Presentation on theme: "Geologic Time—Concepts and Principles"— Presentation transcript:
1 Geologic Time—Concepts and Principles Chapter 4Geologic Time—Concepts and Principles-How do we tell time in geology?...relative dating, absolute datingAbsolute age dating: 4 methods:1. Radiometric age dating: parents…daughters….isotopes…2. Carbon 14 age dating:3. Tree Rings:4. Fission track dating-Basic principles: horizontality, superposition, cross cutting relationships-unconformities- 3 types….
2 Grand CanyonWhen looking down into the Grand Canyon, we are really looking all the way back to the early history of Earth
3 Grand Canyon More than 1 billion years of history are preserved, like pages of a book,in the rock layers of the Grand CanyonReading this rock book we learnthat the area underwent episodes ofmountain buildingadvancing and retreating shallow seasWe know these things byapplying the principles of relative dating to the rocksand recognizing that present-day processeshave operated throughout Earth history
4 What is time? We are obsessed with time, using clockscalendarsappointment booksMostly we don’t have enough of it.Our common time units aresecondshoursdaysweeksmonthsyearsAncient history involveshundreds of yearsthousands of yearsBut geologic time involvesmillions of yearseven billions of years
5 Concept of Geologic Time Geologists use two different frames of referencewhen discussing geologic timeRelative dating involves placing geologic eventsin a sequential order as determinedfrom their position in the geologic recordIt does not tell us how long agoa particular event occurredonly that one event preceded anotherFor hundreds of years geologistshave been using relative datingto establish a relative geologic time scale
6 Relative Geologic Time Scale The relative geologic time scale has a sequence ofeonserasperiodsepochsbut no numbers indicating how long ago each of these times occurred
7 Concept of Geologic Time The second frame of reference for geologic timeis absolute datingAbsolute dating results in specific datesfor rock units or eventsexpressed in years before the presentIt tells us how long ago a particular event occurredgiving us numerical information about timeRadiometric dating is the most common methodof obtaining absolute agesSuch dates are calculatedfrom the natural rates of decayof various natural radioactive elementspresent in trace amounts in some rocks
8 Geologic Time Scale The discovery of radioactivity near the end of the 1800sallowed absolute agesto be accurately appliedto the relative geologic time scaleThe geologic time scale is a dual scalea relative scaleand an absolute scale
9 Changes in the Concept of Geologic Time The concept and measurement of geologic timehas changed through human historyEarly Christian theologiansconceived of time as linear rather than circularJames Usher ( ) in Irelandcalculated the age of Earth basedon recorded history and genealogies in GenesisHe 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.
10 Changes in the Concept of Geologic Time During the 1700s and 1800s Earth’s agewas estimated scientificallyGeorges Louis de Buffon ( )calculated how long Earth took to cool graduallyfrom a molten beginningusing melted iron balls of various diametersExtrapolating their cooling rateto an Earth-sized ball,he estimated Earth was 75,000 years old
11 Changes in the Concept of Geologic Time Others used different techniquesUsing rates of deposition of various sedimentsand thickness of sedimentary rock in the crustgave estimates of <1 millionto more than 2 billion years.Using the amount of salt carriedby rivers to the oceanand the salinity of seawaterJohn Joly in 1899obtained a minimum age of 90 million years
12 Relative-Dating Principles Six fundamental geologic principles are used in relative datingPrinciple of superpositionNicolas Steno ( )In an undisturbed succession of sedimentary rock layers,the oldest layer is at the bottomand the youngest layer is at the topThis method is used for determining the relative ageof rock layers (strata) and the fossils they contain
13 Principle of Superposition Illustration of the principles of superpositionand original horizontalitySuperposition: The youngestrocks are at the topof the outcropand the oldest rocks are at the bottom
14 Relative-Dating Principles Principle of original horizontalityNicolas StenoSediment is depositedin essentially horizontal layersTherefore, a sequence of sedimentary rock layersthat is steeply inclined from horizontalmust have been tiltedafter deposition and lithification
15 Principle of Horizontality Illustration of the principles of superpositionand original horizontalityHorizontality: These sediments were originallydeposited horizontallyin a marine environmentThis outcrop is Chattanooga Shale, Tennessee
16 Principle of lateral continuity In 1669, Nicolas Steno proposedhis principle of lateral continuity,meaning that layers of sediment extend outwardin all directions until they terminateTerminations may be abruptat the edge of a depositional basinwhere erodedwhere truncated by faults
17 Gradual Terminations or they may be gradual where a rock unit becomes progressively thinneruntil it pinches outor where it splits intothinner unitseach of which pinches out,called intertongingwhere a rock unit changesby lateral gradationas its composition and/or texturebecomes increasingly different
18 Principle of cross-cutting relationships James Hutton ( )An igneous intrusion or a faultmust be younger than the rocksit intrudes or displacesNorth shore of Lake Superior, Ontario CanadaA dark-colored dike has intruded into older light colored granite. The dike is younger than the granite.
19 Cross-cutting Relationships Templin Highway, Castaic, CaliforniaA small fault displaces tilted beds.The fault is younger than the beds.
20 Principle of Inclusions According to the principle of inclusions,which also helps to determine relative ages,inclusions or fragments in a rockare older than therock itselfLight-colored granitein northern Wisconsinshowing basalt inclusions (dark)Which rock is older?Basalt, because the granite includes it
21 Relative-Dating Principles Principle of fossil successiondiscussed later in the term
22 Early Geologic Concepts/Thoughts: Neptunism All rocks, including granite and basalt,were precipitated in an orderly sequencefrom a primeval, worldwide ocean.proposed in 1787 by Abraham Werner ( )Werner was an excellent mineralogist,but is best rememberedfor his incorrect interpretation of Earth history
23 Neptunism Werner’s geologic column was widely accepted Alluvial rocks unconsolidated sediments, youngestSecondary rocksrocks such as sandstones, limestones, coal, basaltTransition rockschemical and detrital rocks, some fossiliferous rocksPrimitive rocksoldest including igneous and metamorphic
24 Catastrophism Catastrophism proposed by Georges Cuvier ( )dominated European geologic thinkingThe physical and biological history of Earthresulted from a series of sudden widespread catastropheswhich accounted for significant and rapid changes in Earthand exterminated existing life in the affected areaSix major catastrophes occurred,corresponding to the six days of biblical creationThe last one was the biblical flood
25 Neptunism and Catastrophism Were Eventually abandoned These hypotheses were abandoned becausethey were not supported by field evidenceBasalt was shown to be of igneous originVolcanic rocks interbedded with sedimentaryand primitive rocks showed that igneous activityhad occurred throughout geologic timeMore than 6 catastrophes were neededto explain field observationsThe principle of uniformitarianismbecame the guiding philosophy of geology
26 Uniformitarianism Principle of uniformitarianism Present-day processes have operated throughout geologic time.Developed by James Hutton, advocated by Charles Lyell ( )Term uniformitarianism was coinedby William Whewell in 1832Hutton appliedthe principle of uniformitarianismwhen interpreting rocks at Siccar Point ScotlandWe now call what he observed an unconformitybut he properly interpreted its formation
27 Angular Unconformity at Siccar Point Hutton explained thatthe tilted, lower rocksresulted from severe upheavals that formed mountainsthese were then worn awayand covered by younger flat-lying rocksthe erosional surfacerepresents a gap in the rock record= Hiatus
28 3 Types of Unconformities Unconformities of regional extentmay change from one type to anotherThey may not represent the same amountof geologic time everywhere
29 A Disconformity A disconformity between sedimentary rocks in California, with conglomerate deposited uponan erosion surface in the underlying rocks
30 A Nonconformity A nonconformity in South Dakota separating Precambrian metamorphic rocks fromthe overlying Cambrian-aged Deadwood Formation
31 Uniformitarianism Hutton viewed Earth history as cyclical erosionerosionHutton viewed Earth history as cyclicaldepositionupliftHe also understoodthat geologic processes operate over a vast amount of timeModern view of uniformitarianismToday, geologists assume that the principles or laws of nature are constantbut the rates and intensities of change have varied through time
32 Crisis in Geology Lord Kelvin (1824-1907) knew about high temperatures inside of deep minesand reasoned that Earthis losing heat from its interiorAssuming Earth was once molten, he usedthe melting temperature of rocksthe size of Earthand the rate of heat lossto calculate the age of Earth asbetween 400 and 20 million years- too Young!!
33 Absolute-Dating Methods: 1. Radiometric Age Dating The discovery of radioactivitydestroyed Kelvin’s argument for the age of Earthand provided a clock to measure Earth’s ageRadioactivity is the spontaneous decayof an atom’s nucleus to a more stable formThe heat from radioactivityhelps explain why the Earth is still warm insideRadioactivity provides geologistswith a powerful tool to measureabsolute ages of rocks and past geologic events
34 Atoms Understanding absolute dating requires knowledge of atoms and isotopesAll matter is made up of atomsThe nucleus of an atom is composed ofprotons – particles with a positive electrical chargeneutrons – electrically neutral particleswith electrons – negatively charged particles – encircling the nucleusThe number of protons (= the atomic number)helps determine the atom’s chemical propertiesand the element to which it belongs
35 Isotopes Atomic mass number The different forms of an element’s atoms = number of protons + number of neutronsThe different forms of an element’s atomswith varying numbers of neutronsare called isotopesDifferent isotopes of the same elementhave different atomic mass numbersbut behave the same chemicallyMost isotopes are stable,but some are unstableGeologists use decay rates of unstable isotopesto determine absolute ages of rocks
36 Radioactive Decay Radioactive decay is the process whereby an unstable atomic nucleus spontaneously changesinto 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.
37 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.
38 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
40 Half-Lives: key to understanding The half-life of a radioactive isotopeis the time it takes forone half of the atomsof the original unstable parent isotopeto decay to atomsof a new more stable daughter isotopeThe half-life of a specific radioactive isotopeis constant and can be precisely measured
41 Half-Lives The length of half-lives for different isotopes of different elementscan vary fromless than 1/billionth of a secondto 49 billion yearsRadioactive decayis geometric not linear,so has a curved graph
42 Geometric Radioactive Decay In radioactive decay,during each equal time unitone half-life,the proportion of parent atomsdecreases by 1/2
43 Determining Age By measuring the parent/daughter ratio and knowing the half-life of the parentwhich has been determined in the laboratorygeologists can calculate the age of a samplecontaining the radioactive elementThe parent/daughter ratiois usually determined by a mass spectrometeran instrument that measures the proportionsof atoms with different masses
44 Determining Age For example: how old is the rock? 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.
45 What Materials Can Be Dated? Most radiometric dates are obtainedfrom igneous rocksAs magma cools and crystallizes,radioactive parent atoms separatefrom previously formed daughter atomsBecause they fit, some radioactive parentsare included in the crystal structureof certain minerals
46 Not Sedimentary RocksGenerally, sedimentary rocks cannot be radiometrically datedbecause the date obtainedwould correspond to the time of crystallization of the mineral,when it formed in an igneous or metamorphic rock,not the time that it was deposited as a sedimentary particleException: dating the mineral glauconite,because it forms in certain marine environments as a reaction with clayduring the formation of the sedimentary rock
47 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 atomscan have been added or removedfrom the sample since crystallizationIf leakage of daughters has occurredit partially resets the radiometric clockand the age will be too youngIf parents escape, the date will be too old.The most reliable dates use multiple methods.
48 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
49 Long-Lived Radioactive Isotope Pairs Used in Dating The isotopes used in radiometric datingneed to be sufficiently long-livedso the amount of parent material left is measurableSuch isotopes include:Parents Daughters Half-Life (years)Most of these are useful for dating older rocksUranium Lead billionUranium Lead millionThorium Lead billionRubidium Strontium billionPotassium Argon billion
50 2. Fission Track Dating Uranium in a crystal will damage the crystal structure as it decaysThe damage can be seen as fission tracksunder a microscope after etching the mineralThe age of the sample is related tothe number of fission tracksand the amount of uraniumwith older samples having more tracksThis method is useful for samples between 1.5 and 0.04 million years old
51 3. 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 ratioof 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
52 4. Tree-Ring Dating Method The age of a tree can be determinedby counting the annual growth ringsin lower part of the stem (trunk)The width of the rings are related to climateand can be correlated from tree to treea procedure called cross-datingThe tree-ring time scalenow extends back 14,000 years
53 Tree-Ring Dating Method In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans
54 Summary Early Christian theologians viewed time as linear and decided that Earthwas very young (about 6000 years old)A variety of ages for Earth were estimatedduring the 18th and 19th centuriesusing scientific evidence,ages now known to be too youngNeptunism and catastrophism were popularduring the 17th, 18th and early 19th centuriesbecause of their consistency with scripture,but were not supported by evidence
55 Summary James Hutton viewed Earth history as cyclical and very longHis observations were instrumentalin establishing the principle of uniformitarianismCharles Lyell articulated uniformitarianismin a way that soon made itthe guiding doctrine of geologyUniformitarianism holds thatthe laws of nature have been constant through timeand that the same processes operating todayhave operated in the past,although not necessarily at the same rates
56 Summary The principles of superposition, Radioactivity was discovered original horizontality,lateral continuityand cross-cutting relationshipsare basic for determining relative geologic agesand for interpreting Earth historyRadioactivity was discoveredduring the late 19th centuryand lead to radiometric dating,which allowed geologiststo determine absolute ages for geologic events
57 Summary Geologists determine how many half-lives of a radioactive parent isotopehave elapsed since the sample crystallizedHalf-life is the length of timeit takes for one-halfof the radioactive parent isotopeto decay to a stable daughter isotopeof a different element
58 Summary The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairsin igneous rocksCommon pairs include:uranium 238 – lead 206uranium 235 – lead 207thorium 232 – lead 208rubidium87 – strontium 87potassium 40 – argon 40
59 Summary The most reliable radiometric ages are obtained using two different pairsin the same rockCarbon 14 dating can be usedonly for organic matter such aswood, bones, and shellsand is effective backto about 70,000 years
60 Sedimentary Facies On a continental shelf, sand may accumulate in the high-energy nearshore environmentwhile mud and carbonate deposition takes placeat the same timein offshore low-energy environments
61 Uniform Linear Change In this example of uniform linear change, water is dripping into a glassat a constant rate
62 Carbon 14 Carbon 14 is constantly forming When a high-energy neutron in the upper atmosphereWhen a high-energy neutrona type of cosmic raystrikes a nitrogen 14 atomit may be absorbedby the nucleus and eject a protonchanging it to carbon 14The 14C formation rateis fairly constantand has been calibratedagainst tree rings
63 Carbon 14 The carbon 14 becomes While the organism lives part of the natural carbon cycleand becomes incorporated into organismsWhile the organism livesit continues to take in carbon 14but when it diesthe carbon 14 begins to decaywithout being replenishedThus, carbon 14 datingmeasures the time of death
64 What Materials Can Be Dated? The daughter atoms are different elementswith different sizesand, therefore,do not generally fitinto the same minerals as the parentsGeologists can use the crystals containingthe parents atomsto date the time of crystallization
65 Dating Metamorphism a. 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.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.
66 Crisis in Geology This age was too young for the geologic processes envisionedby other geologists at that time,leading to a crisis in geologyKelvin did not know about radioactivityas a heat source within the Earth
67 Igneous Crystallization Crystallization of magma separates parent atomsfrom previously formed daughtersThis resets the radiometric clock to zero.Then the parents gradually decay.