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

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Presentation on theme: "William Smith, 1815 Geologic map of England, Wales, part of Scotland The Map That Changed the World."— Presentation transcript:



3 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 Relative Geologic Time Scale

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

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

6 How was the timescale created?

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

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

9 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 Relative-Dating Principles

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

11 A small fault displaces tilted beds: the fault is younger than the beds Cross-cutting Relationships 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

15 Back to Steno

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

18 Faulting

19 Relative Dating Principles Unconformities Deformation

20 Relative Dating – Cross-section

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

22 Relative Dating – Cross-section

23 Relative Dating – Cross-section Key: H F D G I Erosion C Erosion A B M L K Erosion J

24 Grand Canyon: history revealed

25 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 Grand Canyon

26 Radiometric dating is 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 Absolute Dating

27 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 Geologic Time Scale

28 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 Changes in the Concept of Geologic Time

29 During the 1700s and 1800s Earths 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 Changes in the Concept of Geologic Time

30 –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 Changes in the Concept of Geologic Time

31 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 History of Historical Geology

32 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 History of Historical Geology

33 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 History of Historical Geology

34 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 Uniformitarianism

35 Unconformity at Siccar Point

36 Hutton viewed Earth history as cyclical Uniformitarianism 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 erosion depositionuplift erosion

37 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 Crisis in Geology

38 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 Crisis in Geology

39 The discovery of radioactivity destroyed Kelvins argument for the age of Earth Radioactivity is the spontaneous decay of an atoms 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 Absolute-Dating Methods

40 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 Absolute-Dating Methods

41 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 Radioactive Decay

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

43 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 Radioactive Decay

44 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 Radioactive Decay

45 Uranium 238 decay


47 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 Half-Lives

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

49 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 Determining Age

50 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? Determining Age –25% means it is 2 half- lives old. –the rock is 57 x 2 =114 million years old.

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

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

53 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 Sources of Uncertainty

54 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 Sources of Uncertainty

55 a. A mineral has just crystallized from magma. 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. 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.

56 The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable –Such isotopes include: Parents DaughtersHalf-Life (years) Long-Lived Radioactive Isotope Pairs Used in Dating Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207704 million Thorium 232 Lead 20814 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 401.3 billion Most of these are useful for dating older rocks

57 Mass Spectrometer

58 How do we know the Earth cant be older than about 6-7 b.y.? Moderate half-life isotopes (1 b.y.) If Earth was > 6-7 b.y. old, there wouldnt be many parents left

59 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 < 70,000 years old It is not useful for most rocks, but is useful for archaeology and young geologic materials Radiocarbon Dating Method

60 Carbon 14 is constantly forming in the upper atmosphere –when a high-energy neutron, a type of cosmic ray, strikes a nitrogen 14 atom it may be absorbed by the nucleus and eject a proton changing it to carbon 14 The 14 C formation rate –is fairly constant –and has been calibrated against tree rings Carbon 14

61 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 Carbon 14

62 The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk) The width of the rings are related to climate and can be correlated from tree to tree –a procedure called cross-dating The tree-ring time scale now extends back 14,000 years! Tree-Ring Dating Method

63 In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans Tree-Ring Dating Method

64 Summary Uniformitarianism holds that –the laws of nature have been constant through time –and that the same processes operating today have operated in the past –although not necessarily at the same rates

65 Summary The principles of superposition –original horizontality, –lateral continuity –and cross-cutting relationships –are basic for determining relative geologic ages and for interpreting Earth history Radioactivity was discovered during the late 19 th century –and lead to radiometric dating –which allowed geologists to determine absolute ages for geologic events

66 Summary Half-life is the length of time it takes for one-half of the radioactive parent isotope to decay to a stable daughter isotope of a different element The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairs –in igneous rocks

67 Summary The most reliable radiometric ages are obtained using two different pairs in the same rock Carbon 14 dating can be used only for organic matter such as –wood, bones, and shells –and is effective back to about 70,000 years


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