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Dike A

What is the sequence of events that shaped this geologic setting?. Dike B. Dike A. Fault 2. Fault 1. TODAY: Chapter 10 Part 2 Geologic Time 3) Fossils as time-pieces 4) Absolute dating w/ radioactivity 5) Geologic time scale 6)Regional correlation of rock layers.

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Dike A

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  1. What is the sequence of events that shaped this geologic setting? Dike B Dike A Fault 2 Fault 1

  2. TODAY: Chapter 10 Part 2 Geologic Time 3) Fossils as time-pieces 4) Absolute dating w/ radioactivity 5) Geologic time scale 6)Regional correlation of rock layers

  3. 3. Fossils as timepieces • Fossils William Smith (late 1700s) noted that sedimentary strata in widely separated area could be identified and correlated by their distinctive fossil content • fossil organisms succeed each other in • systematic fashion, always in order • e.g., age of trilobites, age of fishes, age of • reptiles,age of mammals • any time period can be determined by • fossil content “principle of fossil succession”

  4. 3. Fossils as timepieces • “Index fossils” = Fossils as time-indicators • geographically wide-spread • short span of time Overlapping ranges of fossils help date rocks more exactly (time-period is ‘skinnier’).

  5. 3. Fossils as timepieces Using fossil assemblages, formations I and II of Outcrop A and II and III of Outcrop B can be put in relative sequence of each other (3).

  6. 4. Absolute dating w/ radioactivity • Chemistry review Atom model electrons # protrons = atomic #, defines the element nucleus # neutrons can vary: “isotopes” protrons neutrons See Fig. 10.13 for an example of radioactive decay.

  7. 4. Absolute dating w/ radioactivity Half-life: The time required for half of the nuclei to decay Example: an isotope with a half life of 1 year # of atoms Parent Daughter Now 1000 0 Next year 500 500 2 yrs from now 250 750 3 yrs from now 125 875 10 yrs from now 1 999

  8. 4. Absolute dating w/ radioactivity Fig. 10.14: Radioactive decay is exponential. See Table 10.1 on radioactive elements used for radiometric dating.

  9. What to remember…about radioactivity • ØSome isotopes are unstable • ØParent nucleus breaks up (decays) • ØBy-product (daughter) will be different element • ØRate of decay is constant & can be determined • ØHalf life = Time required for half of parent atoms to decay to daughters • ØIsotopes with long ½ lives (uranium) are used to date old rocks • Short ½ lives (carbon-14) are used for young rocks

  10. 5. Geologic time scale - a calendar of Earth’s history eons Phanerozoic - last 570 million years Cenozoic - recent life Mesozoic - age of middle life Paleozoic - ancient life Precambrian - from birth of Earth up to before complex life forms developed eras 0-66.4 m.y.a (mammals, humans) Massive extinction events 66.4-245 m.y.a (dinosaurs, 1st scrawny mammals) 245-540 m.y.a (first fossils with shells, fish, trilobites, clams, corals, ferns ) (algae, bacteria, some fossils without shells like jellyfish, worms) See Fig. 10.12, 10.17, 1.12

  11. 5. Geologic time scale From Tarbuck and Lutgens, Earth, 7th ed.

  12. 6)Regional correlation of rock layers Using relative dating principles and fossils, strata from different localities can be pieced together to form one continuous record of geologic history. Box. 10.1., pp. 226-227

  13. And now a few sample MC… The principle of superposition states that __________. A. a fault is younger than the rocks it cuts B. sediments are deposited as essentially horizontal layers C. the present is the key to the past D. undisturbed sedimentary layers get progressively younger from bottom to top

  14. And now a few sample MC… What is an unconformity? A. a gap in the geologic record B. a period of deposition C. a sedimentary layer of variable thickness D. a sequence of deformed rocks

  15. And now a few sample MC… The study of faunal succession allows _________. A. absolute dating of fossil-bearing strata B. correlation of marine fossils with modern mammals C. reconstruction of paleoclimates D. rocks to be correlated from different outcrops

  16. And now a few sample MC… Which of the following radioactive isotopes is the most useful for dating very young (<10,000 years old) wood and charcoal? A. carbon-14 B. potassium-40 C. rubidium-87 D. uranium-238

  17. And now a few sample MC… Human beings (homo sapiens) evolved during which geologic era? A. Cenozoic B. Mesozoic C. Paleozoic D. Precambrian

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