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Geologic Time

Geologic Time. 1. Goals for understanding geologic time Relative dating vs. Absolute dating Relative dating principles – how geologists date layers of rock The dynamics of fossils Techniques of radiometric dating Why the earth is believed to be 4.6 b.y. The Geologic Time Scale

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Geologic Time

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  1. Geologic Time 1

  2. Goals for understanding geologic time • Relative dating vs. Absolute dating • Relative dating principles – how • geologists date layers of rock • The dynamics of fossils • Techniques of radiometric dating • Why the earth is believed to be 4.6 b.y. • The Geologic Time Scale • what problems exist with the • geologic time scale 2

  3. Which geologic event took place first and • when? • Which rock layer is older, and how is earth • history deciphered? • How do we assign actual years to rock layers? Geology needs a Time Scale Consider: 1 2 ss Shale (sh) 3 3 Limestone (LS) 4 5 Sandstone (ss)

  4. Relative Dating - placing the geologic occurrence in the proper sequence • Which came first and WHY? • To construct a “relative” time scale, rules were • established (principles of relative dating). • Nicholas Steno (1636-1686) • Principle of Original Horizontality • Law of Superposition • Principle of Cross-Cutting Relations • Principle of Inclusions 11

  5. Let’s unravel some geologic history from observations of various formations and their contacts. Nicholas Steno – 1669 proposed the following relative dating principles: • The Principle of Original Horizontality: • Sedimentary rock layers are deposited as horizontal strata. • Any observed non-horizontal strata have been disturbed. Sediment input C B basin A 12

  6. Original Horizontal Strata Limestone (ls) Shale (sh) Sandstone (ss) granitic rock 13

  7. The Principle of Superposition • In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence, and the youngest stratum • is on top. Unit 1 = old Unit 5 = young 5 4 3 2 1 14

  8. Which strata is older? youngest 5 4 3 2 1 5 4 oldest 3 2 1 15

  9. The principle of Cross-Cutting Relationships • Any geologic feature that cuts across another geologic feature is younger. 5 Unit 1 = older Unit 6 = youngest 4 3 2 6 1 Which came first: Unit 5 or Unit 6? 16

  10. Which is older, the fault or volcanic layer? Which is younger, the dike or country rock? fault dike Volcanic layer What type of fault is this? country rock Normal Determine the relative age of the two dikes. 1 2 17

  11. The Principle of Inclusions • A piece of rock (clast) that has become “included” • in another rock body is older than the rock body • it has become part of – why? Rock body A A A A Older (Rock A was there first.) Intrusion of pluton B 18

  12. Which “granites” are older and younger? OLDER YOUNGER 19

  13. Which rock body is older?: B A ? ? C Can you identify the inclusions found in this Sierra Nevada Mountain batholitic material? 20

  14. Original Horizontality Youngest Superposition Oldest Principle of Inclusions Cross-Cutting Relationship Which granite is older? A B C Asp Vn Older Younger 21

  15. I this geology science class. Discuss with a friend: • Explain the concept of relative dating. • Draw a diagram, and explain each of the • following dating principles: • Original Horizontality • Superposition • Cross-Cutting Relations • Inclusion Principle I will get an A on my exams and quizzes. 22

  16. Ok – given the principles, what is wrong with this stack of rock (strata) youngest 7 6 5 3 2 1 oldest Missing time – or does time really stop? 23

  17. The principle of Unconformities • rock surface that represents a period of erosion or non- deposition • referred to as “missing time” • three major types of unconformities: • disconformity • angular unconformity • non-conformity disconformity – unconformity in non-disturbed sedimentary layers angular unconformity – uncon. lies between angled strata and overlying horizontal strata non-conformity – sedimentary strata overlies crystalline rocks (ig and met) Unconformity Igneous or metamorphic rock 24

  18. disconformity angular unconformity Sedimentary rocks nonconformity Xln rocks 25

  19. I this geology science class. • Explain what an unconformity is and • what it represents • 2. Diagram pictures that represent the • three types of unconformities Discuss with a friend I will get an A on my exams and quizzes 26

  20. Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric life Paleontology – study of fossils • How do we get a fossil? – preservation of past life • 2 conditions must exist for preservation • rapid burial • possession of hard parts Prehistoric bug Rapid burial of sediment covers the bug – fossil Bug dies Bug soft parts are eaten or dissolve 27

  21. Fossils – evidence of past life or “time pieces,” the remains or traces of prehistoric life • Preservation of fossils • Small percentage of fossils preserved • throughout geologic time – WHY? • Most organisms composed of soft parts. • Organisms with hard parts and within • a sedimentary environment are favored. • Very rare to see vast array of other life • forms How do fossils help scientists relatively date layers of rock (strata)? 28

  22. William Smith – Principle of Fossil Succession Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. “Fossils are arranged according to their age by using the law of superposition.” • Fossil succession: • allows geologists to age date wide geographical • areas • documents the evolution of life • Age of mammals • Age of reptiles • Age of fish Youngest Oldest 29

  23. How do fossils help date rocks? 1200 miles 7 7 6 6 Disconformity 5 4 3 3 2 2 Which fossils are the youngest and oldest? 1 30

  24. 31

  25. I this geology science class. • Give 2 reasons why many organisms are • are not fossilized. • Explain the law of fossil succession and • how this law allows dating of strata. • How has fossil succession helped geologists • unravel earth history? Discuss with a friend: I will get an A on my exams and quizzes. 32

  26. OK – We have relative dating and fossils – How do we get “absolute” ages on the rocks (numbers)? • Radiometric dating – applying a number • radioactive atoms (isotopes) decay at a • constant rate over time 33

  27. Radioactive decay of an unstable isotope atom + + + + Decay process + + + Pb206 (lead) + + U238 (Uranium) + + • The time of decay can be measured. • Isotope decay does not vary under various • weathering conditions. • Isotopes decay at a fixed rate. • One isotope will decay into another isotope. 34

  28. How does radiometric dating work, and where does the age (number) come from? 35 Terms: Parent element: the “beginning” element that contains 100% of radioactive particles Daughter element: the element that the parent element decays into (or turns into over time) Half life: the time required for ½ of the parent to decay into the daughter element

  29. U-235 Pb 207 1 half life = 704 million years U-3 Daughter element 1/2 1/2 1/2 704 m.y. 1.4 b.y. 2.1 b.y. Parent element 36

  30. I geology science class. • Specifically define the differences • between relative and absolute dating • techniques. • 2. Define the following absolute dating terms: • parent/daughter elements, half-life • 3. Explain how the half-life is used to • calculate an absolute age. I will get an A on my exams and quizzes. 37

  31. What is the importance of radiometric dating? • produced thousands of dates for earth • events • rocks have been dated at more than 3 b.y. • granite in South Africa dated at 3.2 b.y. • granite contains inclusions of quartzite • quartzite inclusions must be older • Acasta gneiss in Northern Canada – 4.0 b.y. • Earth believed to be 4.55 (4.6) b.y. old • Radiometric dating: • vindicated the ideas of Hutton, Darwin, and others • consistent with relative dating techniques • allowed “absolute” dating on the Geologic Time Scale 38

  32. Lets make a Geologic Time Scale Relative dating + Absolute dating • The Geologic Time Scale: • It combines both relative and absolute dating • Created during the nineteenth century in Western Europe • and Great Britain • Sub-divides the 4.6 billion-year-history of the earth • Eons • Eras • Periods • Epochs 39

  33. 40 • Phanerozoic • “visible life” • fossil record becomes more • detailed • animals have hard shells • and skeletons Building the Geologic Time Scale • Proterozoic • Multi-celled, soft body • organisms • “early life” Precambrian • Archean • Single cell life developed • most “ancient” rocks found • preserved rocks at the base • of the Archean • Hadean • represents the earth’s • time of formation • no rocks are represented • “hellish” conditions

  34. 41 • Cenozoic Era • birds and mammals • flourished • appearance of man • Mesozoic Era • marks the rise in dinosaurs • dominant vertebrates • first flowering plants • first shrew-like • mammals • Paleozoic Era • known as ancient life • life progressed from marine • invertebrates to fish, • amphibians, and reptiles

  35. 42 • Periods based on: • fossil types • massive extinctions • geographical locations • characteristics of strata • Cretaceous, Jurassic, Triassic • age of reptiles • dinosaurs dominant • massive dinosaur extinction • at 65 m.y. –Cretaceous • “Jurassic Park” • Cambrian period • animals with hard shells • diversification of life • “the Cambrian explosion”

  36. 43 • Epochs • not defined by extinction • events, but % of fossils • still living • plants and animals found • in the Pliocene epoch • have living species today • Eocene-few species • surviving today • Holocene • human’s time Age of Reptiles Amphibians Age of fish Invertebrates How accurate is the Geologic Time Scale?

  37. 44 I the geologic Time Scale. • You should be able to draw the Geologic • Time Scale and label it with the following: • Eons, Eras, Periods, and Cenozoic/ • Tertiary epochs. • 2. List major characteristics of each • period. • 3. How did the strength of both absolute and • relative dating techniques contribute the • development of the geologic time scale?

  38. The Geologic Time Scale – How much of Earth history is represented? Geologic Time Scale Cenozoic, Mesozoic, Paleozoic Eras 12% 88% Precambrian Eon 45

  39. Difficulties in dating the Geologic Time Scale • Not all rocks can be dated radiometrically. • all minerals must contain 100% parent atoms. • Sedimentary rocks can only rarely be dated. • some parent atoms come from pre-existing rocks • that have been weathered and transported. • sedimentary rocks are dated in proximity of • igneous bodies. • Metamorphic rocks are challenging. • some minerals do not necessarily represent the • time when the rock was formed 46

  40. How is the age of the Earth determined? • Why is it difficult to determine the age • of the earth? (think rock cycle) • The external and internal forces constantly • recycle earth material, obliterating rock clues to • the earth’s past. What evidence suggests a 4.6 b.y. old earth? • Precambrian rocks (Acasta gneiss, northern • Canada) date at 4.0 billion years. • Mineral grain found in sedimentary rock • (Australia) dates at 4.4 billion years. • What does the mineral grain in a • sedimentary rock indicate about the • 4.4 b.y. age relationship? 47

  41. Acasta gneiss, northern Canada • known as the Acasta gneiss complex • dated at the Hadean Eon (4.0 billion years old) • part of the Canadian Slave craton 48

  42. Evidence from space to age date the earth-- • moon dust and meteorites: • moon dust from Apollo astronauts dated at • 4.55 billion years • the Allende Meteorite: • a carbonaceous chondrite meteorite that was found in • Chihuahua, Mexico, 1969 • contains unaltered material from the formation of • the solar system • composed of tiny amounts of carbon that form the • compounds of amino acids (essential for life) • age dating of this and other meteorites is around • 4.55 billion years • based on earth rocks and interstellar space objects, • earth is believed to be around 4.6 billion years old. 49

  43. Allende Meteorite, Chihuahua, Mexico, Feb. 8, 1969 • unaltered material from our • solar system • contains carbon (3 parts/1000) • some carbon compounds in • the form of amino acids dark areas – olivine with trace amounts of iron and carbon • calcium and aluminum oxide • compounds • first matter to form during • solar system formation • older than earth carbonaceous chondrite 50

  44. I what geology can do for me. Discuss with a friend: • Identify at least 2 problems with the • accuracy of the Geologic Time Scale. • 2. Why does the Geologic Time Scale only • represent about 12% of the earth’s geologic • history (assuming the earth is 4.6 b.y. old)? I will get an A on my exams and quizzes. 51

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