Understanding Geologic Time

1. Understanding Geologic Time The map that changed the world by William Smith (1815) links: fossils rock patterns 3Dapproach

2. Grand Canyon: history revealed

3. Grand Canyon • Preserves more than 1 billion years of history • This rock book shows • mountain building • advancing and retreating seas • evolution of faunas • Determine these things by: • applying the principles of relative dating to the rocks • Uniformitarianism

4. Concepts of Geologic Time • Two frames of reference • Relative dating – describes sequential order • Absolute dating – timing of events in years before present

5. Relative Geologic Time Scale • The relative geologic time scale has a sequence of • eons • eras • periods • epochs • but no numbers indicating how long ago each of these times occurred, just the order of occurrence

6. Absolute Dating - specific dates for rock units or events • expressed in years before the present • gives us numerical information about events

7. Absolute Dating • • Radiometric datingis the most common method of obtaining absolute ages • – calculated from the rates of decay of various natural radioactive elements present in trace amounts in some rocks •Other methods – tree ring counting – varves (layers year sediment accumulations) – ice (count layers of ice for annual scale)

8. Geologic Time Scale • Radioactivity (late 1800s) allowed absolute ages to be accurately applied to the relative geologic time scale • The geologic time scale is a dual scale • a relative scale • and an absolute scale

9. Changes in the Concept of Geologic Time • James Ussher (1581-1665) in Ireland • calculated the age of Earth based on recorded history and genealogies in Genesis • announced that Earth was created on October 22, 4004 B.C. • widely accepted http://star.arm.ac.uk/history/USSHER.GIF

10. Changes in the Concept of Geologic Time Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning • used melted iron balls of various diameters • he estimated Earth was 75,000 years old • considered an "old Earth!" http://www.nceas.ucsb.edu/~alroy/lefa/Buffon.jpg

11. Changes in the Concept of Geologic Time • Rates of deposition of various sediments and thickness of sedimentary rock in the crust • gave estimates of <1 million • to more than 2 billion years • Amount of salt carried by rivers to the ocean and the salinity of seawater • John Joly in 1899 obtained a minimum age of 90 million years

12. Relative-Dating • Six fundamental geologic principles 1) Superposition 2) Original horizontality 3) Lateral continuity 4) Cross-cutting relationships 5) Inclusions 6) Fossil succession

13. Relative-Dating Principles • Principle of superposition • Nicolas Steno (1638-1686) • in an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top • this method is used for determining the relative age of rock layers (strata) and the fossils they contain http://www.science.siu.edu/zoology/king/304/biogrphy.htm

14. Relative-Dating Principles • Principle of original horizontality • Nicolas Steno • sediment is deposited in essentially horizontal layers • a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification • Principle of lateral continuity • Sediment extends laterally in all directions until it thins and pinches out or terminates against the edges of a basin (also Steno)

15. Relative-Dating Principles • Horizontality • sediments were originally deposited horizontally in a marine environment • Superposition • old to young

16. Relative-Dating Principles • Principle of cross-cutting relationships • James Hutton (1726-1797) • an igneous intrusion or a fault must be younger than the rocks it intrudes or displaces http://www.physicalgeography.net/fundamentals/10c.html

17. Cross-cutting Relationships • A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite

18. Cross-cutting Relationships • A small fault displaces tilted beds: the fault is younger than the beds.

19. Relative-Dating Principles • Principle of inclusions • discussed later in the term • Principle of fossil succession • discussed later in the term

20. History of Historical Geology • Neptunism • all rocks, including granite and basalt, were precipitated in an orderly sequence from a primeval, worldwide ocean • proposed in 1787 by Abraham Werner (1749-1817) • Werner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history http://de.wikipedia.org/wiki/Abraham_Gottlob_Werner

21. History of Historical Geology • Catastrophism • proposed by Georges Cuvier (1769-1832) • dominated European geologic thinking • the history of Earth resulted from a series of sudden widespread catastrophes which exterminated existing life in the affected area • six major catastrophes occurred, corresponding to the six days of biblical creation, the last one was the biblical flood http://search.eb.com/dinosaurs/dinosaurs/ocuvier001p1.html

22. History of Historical Geology • Neptunism and Catastrophism were eventually abandoned • they were not supported by field evidence • basalt was shown to be of igneous origin • volcanic rocks interbedded with sedimentary • primitive rocks showed that igneous activity had occurred throughout geologic time • more than 6 catastrophes were needed to explain field observations • The principle of uniformitarianism became the guiding philosophy of geology

23. Uniformitarianism • Present-day processes have operated throughout geologic time • Developed by James Hutton • Advocated by Charles Lyell (1797-1875) • term uniformitarianism was coined by William Whewell in 1832 http://www.stephenjaygould.org/people/charles_lyell.html http://cepa.newschool.edu/het/profiles/whewell.htm

24. Unconformity at Siccar Point • • Hutton applied the principle of uniformitarianism when interpreting rocks • • We now call what he observed an unconformity • .

25. erosion erosion deposition uplift Uniformitarianism • Hutton viewed Earth history as cyclical • He also understood that geologic processes operate over a vast amount of time • Modern view of uniformitarianism • geologists assume that the principles or laws of nature are constant • but the rates and intensities of change have varied through time

26. Crisis in Geology • Lord Kelvin (1824-1907) • knew about high temperatures inside of deep mines and reasoned that Earth is losing heat from its interior • Assuming Earth was once molten, he used • the melting temperature of rocks • the size of Earth • and the rate of heat loss • to calculate the age of Earth as between 400 and 20 million years http://www.energyquest.ca.gov/scientists/kelvin.html

27. Crisis in Geology • This age was too young for the geologic processes envisioned by other geologists at that time • Kelvin did not know about radioactivity as a heat source within the Earth

28. Absolute-Dating Methods • The discovery of radioactivity • destroyed Kelvin’s argument for the age of Earth • Radioactivity is the spontaneous decay of an atom’s nucleus to a more stable form • The heat from radioactivity helps explain why the Earth is still warm inside • Radioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events

29. Absolute-Dating Methods • Understanding absolute dating requires knowledge of atoms and isotopes: • Atomic mass number = number of protons + number of neutrons • Isotopes: different numbers of neutrons, same number of protons • Different isotopes have different atomic mass numbers but behave the same chemically • Most isotopes are stable • but some are unstable • Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

30. Radioactive Decay • Radioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element • Three types of radioactive decay: • alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus

31. Radioactive Decay • beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton • electron capture decay, a proton captures an electron and converts to a neutron

32. Radioactive Decay • Some isotopes undergo only one decay step before they become stable. • rubidium 87 decays to strontium 87 by a single beta emission • potassium 40 decays to argon 40 by a single electron capture

33. Radioactive Decay • Other isotopes undergo several decay steps • uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps • uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps

34. Age Dating with Half-Lives • Half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope • The half-life of a specific radioactive isotope is constant and can be precisely measured

35. Half-Lives • The length of half-lives for different isotopes of different elements can vary from • < 1/1000000000 of a second up to 49 billion years • Radioactive decay • is geometric not linear • a curved graph

36. Uniform Linear Change • In this example of uniform linear change, water is dripping into a glass at a constant rate

37. Geometric Radioactive Decay During each half-life, the proportion of parent atoms decreases by 1/2

38. Determining Age • By measuring the parent/daughter ratio and knowing the half-life of the parent, geologists can calculate the age of a sample containing the radioactive element • The parent/daughter ratio is usually determined by a mass spectrometer • an instrument that measures the proportions of atoms with different masses

39. Determining Age • For example: • If a rock has a parent/daughter ratio of 1:3 , the remaining parent proportion is 25% • 25% = 2 half lives • If half life is 57 milliion years then the rock is 57 million years x 2 = 114 million years old

40. What Materials Can Be Dated? • Most radiometric dates are obtained from igneous rocks • As magma cools and crystallizes, radioactive parent atoms separate from daughter atoms • Parent and daughter fit differently into the crystal structure of certain minerals • Geologists can use the crystals containing the parent atoms to date the time of crystallization

41. Igneous Crystallization • Crystallization of magma separates parent atoms from previously formed daughters • This resets the radiometric clock to zero • Then the parents gradually decay

42. Sedimentary Rocks • Generally, sedimentary rocks cannot be radiometrically dated • the date obtained would correspond to the time of crystallization of the mineral, not the time that it was deposited as a sedimentary particle

43. Dating Metamorphism b. As time passes, parent atoms decay to daughters. c. Metamorphism drives the daughters out of the mineral (to other parts of the rock) as it recrystallizes. a. A mineral has just crystallized from magma. d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time. •Dating the whole rock yields a date of 700 million years = time of crystallization.

44. Sources of Uncertainty • Closed system is needed for an accurate date • neither parent nor daughter atoms can have been added or removed from the sample since crystallization • If leakage of daughters has occurred • it partially resets the radiometric clock and the age will be too young • If parents escape, the date will be too old • Most reliable dates use multiple methods

45. Sources of Uncertainty • Dating techniques are always improving. • Presently measurement error is typically <0.5% of the age, and even better than 0.1% • A date of 540 million might have an error of ±2.7 million years or as low as ±0.54 million

46. Long-Lived Radioactive Isotope Pairs Used in Dating • The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable Parents Daughters Half-Life (years) Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion

47. Fission Track Dating • Uranium in a crystal will damage the crystal structure as it decays • The damage can be seen as fission tracks under a microscope after etching the mineral • The age of the sample is related to • the number of fission tracks • and the amount of uranium • with older samples having more tracks • This method is useful for samples between 1.5 and 0.04 million years old

48. Radiocarbon Dating Method • Carbon is found in all life • It has 3 isotopes • carbon 12 and 13 are stable but carbon 14 is not • carbon 14 has a half-life of 5730 years • carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living • The short half-life of carbon 14 makes it suitable for dating material < 50,000 years old

49. Carbon 14 • Carbon 14 is constantly forming in the upper atmosphere • The 14C formation rate • is fairly constant • and has been calibrated against tree rings

50. Carbon 14 • The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organisms • While the organism lives it continues to take in carbon 14 • when it dies the carbon 14 begins to decay without being replenished • Thus, carbon 14 dating measures the time of death