07_fossils_num_dating_09_post

07_fossils_num_dating_09_post - 07: Geologic Time 2:...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 07: Geologic Time 2: Fossils, Absolute Dating, Age of Earth Why this is important • The changing fossil record allows geologists to determine the relative ages of rocks and to demonstrate the timeequivalence of rock units from around the world. http://www.nrc.gov/ Fig. E.2b Lecture 07: 1 Lecture 07: 2 Why this is important Why this is important • The fossil record, the length of geologic time, the gradual nature of many geological processes, and Charles Darwin’s own geological investigations were instrumental in the development of his theory of natural selection. Cambrian (530 million years ago) trilobites. Photo: C. Clark Lecture 07: 3 Ordovician (~475 million years ago) trilobite & brachiopod. Photo: R.M. Busch Devonian (~385 million years ago) trilobite & brachiopod. Photo: R.M. Busch • The decay of radioactive isotopes provides the ticking clock for dating geologic events, including the age of the Earth and the solar system. Lecture 07: 4 Basics: Fossils Fossil: the remnant or trace of an ancient living organism that has been preserved in rock or sediment Basics: Types of fossils 1. Frozen or dried body fossils Fig. E.4 Frozen wooly mammoth news.bbc.co.uk Lecture 07: 5 Lecture 07: 6 Basics: Types of fossils 2. Body fossils preserved in amber (hardened tree sap) Basics: Types of fossils 4. Replaced hard parts as a result of permineralization (process by which minerals precipitate in porous material like wood (forming petrified wood) or bone from groundwater solutions that have seeped into the pores. Fig. E.5b 3. Preserved hard parts • bones • teeth • shells Fig. E.5c Lecture 07: 7 Fig. E.5d Lecture 07: 8 Basics: Types of fossils Basics: Types of fossils 5. Molds and casts of body parts: if a shell, for example, dissolves out of the enclosing rock, the cavity is a mold; if the mold later fills in with material, it becomes a cast. cast 6. Carbonized impressions of bodies: feathers, insects, leaves, etc., leave an imprint when pressed between layers of sediment; although most of the original organic material is later removed, insoluble carbon remains as a thin film. mold cast mold Lecture 07: 9 Lecture 07: 10 Basics: Factors favoring fossilization 1. Death in an oxygen-poor environment prevents organic decay 2. Rapid burial: organism is enclosed in sediment before it can decay, be eaten, or disturbed by other processes. 3. Presence of hard parts (bones, shells, etc.). 4. Lack of subsequent deformation and/or metamorphism Basics: Mechanism of evolution ‘Survival of the fittest.’ Because these conditions do not always occur, the fossil record is incomplete, but transitional forms do occur; this is evidence for evolution. http://evolution.berkeley.edu/ http://evolution.berkeley.edu/ Lecture 07: 11 Lecture 07: 12 Basics: Evidence for evolution Bacteria reproduce very quickly, and new strains evolve through mutation in a matter of minutes and hours Basics: Evidence for evolution Breeding of new animal or plant varieties Dogs have evolved through natural selection and breeding from wolves. http://evolution.berkeley.edu/ Development of antibiotic-resistance bacteria is evolution in action. ‘Survival of the fittest.’ http://evolution.berkeley.edu/ Lecture 07: 13 Lecture 07: 14 Basics: Evidence for evolution Differences among similar species separated by geography Basics: Evidence for evolution Differences among similar species separated by geography A visit to the Galapagos Islands in 1835 helped Charles Darwin formulate his ideas on natural selection. He found several species of finch adapted to different environmental niches. The finches also differed in beak shape, food source, and how food was captured. http://evolution.berkeley.edu/ http://evolution.berkeley.edu/ http://rapidfire.sci.gsfc.nasa.gov/gallery/?20020710312/Galapagos.A2002071.1625.250m.jpg Lecture 07: 15 http://forum.myspace. com/index.cfm?fusea ction=messageboard. viewThread&entryID= 57762987&groupID= 100270914&adTopicI D=30&Mytoken=2B18 0BCC-BB6C-461CB6BC3FD30A73B1C 2462552764 Lecture 07: 16 Basics: Evidence for evolution Review Questions 7-1. Which of the following organisms is most likely to be fossilized? A. Leaves from trees growing near a shallow, oxygen-rich lake. B. Dinosaur living near a mountain-top where little sedimentation occurs. C. Clams living in the ocean. Fossil record showing changes through time. 7-2. Refer to slide 16. Which of the following is not an evolutionary change in horses? A. decrease in the number of toes B. decrease in body size C. increase in the complexity of the teeth Detailed family tree of horses is based on fossil record, which reveals a number of transitional forms between Hyracotherium and the modern Equus. Names in blue are specific intervals of geologic time. The numbers in red are the absolute ages of the boundaries between these intervals. Details of the forelimb and teeth of the species shown in bold in the family tree are shown at right. [Modified from Dolgoff (1998) and Campbell (1987).] 7-3 Refer to the diagram in slide 10. The youngest whales have nostrils at the ___ of the skull. A. front B. middle C. top 7-4. Which of the following is not evidence for evolution? A. development of new strains of bacteria as a result of the use of antibiotics B. different assemblages of fossils in rocks of the same age C. differences in the morphology of species of finches on the different Galapagos islands Lecture 07: 17 Lecture 07: 18 Fossil Succession ! ! ! ! Species evolve, exist for a time, and then go extinct. First appearance, range, and extinction dates rocks. Fossils succeed one another in a known order. A time period is recognized by its fossil content. Lecture 07: 19 Fossils & Dating Fig. 12.7 ! Fossil range – first and last appearance. " " Each fossil has a unique range. Overlapping ranges provide distinctive time markers. Index fossil: fossil with a narrow range & widespread distribution Lecture 07: 20 Fig. 12.7 In-class exercise Exercise 1: In the diagram at right, red arrows show the range of fossils (names are in italics). The other names are of geologic time periods. a. A rock layer contains fossils of Primitia. It’s age is NOT: A. Cambrian B. Devonian C. Ordovician D. Permian Fossils & Correlation Correlation: the process of defining the age relations between strata at one locality and strata at another b. A rock layer contains fossils of Primitia, Drepanodus, and Phacops. Its possible age is: A. Ordovician B. Silurian C. Devonian D. Carboniferous. c. The most likely index fossil is: A. Drepanodus B. Monograptus C. Phacops D. Primitia Fig. 12.12 d. The least likely index fossil is: A. Drepanodus B. Monograptus C. Phacops D. Primitia Lecture 07: 21 Lecture 07: 22 Correlation !National Correlation parks of Arizona and Utah. " Units can be traced long distances. " Overlap is seen in the sequences of rock types. " Overlapping partial sections are used to build composite. Fig. 12.17 Lecture 07: 23 Lecture 07: 24 Fossils: most useful for correlation Fig. 12.17 Geologic column Fig. 12.15 • Subdivision of Earth's history into discrete intervals of geologic time based on relative dating " " Geologic time scale • Subdivision of Earth's history into discrete intervals of geologic time based on relative and absolute dating Constructed from partial sections across the globe. Brackets almost the entirety of Earth history. Eon-->Era--> Period-->Epoch Fig. 12.15 Lecture 07: 25 Lecture 07: 26 Absolute dating Absolute dating • Based on decay of radioactive isotopes • Half-life is the time it takes for half the parent isotope to decay into daughter product. • Parent isotope breaks down to daughter isotope at some known (and constant) rate • Measuring parent-todaughter ratio & using decay rate --> absolute age • Unaffected by pressure, temperature & chemistry. Fig. 12.18 Lecture 07: 27 • Assumption: no parent or daughter products have entered or escaped since rock formation Lecture 07: 28 Fig. 12.18 Absolute dating Absolute dating • Measuring parent-to-daughter ratio requires mass spectrometer . 14Carbon-->14Nitrogen 5730 Effective range: 100-70,000 years ago. Lecture 07: 29 Lecture 07: 30 Exercise 2a: Using the Pb-207/U-235 dating method, the Nd/Np ratio for a granite is 3.7. The number of half-lives for U-235 is: A. 2.2 B. 3.7 C. 4.0 D. 15 30 25 20 Nd/Np Exercise 2: The graph at right shows how the Nd/Np ratio changes as a function of the number of elapsed half lives. Nd is the number of daughter atoms; Np is the number of parent atoms. In-Class Exercise: Dating 15 10 5 0 Lecture 07: 31 0 1 2 3 4 Number of elapsed half lives 30 25 20 Nd/Np In-Class Exercise: Dating Wood, charcoal, peat; bone and tissue; shells containing calcium carbonate (CaCO3); groundwater, ocean water, and glacier ice containing dissolved CO2. 15 10 5 0 5 Lecture 07: 32 0 1 2 3 4 Number of elapsed half lives 5 Exercise 2c: If the halflife for the Pb-207/U235 system is 1,273,000,000 years and the half-life for the Ar40/K-40 system is 700,000,000 years, determine the age of the granite for the two systems. 30 25 20 Nd/Np Exercise 2b: Using the Ar-40/K-40 dating method, the Nd/Np ratio for the same granite is 15. The number of halflives for K-40 is: A. 2.2 B. 3.7 C. 4.0 D. 15 In-Class Exercise: Dating 15 10 5 0 0 1 2 3 4 Number of elapsed half lives 5 Lecture 07: 33 Lecture 07: 34 Geologic time scale In-Class Exercise: Dating Exercise 2d: Are the two systems in agreement? 30 25 Nd/Np 20 15 10 5 0 Lecture 07: 35 The age is: A. 1,273,000,000 years B. 2,800,000,000 years C. 1,400,000,000 years D. 5,100,000,000 years E. 2,650,000,000 years 0 1 2 3 4 Number of elapsed half lives 5 • Subdivision of Earth's history into discrete intervals of geologic time based on relative and absolute dating Lecture 07: 36 30 25 20 Nd/Np In-Class Exercise: Dating 15 10 5 0 0 1 2 3 4 Number of elapsed half lives 5 Geologic time scale / iClicker Geologic time scale • Bracketing: dating of the geologic time scale using multiple age dates on igneous rocks interlayered with or intruding sedimentary rocks Exercise 3a. What is the age of the unit X? A. >125 Ma B. 80-125 Ma C. 50-80 Ma D. <50 Ma X Fig. 12.23 Fig. 12.23 Lecture 07: 37 Lecture 07: 38 Geologic time scale / iClicker Exercise 3b: What is the age of the Paleocene sandstone? A. >125 Ma B. 80-125 Ma C. 50-80 Ma D. <50 Ma Geologic time scale / iClicker Exercise 3c: What is the age of the unit Y? A. >125 Ma B. 80-125 Ma C. 50-80 Ma D. <50 Ma X Fig. 12.23 Lecture 07: 39 Y Lecture 07: 40 Fig. 12.23 Exercise 3d: What is the age of the Cretaceous sandstone? A. >125 Ma B. 80-125 Ma C. 50-80 Ma D. <50 Ma Age of Earth 1956: Clair Patterson (1922-1995, USA) uses U/Pb dating of meteorites to determine the solar system’s age as 4.55 billion years. Assumption: Earth & meteorites formed at Assumption: same time. www.todayinsci.com Geologic time scale / iClicker Fig. 12.23 http://www.astro.virginia.edu/~mnc3z/images/astro121/asteroid_belt.gif Lecture 07: 41 Lecture 07: 42 Earth’s oldest rocks & minerals Earth’s oldest rocks & minerals Imataca Complex, Venezuela: 3.77 Ga Rhodesian Craton, Zimbabwe: 3.52 Ga Mount Stones, Antarctica: 3.53 Ga Northwest Territories, Canada: 3.96 Ga Zircons in sedimentary rocks: 4.15 Ga Press & Sevier Earth was too hot (possibly mostly molten) from 4.55 to 4.15 Ga …..Why no 4.55 Ga? Oldest rocks not found or were destroyed and/or recycled Press & Sevier Lecture 07: 43 Lecture 07: 44 Review Questions Review Questions 7-5. Which type of investigation gave scientists the best estimate of the age of the Earth? A. paleontologic dating B. archeological dating C. radiometric dating D. carbon dating 7-6. Radiometric dating is possible if a rock contains a measurable amount of: A. only parent isotopes B. only daughter isotopes C. both parent and daughter isotopes D. either parent or daughter isotopes 7-7. If a sample contains 25% parent isotope and 75% daughter isotope and the half-life is 10 million years, the age of the sample is ___. A. 1 million years B. 10 million years C. 20 million years D. 100 million years 7-8. A rock contains 75 atoms of a radioactive parent element, and 150 atoms of parent plus daughter elements. If the half-life for the radioactive element is 1 million years, how old is the rock? A. 250,000 years B. 500,000 years C. 1,000,000 years D. 2,000,000 years Lecture 07: 45 7-18. Unit Z is a ___. A. dike B. sill 7-19. Contact X is ___ . A. angular unconformity B. disconformity C. nonconformity 7-20. Contact X is ___ . A. angular unconformity B. disconformity C. nonconformity Lecture 07: 47 7-10. Two atoms of a single element that differ in number of neutrons are said to represent two distinct ____________ of that element. A. isomers B. isotherms C. isotopes D. atomic species 7-11. What fraction of a radioactive isotope is expected to remain in an unaltered (unreacted) state after the passage of three half-lives? A. one-third B. one-eighth C. three-halves D. one-sixth 7-12. A. True / B. False: If the numerical (absolute) ages of two formations are known, then the relative age of each with respect to one another can be inferred. 7-13. A. True / B. False: If the relative (absolute) age of two formations (with respect to one another) is known, then the numerical ages for each formation can be inferred. Lecture 07: 46 Review Questions Use bracketing to answer questions 13-16. Units C and D are lava flows. 7-14. The age of the Oligocene unit is _____. A. >60 Ma B. 60-34 Ma C. 34-30 Ma D. 30-20 Ma 7-15. The age of the Eocene unit is _____. A. >60 Ma B. 60-34 Ma C. 34-30 Ma D. 30-20 Ma 7-16. The age of the Paleocene unit is _____. A. >60 Ma B. 60-34 Ma C. 34-30 Ma D. 30-20 Ma 7-17. The age of the Triassic unit is _____. A. >60 Ma B. 60-34 Ma C. 34-30 Ma D. 30-20 Ma 7-9. Why is radiocarbon (carbon-14) dating only rarely applied in geological work? A. No substances on Earth contain significant amounts of carbon-14. B. The half-life of carbon-14 is so long that it is effectively a stable isotope. C. The half-life of carbon-14 is so short that it can only be used to date materials that are less than 70,000 years old. E. <20 Ma E. <20 Ma E. <20 Ma E. <20 Ma ...
View Full Document

Ask a homework question - tutors are online