Presentation on theme: "Chapter 4: Geologic Time— Concepts and Principles"— Presentation transcript:
1 Chapter 4: Geologic Time— Concepts and Principles Grand CanyonWhen looking down into the Grand Canyon, we are really looking all the way back to the early history of EarthChapter 4: Geologic Time— Concepts and Principles
2 Relative Geologic Time Scale The relative geologic time scale has a sequence ofeonserasperiodsepochsbut no numbers indicating how long ago each of these times occurred
3 Concept of Geologic Time Relative Dating – putting rock layers and events in order relative to when they occurred.Absolute dating which results in specific numerical dates for rock units or eventsSuch dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks
4 Geologic Time Scale -- today The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scaleThe most recent geologic time scale model is a dual scalea relative scale and an absolute scaleFigure 4.1: The Geologic Time Scale.Some of the major geologic and biologic events are indicated along the right-hand margin. Dates are from Gradstein, F., Ogg, J. and Smith, A., A Geologic Timescale 2004 (Cambridge, UK: Cambridge University Press, 2005), Figure 1.2.Fig. 4-1, p. 62
5 Changes in the Concept of Geologic Time Attempts to give an age to the earthJames Usher ( ) in Irelandcalculated the age of Earth based on genealogies in GenesisAnnounced that Earth was created onOctober 22, 4004 B.C.A century later it was still considered heresy to say Earth was more than about 6000 years old.
6 Changes in the Concept of Geologic Time Georges Louis de Buffon ( )calculated how long Earth took to cool gradually from molten iron ballsEarth about 75,000 yearsOthers calculated the rate of sediment depositionAlso the rate of salt build-up in the oceans from the continental riversAges in millions to billions of years In 1953, the dating of meteorites was accomplished. Age ~ 4.5 by
7 Relative-Dating Principles Six fundamental geologic principles are used today in relative dating1. Principle of superpositionNicolas Steno ( )In an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top(note: Steno lived contemporaneously with Usher)This method is used for determining the relative age of rock layers (strata) and the fossils they contain
9 Relative-Dating Principles 2. Principle of original horizontalityNicolas StenoSediment is deposited in essentially horizontal layersTherefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification
10 Illustration of the principles of original horizontality
11 Principle of Lateral Continuity Nicholas Steno Sediment extends laterally in all direction until it thins and pinches out or terminates against the edges of the depositional basin
12 Principle of Cross-Cutting Relationships James Hutton ( An igneous intrusion or a fault event must be younger than the rocks it intrudes or cuts across
13 Cross-cutting Relationships North shore of Lake Superior, Ontario CanadaA dark-colored dike has intruded into older light colored granite.The dike is younger than the granite.
14 Cross-cutting Relationships Templin Highway, Castaic, CaliforniaA small fault displaces tilted beds.The fault is younger than the beds.
15 Principle of inclusions That which is included is _________ (older Principle of inclusions That which is included is _________ (older? Younger?)
18 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 areaSix major catastrophes occurred, corresponding to the six days of biblical creation.The last one was the biblical flood(also relatively modern, and built on Usher’s Biblical age of the Earth)
19 Uniformitarianism Principle of uniformitarianism Present-day processes have operated throughout geologic time. This includes the physical, chemical and biological processesDeveloped by James Hutton, advocated by Charles Lyell ( )Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point ScotlandWe now call what he observed an unconformity but he properly interpreted its formationTerm uniformitarianism was coined by William Whewell in 1832
20 Unconformity at Siccar Point the tilted, lower rocks resulted from severe upheavals that formed mountainsThe mountains were then worn away and covered by younger flat-lying rocksthe erosional surface represents a gap in the rock record
21 Uniformitarianism Hutton viewed Earth history as cyclical erosion upliftdepositionOld Earth: geologic processes operate over a vast amount of timeModern view of uniformitarianismToday, geologists assume that the principles or laws of nature are constant but the rates and intensities of change have varied through time
24 Using Radioactive Decay to obtain numerical age Understanding absolute dating requires knowledge of atoms and isotopesThe nucleus of an atom is composed ofprotons – particles with a positive electrical chargeneutrons – electrically neutral particleselectrons – the negatively charged particles – encircling the nucleusatomic numberEqual to the number of protonshelps determine the atom’s chemical properties and the element to which it belongs
25 Isotopes Atomic mass number = number of protons + number of neutrons The different forms of an element’s atoms with varying numbers of neutrons are called isotopesDifferent isotopes of the same element have different atomic mass numbers but behave the same chemicallyMost isotopes are stable, but some are unstableGeologists use decay rates of unstable isotopes to determine absolute ages of rocks
26 Radioactive DecayRadioactive decay -the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different elementThree types of radioactive decay:In alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus.
27 Radioactive DecayIn beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton.In electron capture decay, a proton captures an electron and converts to a neutron.
28 Radioactive DecaySome isotopes undergo only one decay step before they become stable.Examples:rubidium 87 decays to strontium 87 by a single beta emissionpotassium 40 decays to argon 40 by a single electron captureBut other isotopes undergo several decay stepsuranium 235 decays to lead 207 by 7 alpha steps and 6 beta stepsuranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps
30 Half-LivesThe 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 isotopeThe half-life of a specific radioactive isotope is constant and can be precisely measuredCan vary from less than 1/billionth of a second to 49 billion yearsIs geometric not linear, so has a curved graph
31 Uniform Linear ChangeIn this example of uniform linear change, water is dripping into a glass at a constant rate
32 Geometric Radioactive Decay In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2
33 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,25% means it is 2 half-lives old.the rock is 57 x 2 =114 million years old.
34 What Materials Can Be Dated? Most radiometric dates are obtained from igneous rocksAs magma cools and crystallizes,radioactive parent atoms separate from previously formed daughter atomsSome radioactive parents are included in the crystal structure of certain minerals Dating of sedimentary rocks RARE: dating the mineral glauconite, because it forms in certain marine environments as a reaction with clay during the formation of the sedimentary rock
35 Igneous Crystallization Crystallization of magma separates parent atomsfrom previously formed daughtersThis resets the radiometric clock to zero.Then the parents gradually decay.
36 Sources of Uncertainty In glauconite, potassium 40 decays to argon 40because argon is a gas, it can easily escape from a mineralA closed system is needed for an accurate datethat is, neither parent nor daughter atoms can have been added or removed from the sample since crystallizationIf leakage of daughters has occurredit partially resets the radiometric clock and the age will be too youngIf parents escape, the date will be too old.The most reliable dates use multiple methods.
37 Sources of Uncertainty During metamorphism, some of the daughter atoms may escapeleading to a date that is too young.However, if all of the daughters are forced out during metamorphism, then the date obtained would be the time of metamorphism—a useful piece of information.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
38 Dating Metamorphisma. A mineral has just crystallized from magma.b. As time passes, parent atoms decay to daughters.c. Metamorphism drives the daughters out of the mineral as it recrystallizes.Dating the whole rock yields a date of 700 million years = time of crystallization.d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time.
39 Long-Lived Radioactive Isotope Pairs Used in Dating The isotopes used in radiometric datingneed to be sufficiently long-lived so the amount of parent material left is measurableSuch isotopes include:Parents Daughters Half-Life (years)Uranium Lead billionUranium Lead millionThorium Lead billionRubidium Strontium billionPotassium Argon billion
40 Fission Track DatingUranium in a crystal will damage the crystal structure as it decaysThe damage can be seen as fission tracks under a microscope after etching the mineralThe age of the sample is related tothe number of fission tracksthe amount of uranium
41 Radiocarbon Dating Method Carbon is found in all lifeIt has 3 isotopescarbon 12 and 13 are stable but carbon 14 is notCarbon 14 has a half-life of 5730 yearsCarbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once livingThe short half-life of carbon 14makes it suitable for dating material < 70,000 years oldIt is not useful for most rocks,but is useful for archaeologyand young geologic materials
42 Carbon 14 Carbon 14 is constantly forming in the upper atmosphere When a high-energy neutrona type of cosmic ray strikes a nitrogen 14 atomit may be absorbed by the nucleus and eject a proton changing it to carbon 14The 14C formation rateis fairly constanthas been calibrated against tree rings
43 Carbon 14The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organismsWhile the organism lives it continues to take in carbon 14 but when it dies the carbon 14 begins to decaywithout being replenishedThus, carbon 14 datingmeasures the time of death
44 Tree-Ring Dating Method 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 can be correlated from tree to treea procedure called cross-datingThe tree-ring time scale now extends back 14,000 years
45 Tree-Ring Dating Method In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans
46 Summary James Hutton viewed Earth history as cyclical and very long His observations were instrumental in establishing the principle of uniformitarianismCharles Lyell articulated uniformitarianism in a way that soon made it the guiding doctrine of geologyUniformitarianism holds thatthe 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
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