Presentation on theme: "The Rock and Fossil Record"— Presentation transcript:
1The Rock and Fossil Record Chapter 6Earth’s Story and Those Who First ListenedRelative Dating: Which Came First?Absolute Dating: A Measure of TimeLooking at FossilsTime Marches On
2Terms You Must Know Fossil Uniformitarianism Trace fossil MoldCastIndex fossilGeologic time scaleEonEraPeriodEpochextinctionUniformitarianismCatastrophismPaleontologyRelative datingSuperpositionGeologic columnUnconformityAbsolute datingIsotopeRadioactive decayRadiometric datingHalf-life
3People To KnowJames HuttonCharles LyellGeorges Cuvier
4UniformitarianismDeveloped by James Hutton, advocated by Charles Lyell ( )James Hutton wrote Theory of the EarthHutton stated that present-day processes, such as erosion and deposition, have operated throughout geologic timeUniformitarianism is a principle that states that geologic processes that occurred in the past can be explained by current geologic processesHutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point ScotlandWe now call what he observed an unconformitybut he properly interpreted its formation
5Siccar Point Hutton’s theories sparked a scientific debate In Hutton’s time, people believed that the Earth was only a few thousand years old.What Hutton proposed could not happen in just a few thousand yearsHe formed his theories by observing the geologic processes at Siccar PointDeposition and folding were observed
6UnconformityHutton first described the evidence for this break in the rock record. He was impressed by a placed called Siccar Point in Scotland where some of the rocks were laid down horizontally, but the rocks underneath them were at an angle. The only explanation Hutton felt was plausible was that the underlying rocks had originally been laid down horizontally, then were tilted and eroded and more rocks were then deposited on top of them. Hutton realized that this scenario represented a large gap in time between the first set of rocks being laid down, then tilted, and the second set of rocks deposited. These gaps in the rock record are called unconformities.
8UniformitarianismerosiondepositionupliftHutton viewed Earth history as cyclicalHe also understood that geologic processes operate over a vast amount of timeModern view of uniformitarianismgeologists assume that the principles or laws of nature are constantbut the rates and intensities of change have varied through time
10Grand CanyonMore than 1 billion years of history are preserved in the rock layers of the Grand CanyonReading this rock book shows:periods of mountain buildingadvancing and retreating shallow seasevolution of plant and animal speciesDetermine these things by:applying the principles of relative dating to the rocksand recognizing that present-day processes have operated throughout Earth history - Uniformitarianism
11Catastrophism Catastrophism proposed by Georges Cuvier (1769-1832) dominated European geologic thinkingthe 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, he last one was the biblical flood
12Uniformitarianism vs Catastrophism Catastrophism remained the guiding principle until the work of Charles LyellLyell published Principles of Geology, in which he reintroduced uniformitarianism.He believed that geologic change happened at the same rate in the past as it happens in the present-- gradually!
13Lyell and Darwin Charles Darwin and Charles Lyell were good friends. Darwin accepted and supported uniformitarianismDarwin had read Lyell’s book Principles of Geology before his famous 1831 voyage on HMS BeagleDespite being friends, Lyell did not embrace Darwin’s theories of natural selection.Much later, Lyell finally accepted Darwin’s theories.
14Modern GeologyModern scientists like Stephen Gould have challenged Lyell’s uniformitarianism.Today scientists beleive that catastrophes do at times play an important role in shaping Earth’s history.Most geologic change is gradual and uniform but catastrophes have caused geologic change.Ex. Craters formed due to asteroids and comets
15PaleontologyPaleontology is thee scientific study of fossils- plant and animal.Fossils are the remains of organisms preserved by geologic processes.Vertebrate paleontologists study fossils of animals that have backbones.Invertebrate paleontologists study fossils of animals that do not have backbones.Paleobotanists study fossils of plants
16Relative Dating"Relative Dating" This phrase may conjure up odd jokes and images of kissing cousins to some, but to geologists the phrase refers to distinguishing the age relationships between contiguous rock layers.
17Relative-Dating Principles Relative dating is any method of determining whether an event or object is older or younger than other events or objects.SuperpositionOldest on bottom, youngest on topChattanooga Shale, TN
18Relative Dating Relative dating Can only be used when the rock layers have been preserved in their original sequence – top layer being the youngest layerHelps scientists determine whether one fossil is older than the other
19Disturbing ForcesNot all rock formations are arranged with the oldest layers on the bottom.Natural forces can fold, tilt, break, or remove parts of the rock layerGeologists use a geologic column to help themRelative dating assumes that if rock layers are not horizontal, then something must have disturbed them after they formed.
20Unconformities What is unconformity? Unconformity is a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.A surface of erosion or non-depositionRecognizable surface in the rock record
21UnconformitiesUnconformities involve time gaps, typically on the order of tens of millions of years or more. A time gap refers to missing time (as in taking a vacation). A time gap may be due to a time of "nondeposition", meaning that no sediments were deposited for an interval of time. More likely, unconformities indicate a time when uplift and erosion have occurred such that layers deposited at an earlier time have been stripped away. Typically, unconformities involve:Major sea level changesMajor tectonic events
22UnconformityWhen a geologists finds an unconformity, they must question whether the “missing layer” was never present or whether is was somehow removedNondeposition- stopage of deposition when a supply of sediment is cut offErosion can create unconformities.
233 Types of Unconformity Disconformities Nonconformities Most commonPart of a sequence of parallel rock layers is missingNonconformitiesAngular unconformities
24disconformityDisconformities are much harder to recognize in the field, because often there is no angular relationship between sets of layers. Disconformities are usually recognized by correlating from one area to another and finding that some strata is missing in one of the areas.
25nonconfomityNonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface. The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion.
26NonconfomityNotice that there is an "intersection" of a vertical rock butting up against a horizontal sock. In an unconformity, it is two of the same type of rock (e.g. sedimentary & sedimentary). A nonconformity is two layers of different types (e.g. igneous & sedimentary).
27Angular unconformityAngular unconformities are easy to recognize in the field because of the angular relationship of layers that were originally deposited horizontally.
28Example of an Unconformity Tilted sandstone and siltstone below, conglomerate above
29Dating rocks Relative dating Using a set of principles to put rocks in their proper sequences of formationAbsolute datingUsing radioactive decay to determine the exact age of rocks
30Absolute Dating: any method of measuring the age of an event or object in years Most common:based on Radioactive DecayParentdaughterWhy does it work?1. The decay rate is CONSTANT,independent of external conditions in the earth.2. The daughter/Parent ratio can be precisely measured.
31Radioactive DecayThe process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.Sounds great but what is an isotope?An isotope is an atom that has the same number of protons (atomic #) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)
32IsotopesMost isotopes are stable, meaning they stay in their original formBut some are unstableUnstable isotopes are radioactiveRadioactive decay is the process in which a radioactive isotope tends to bread down into a stable isotope of the same or another element
33How does it work? What does this have to do with the age of rocks? Unstable isotope is called parent isotopeThe stable isotope produced by radioactive decay is the daughter isotope.Decay is constantThe more daughter isotope- the older the rock!
34Radiometric datingA method of determining the age of an object by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotopeRatio or parent material to daughter material
35Absolute datingHelps scientists determine actual age of fossilsRocks near fossils contain radioactive elements – unstable elements that break down into different elementsHalf-life of a radioactive element is the time it takes for half of the atoms in a sample to decayScientists compare the amount of radioactive element in a sample to the amount of the element into which it breaks downScientists use this info to calculate the age of the rock, which then tells the age of the fossil
36Half-life – the time required for one-half of the radioactive nuclei in a sample to decay
37Dating with carbon-14 (radiocarbon dating) Carbon is normally found in three forms: stable C-12, stable C-14 & radioactive C-14All combine with oxygen to form CO2Half-life of only 5730 yearsUsed to date very recent eventsCarbon-14 is produced in the upper atmosphereUseful tool for anthropologists, archeologists, and geologists who study very recent Earth history
38Carbon-14Carbon-14 is continuously created in the atmosphere by cosmic radiation.There is one atom of radioactive C-14 for every trillion atoms of C-12 in the atmospherePlants absorb C-14 directly through their leaves in the form of carbon dioxideAnimals take in C-14 indirectly when they eat plantsAlthough C-14 disintegrates at a constant rate, it is continuously renewed as long as an organism remains alive.When an organism dies, it stops absorbing new C-14 and its radiocarbon “clock” is set.
39Types of radiometric dating Potassium-argon methodK-40 half-life 1.3 billion yearsDecays into argon and calciumUsed to date rocks older than 100,00 years oldUranium-lead methodU-238 half-life 4.5 billion yearsDecays into lead-206Used for rocks more than 10 million years oldRubidium-strontium methodRb-87 half-life 49 billion yearsDecays into strontium-87Carbon-14 method
40Relative-Dating Principles Lateral continuitysediment extends laterally in all directions until it thins and pinches out or terminates against the edges of the depositional basinCross-cutting relationshipsan igneous intrusion or a fault must be younger than the rocks it intrudes or displaces
41Cross-cutting Relationships A dark-colored dike has intruded into older light colored granite: the dike is younger than the graniteNorth shore of Lake Superior, Ontario Canada
42Cross-cutting Relationships A small fault displaces tilted beds: the fault is younger than the bedsTemplin Highway, Castaic, California
44Why are layers tilted? Deformation of rocks Folding Faulting Occurs after they are depositedImportant factor in relative datingFoldingAnticlines, synclinesRock bends, but does not breakFaultingNormal, reverse, transformRock breaks
52How was the timescale created? Mapping in 1800s using the principles ofSuperpositionOriginal HorizontalityOriginal Lateral ContinuityCross-cutting relationshipsAlso Fossil Correlation
53Absolute DatingRadiometric dating is the most common method of obtaining absolute agescalculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocksOther methods?Tree ring countingVarvesIce cores
54Geologic Time ScaleThe discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scaleThe geologic time scale is a dual scalea relative scaleand an absolute scale
55Changes in the Concept of Geologic Time The concept and measurement of geologic time has changed through human historyJames Ussher ( ) in Irelandcalculated the age of Earth based on 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
56Changes in the Concept of Geologic Time During the 1700s and 1800s Earth’s age was estimated scientificallyGeorges Louis de Buffon ( ) calculated how long Earth took to cool gradually from a molten beginningused melted iron balls of various diametershe estimated Earth was 75,000 years old
57Changes in the Concept of Geologic Time Others used rates of deposition of various sediments and thickness of sedimentary rock in the crustgave estimates of <1 millionto more than 2 billion yearsOr the amount of salt carried by rivers to the ocean and the salinity of seawaterJohn Joly in 1899 obtained a minimum age of 90 million years
58History of Historical Geology Neptunismproposed in 1787 by Abraham Werner ( )all rocks, including granite and basalt, were precipitated in an orderly sequence from a primeval, worldwide oceanWerner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history
59History of Historical Geology Neptunism and Catastrophism were eventually abandonedthey were not supported by field evidencebasalt was shown to be of igneous originvolcanic rocks interbedded with sedimentaryprimitive rocks showed that igneous activity had occurred throughout geologic timemore than 6 catastrophes were needed to explain field observationsThe principle of uniformitarianism became the guiding philosophy of geology
60Crisis in Geology Lord Kelvin (1824-1907) knew about high temperatures inside of deep mines and reasoned that Earth is 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 as between 400 and 20 million years
61Crisis in GeologyThis age was too young for the geologic processes envisioned by other geologists at that timeleading to a crisis in geologyKelvin did not know about radioactivity as a heat source within the Earth
62Absolute-Dating Methods The discovery of radioactivity destroyed Kelvin’s argument for the age of EarthRadioactivity is the spontaneous decay of an atom’s nucleus to a more stable formThe heat from radioactivity helps explain why the Earth is still warm insideRadioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events
63Absolute-Dating Methods Understanding absolute dating requires knowledge of atoms and isotopes: we have it!Atomic mass number= number of protons + number of neutronsIsotopes: different numbers of neutronsDifferent isotopes have different atomic mass numbers but behave the same chemicallyMost isotopes are stablebut some are unstableGeologists use decay rates of unstable isotopes to determine absolute ages of rocks
64Radioactive DecayRadioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different elementThree types of radioactive decay:alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus
65Half-LivesHalf-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 measured
66Half-LivesThe length of half-lives for different isotopes of different elements can vary fromless than 1/billionth of a secondto 49 billion yearsRadioactive decayis geometric not lineara curved graph
67Geometric Radioactive Decay In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2
68Determining AgeBy 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 elementThe parent/daughter ratio is usually determined by a mass spectrometeran instrument that measures the proportions of atoms with different masses
69Determining 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.
70What Materials Can Be Dated? Most radiometric dates are obtained from igneous rocksAs magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atomsthey fit differently into the crystal structure of certain mineralsGeologists can use the crystals containing the parents atoms to date the time of crystallization
71Igneous Crystallization Crystallization of magma separates parent atoms from previously formed daughtersThis resets the radiometric clock to zeroThen the parents gradually decay
72Sources of Uncertainty Closed system is needed for an accurate dateneither 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 oldMost reliable dates use multiple methods
73Sources of Uncertainty Dating techniques are always improvingPresently 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
74Dating 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 (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.
75Long-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 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
77How do we know the Earth can’t be older than about 6-7 b.y.? Moderate half-life isotopes (1 b.y.)If Earth was > 6-7 b.y. old, there wouldn’t be many parents left
78Radiocarbon 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 14 makes it suitable for dating material < 70,000 years oldIt is not useful for most rocks, but is useful for archaeology and young geologic materials
79Carbon 14 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 14The 14C formation rateis fairly constantand has been calibrated against tree rings
80Carbon 14The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organismsWhile the organism lives it continues to take in carbon 14when it dies the carbon 14 begins to decay without being replenishedThus, carbon 14 dating measures the time of death
81Tree-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 and can be correlated from tree to treea procedure called cross-datingThe tree-ring time scale now extends back 14,000 years!
82Tree-Ring Dating Method In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans
83Summary Uniformitarianism holds that the laws of nature have been constant through timeand that the same processes operating today have operated in the pastalthough not necessarily at the same rates
84Summary The principles of superposition original horizontality,lateral continuityand cross-cutting relationshipsare basic for determining relative geologic ages and for interpreting Earth historyRadioactivity was discovered during the late 19th centuryand lead to radiometric datingwhich allowed geologists to determine absolute ages for geologic events
85SummaryHalf-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 elementThe most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairsin igneous rocks
86SummaryThe most reliable radiometric ages are obtained using two different pairs in the same rockCarbon 14 dating can be used only for organic matter such aswood, bones, and shellsand is effective back to about 70,000 years