William Smith, 1815 Geologic map of England, Wales, part of Scotland

1. The Map That Changed the World • William Smith, 1815 • Geologic map of England, Wales, part of Scotland

2. The Map That Changed the World

3. 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

4. Geologic Time Scale • Large divisions based on…? • Paleozoic Era – • Mesozoic Era – • Cenozoic Era –

5. Geologic Time Scale • Large divisions based on characteristics of fossils • Paleozoic Era – early life dominated by invertebrate animals • Mesozoic Era – middle life • Cenozoic Era – recent life

6. How was the timescale created?

7. How was the timescale created? • Mapping in 1800s using the principles of • Superposition • Original Horizontality • Original Lateral Continuity • Cross-cutting relationships • Also Fossil Correlation

8. Relative-Dating Principles • Superposition • Oldest on bottom, youngest on top • Original Horizontality • Sediment originally deposited in flat parallel layers Chattanooga Shale, TN

9. Relative-Dating Principles • Lateral continuity • sediment extends laterally in all directions until it thins and pinches out or terminates against the edges of the depositional basin • Cross-cutting relationships • an igneous intrusion or a fault must be younger than the rocks it intrudes or displaces

10. Cross-cutting Relationships • A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite North shore of Lake Superior, Ontario Canada

11. Cross-cutting Relationships • A small fault displaces tilted beds: the fault is younger than the beds Templin Highway, Castaic, California

12. Unconformities • What is an unconformity?

13. Unconformities • What is an unconformity? • A surface of erosion or non-deposition • Recognizable surface in the rock record

14. Example of an Unconformity • Tilted sandstone and siltstone below, conglomerate above www.geology.sdsu.edu/visualgeology/geology101/erosion6.htm

15. Back to Steno www.gly.uga.edu/railsback/1121Steno.jpg

16. Why are layers tilted? • Deformation of rocks • Occurs after they are deposited • Important factor in relative dating • Folding • Anticlines, synclines • Rock bends, but does not break • Faulting • Normal, reverse, transform • Rock breaks

17. Folding www.hill.anorak.org.uk/dhtml/glgchap5.html

18. Faulting www.stmarys.ca/academic/science/geology/structural/faults.html

19. Relative Dating • Principles • Unconformities • Deformation

20. Relative Dating – Cross-section http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/

21. Relative Dating – Cross-section Key: E Erosion G L C H Tilting Erosion M D J A Erosion N K B Tilting Erosion F Erosion http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/

22. Relative Dating – Cross-section http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm

23. Relative Dating – Cross-section Key: H F D G I Erosion C Erosion A B M L K Erosion J http://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htm

24. Grand Canyon: history revealed

25. Grand Canyon • More than 1 billion years of history are preserved in the rock layers of the Grand Canyon • Reading this rock book shows: • periods of mountain building • advancing and retreating shallow seas • evolution of faunas • Determine these things by: • applying the principles of relative dating to the rocks • and recognizing that present-day processes have operated throughout Earth history - Uniformitarianism

26. Absolute Dating • Radiometric datingis the most common method of obtaining absolute ages • calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks • Other methods? • Tree ring counting • Varves • Ice cores

27. Geologic Time Scale • The discovery of radioactivity near the end of the 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

28. Changes in the Concept of Geologic Time • The concept and measurement of geologic time has changed through human history • James Ussher (1581-1665) in Ireland • calculated the age of Earth based on recorded history and genealogies in Genesis • he announced that Earth was created on October 22, 4004 B.C. • a century later it was considered heresy to say Earth was more than about 6000 years old

29. Changes in the Concept of Geologic Time • During the 1700s and 1800s Earth’s age was estimated scientifically • 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

30. Changes in the Concept of Geologic Time • Others used 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 • Or the 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

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

32. History of Historical Geology • Catastrophism • proposed by Georges Cuvier (1769-1832) • dominated European geologic thinking • the physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area • six major catastrophes occurred, corresponding to the six days of biblical creation, he last one was the biblical flood

33. 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

34. Uniformitarianism • Developed by James Hutton, advocated by Charles Lyell (1797-1875) • Present-day processes have operated throughout geologic time • Term uniformitarianism was coined by William Whewell in 1832 • Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point Scotland • We now call what he observed an unconformity • but he properly interpreted its formation

35. Unconformity at Siccar Point

36. 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

37. 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

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

39. 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

40. Absolute-Dating Methods • Understanding absolute dating requires knowledge of atoms and isotopes: we have it! • Atomic mass number = number of protons + number of neutrons • Isotopes: different numbers of neutrons • 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

41. 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

42. 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

43. 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

44. 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

45. Uranium 238 decay

46. 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

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

48. Geometric Radioactive Decay • In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2

49. Determining Age • By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined in the laboratory 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

50. Determining Age • For example: • If a rock has a parent/daughter ratio of 1:3  a parent proportion of 25% • and the half-live is 57 million years, how old is the rock? • 25% means it is 2 half-lives old. • the rock is 57 x 2 =114 million years old.