

15. Nonconformity in the Grand Canyon - Sediment deposited over Schist

16. Nonconformity in the Grand Canyon - sediment deposited over schist What evidence tells you this is a nonconformity? A)Non-sedimentary rocks located beneath the contact B)Angular discordance in sediments across the contact C)Weathering profile at contact D)Loss of time across the contact

17. Nonconformity on Flagstaff Mountain - Fountain Formation over granite Weathered Boulder Creek Granodiorite (1700 Ma) Fountain Formation (300 Ma) Silver Plume Dikes (1400 Ma)

18. What is the geologic history of the Boulder region as revealed by this roadcut? A)Intrusion at 1700 & 1400, Deposition at 300 B)Uplift at 2000, Intrusion at 1700 & 1400, Erosion at 1400-300, Deposition at 300 C)Intrusion at 1700 & 1400, Uplift & Erosion between 1400-300, Deposition at 300 D)Intrusion at 1700 & 1400, Deposition from sometime before 300 to-300, Uplift and Erosion just before 300, then Deposition after 300 Weathered Boulder Creek Granodiorite (1700 Ma) Fountain Formation (300 Ma) Silver Plume Dikes (1400 Ma)

19. What is the geologic history of the Boulder region as revealed by this roadcut? A)Intrusion at 1700 & 1400, Deposition at 300 B)Uplift at 2000, Intrusion at 1700 & 1400, Erosion at 1400-300, Deposition at 300 C)Intrusion at 1700 & 1400, Uplift & Erosion between 1400-300, Deposition at 300 D)Intrusion at 1700 & 1400, Deposition from sometime before 300 to-300, Uplift and Erosion just before 300, then Deposition after 300 Weathered Boulder Creek Granodiorite (1700 Ma) Fountain Formation (300 Ma) Silver Plume Dikes (1400 Ma)

20. Correlation of rock layers Matching strata of similar ages in different regions is called correlation

21. Correlation of strata in southwestern United States Sections are incomplete

22. Correlation of rock layers with fossils Correlation relies upon fossils Principle of fossil succession – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

23. Determining the ages of rocks using fossils

24. 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 – the greatest expanse of time

25. Geologic Timescale Names based on fossils Numbers based on radio metric age dates Numbers based on absolute age dates For exam Know Eras Periods and Epochs and Their order

26. Geologic Times Scale Mnemonic’s… A.Parents Can Only Sit Down Carefully; Perhaps Their Joints Can't Tolerate Quickness winner by a longshot B. Please Come Over, Sally Dean, Might Prepare Pasta Then Juice Cantaloupes Too Quickly C. Quoting Tim Curry Just Takes Patience Most Dancers Seem Outstanding Counting Presley D. Quadriplegics Tantalize Congressmen Just To Persuade Paralyzed Men Disregarding Strippers Or Coloradans 2nd E. Poor clueless onlookers slowly discussed many pretty perturbed thirsty joggers cursing their quest 3 rd F. Penetration causes orgasms strongly developing many phenomenally Pleasurable Times Justifiably Causing Thrilling Quivers "Cold Oysters Seldom Develop Many Precious Pearls, Their Juices Congeal Too Quickly." Another device for remembering the order of the Epochs of the Cenozoic era is, "Pigeon Egg Omelets Make People Puke Heartily."

28. Geologic time scale Structure of the geologic time scale Names of the eons –Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago –Proterozoic –Archean –Hadean – the oldest eon

29. This, believe it or not, is the rock with the oldest known minerals ever found. From NW Australia, the rock (a conglomerate) is about 3.0 Billion years old. The rock contains detrital grains of zircon (a mineral formed in granite in the crust) that is 4.4 Billion years old. Age of the Earth is 4.567 Billion ( ancient zircons first discovered by Simon Wilde, Curtin Institute of Technology, Australia). Rock and additional age dating courtesy of Steve Mojzsis) Amazing because Earth must have differentiated quickly (<100Ma)

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

31. Geologic time scale Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into small time units because the events of Precambrian history are not know in detail Immense space of time (Earth is ~ 4.5 Ga)

32. Radioactivity (Used to age date rocks) Spontaneous changes (decay) in structure of atomic nuclei Types of radioactive decay Alpha emission –Emission of 2 protons and 2 neutrons (an alpha particle) Beta emission –An electron (beta particle) is ejected from the nucleus Electron capture –An electron is captured by the nucleus –The electron combines with a proton to form a neutron

33. Neutron capture (A) and Beta emission (B)

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

35. A radioactive decay curve

37. Dating with carbon-14 (radiocarbon 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 (for me this is the history of earthquakes).

38. The daughter-to-parent ratio (D/N) of two isotopes in a rock is 7:1 and the half-life for conversion between the parent and daughter is 100 million years. How old is the rock? A)200 million years B)300 million years C)500 million years D) 700 million years E) none of the above

39. The daughter-to-parent ratio (D/N) of two isotopes in a rock is 7:1 and the half-life for conversion between the parent and daughter is 100 million years. How old is the rock? A)200 million years B)300 million years C)500 million years D) 700 million years E) none of the above

40. Question worth 3 points if correct, 1 point if not… Please work by yourself on this one… A 400 million year old rock is dated using an isotopic technique where the parent isotope has a half life of 100 million years. What are the percentages of parent and daughter isotope left in the rock Parent Daughter A)50% 50% B)25% 75% C)12.5% 87.5% D) 6.25% 93.75% E) 3.12% 96.88%

41. Question worth 3 points if correct, 1 point if not… Please work by yourself on this one… A 400 million year old rock is dated using an isotopic technique where the parent isotope has a half life of 100 million years. What are the percentages of parent and daughter isotope left in the rock Parent Daughter A)50% 50% B)25% 75% C)12.5% 87.5% D) 6.25% 93.75% E) 3.12% 96.88%

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

43. How do we actually “date” a rock? 1.Collect sample (geologist = pack animal) 2.Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids 3.Measure parent/daughter ratio of mineral separates with a mass spectrometer

44. Dating sediments without fossils Use volcanic rocks instead…

45. Question: What is the age of the Dakota Sandstone? A) Older than 160 Million B) Younger than 160 Million C) Younger than 160 Million and older than 66 Million D) Younger than 66 Million E) None of the above

46. End of Chapter 8