Geologic Time—Geologic Time—Concepts and Concepts and

PrinciplesPrinciples

Chapter 4Chapter 4

►When looking down into the Grand When looking down into the Grand Canyon, we are really looking all the Canyon, we are really looking all the way back to the early history of Earthway back to the early history of Earth

Grand CanyonGrand Canyon

► Geologists use two different frames of Geologists use two different frames of reference when discussing geologic timereference when discussing geologic time

Relative datingRelative dating involves placing geologic events in involves placing geologic events in a sequential order as determined from their position a sequential order as determined from their position in the geologic recordin the geologic record

► It does not tell us how long ago a particular event occurred It does not tell us how long ago a particular event occurred only that one event preceded anotheronly that one event preceded another

► For hundreds of years geologists have been using relative For hundreds of years geologists have been using relative dating to establish a relative geologic time scaledating to establish a relative geologic time scale

Concept of Geologic TimeConcept of Geologic Time

►The relative geologic The relative geologic time scale has a time scale has a sequence of sequence of eons eons eras eras periods periods epochs epochs but no numbers but no numbers

indicating how long indicating how long ago each of these ago each of these times occurredtimes occurred

Relative Geologic Time Relative Geologic Time ScaleScale

► The second frame of reference for geologic The second frame of reference for geologic time is absolute datingtime is absolute dating Absolute datingAbsolute dating results in specific dates for results in specific dates for

rock units or eventsrock units or events►expressed in years before the presentexpressed in years before the present

► Radiometric datingRadiometric dating is the most common is the most common method of obtaining absolute agesmethod of obtaining absolute ages Such dates are calculated Such dates are calculated from the natural from the natural

rates of decay of various natural radioactive rates of decay of various natural radioactive elements present in trace amounts in some elements present in trace amounts in some rocksrocks

Concept of Geologic TimeConcept of Geologic Time

► The discovery of The discovery of radioactivity near the end of radioactivity near the end of the 1800s allowed absolute the 1800s allowed absolute ages to be accurately ages to be accurately applied to the relative applied to the relative geologic time scalegeologic time scale The geologic time scale is The geologic time scale is

a dual scalea dual scale►a relative scalea relative scale►and an absolute scaleand an absolute scale

Geologic Time ScaleGeologic Time Scale

► The concept and measurement of geologic The concept and measurement of geologic time has changed through human historytime has changed through human history Early Christian theologians conceived of time as Early Christian theologians conceived of time as

linear rather than circularlinear rather than circular► James UsherJames Usher (1581-1665) in Ireland (1581-1665) in Ireland

calculated the age of Earth based on recorded calculated the age of Earth based on recorded history and genealogies in Genesishistory and genealogies in Genesis

►Announced that Earth was created on October 22, Announced that Earth was created on October 22, 4004 B.C.4004 B.C.

►A century later it was considered heresy to say A century later it was considered heresy to say Earth was more than about 6000 years old.Earth was more than about 6000 years old.

Changes in the Concept of Changes in the Concept of Geologic TimeGeologic Time

►During the 1700s and 1800s Earth’s During the 1700s and 1800s Earth’s age was estimated scientificallyage was estimated scientifically Georges Louis de Buffon (1707-1788)Georges Louis de Buffon (1707-1788)

►calculated how long Earth took to cool calculated how long Earth took to cool gradually from a molten beginning using gradually from a molten beginning using melted iron balls of various diametersmelted iron balls of various diameters

►Extrapolating their cooling rate to an Earth-Extrapolating their cooling rate to an Earth-sized ball, he estimated Earth was 75,000 sized ball, he estimated Earth was 75,000 years oldyears old

Changes in the Concept of Changes in the Concept of Geologic TimeGeologic Time

►Others used different techniquesOthers used different techniques Using rates of deposition of various Using rates of deposition of various

sediments and thickness of sedimentary sediments and thickness of sedimentary rock in the crust gave estimates of 1 rock in the crust gave estimates of 1 million to more than 2 billion years.million to more than 2 billion years.

Using the amount of salt carried by Using the amount of salt carried by rivers to the ocean and the salinity of rivers to the ocean and the salinity of seawater John Joly in 1899 obtained a seawater John Joly in 1899 obtained a minimum age of 90 million yearsminimum age of 90 million years

Changes in the Concept of Changes in the Concept of Geologic TimeGeologic Time

►Six fundamental geologic principles Six fundamental geologic principles are used in relative datingare used in relative dating Principle of superpositionPrinciple of superposition

►Nicolas Steno (1638-1686)Nicolas Steno (1638-1686) In an undisturbed succession of sedimentary rock In an undisturbed succession of sedimentary rock

layers, the oldest layer is at the bottom and the layers, the oldest layer is at the bottom and the youngest layer is at the topyoungest layer is at the top

This method is used for determining the This method is used for determining the relative age of rock layers (strata) and relative age of rock layers (strata) and the fossils they containthe fossils they contain

Relative-Dating PrinciplesRelative-Dating Principles

►Principle of original horizontalityPrinciple of original horizontality Nicolas Steno Nicolas Steno

►Sediment is deposited in essentially Sediment is deposited in essentially horizontal layershorizontal layers

Therefore, a sequence of sedimentary rock layers Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have that is steeply inclined from horizontal must have been tilted after deposition and lithificationbeen tilted after deposition and lithification

Relative-Dating PrinciplesRelative-Dating Principles

Illustration of the principles of original Illustration of the principles of original horizontalityhorizontality

Illustration of the principles of Illustration of the principles of superpositionsuperposition

►Principle of lateral continuityPrinciple of lateral continuity Nicolas StenoNicolas Steno

►Sediment extends laterally in all direction Sediment extends laterally in all direction until it thins and pinches out or terminates until it thins and pinches out or terminates against the edges of the depositional basinagainst the edges of the depositional basin

►Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships James Hutton (1726-1797)James Hutton (1726-1797)

►An igneous intrusion or a fault must be An igneous intrusion or a fault must be younger than the rocks it intrudes or younger than the rocks it intrudes or displacesdisplaces

Relative-Dating PrinciplesRelative-Dating Principles

►Principle of inclusionsPrinciple of inclusions discussed later in the term discussed later in the term

►Principle of fossil succession Principle of fossil succession discussed later in the term discussed later in the term

Relative-Dating PrinciplesRelative-Dating Principles

North shore of Lake Superior, North shore of Lake Superior,

Ontario CanadaOntario Canada ►A dark-colored A dark-colored

dike has dike has intruded into intruded into older light older light colored granite.colored granite.

Cross-Cross-cutting cutting

RelationshipRelationshipss

►The dike is The dike is younger than younger than the granite.the granite.

Templin Highway, Templin Highway, Castaic, CaliforniaCastaic, California

►A small fault A small fault displaces displaces tilted beds.tilted beds.

Cross-cutting Cross-cutting RelationshipsRelationships

►The fault is The fault is younger than younger than the beds.the beds.

►NeptunismNeptunism All rocks, including granite and basalt, All rocks, including granite and basalt,

were precipitated in an orderly sequence were precipitated in an orderly sequence from a primeval, worldwide ocean.from a primeval, worldwide ocean.

►proposed in 1787 by Abraham Werner (1749-proposed in 1787 by Abraham Werner (1749-1817)1817)

►Werner was an excellent mineralogist, but is Werner was an excellent mineralogist, but is best remembered for his incorrect best remembered for his incorrect interpretation of Earth historyinterpretation of Earth history

NeptunismNeptunism

►Werner’s geologic column was widely Werner’s geologic column was widely acceptedaccepted Alluvial rocksAlluvial rocks

►unconsolidated sediments, youngestunconsolidated sediments, youngest Secondary rocksSecondary rocks

►rocks such as sandstones, limestones, coal, basaltrocks such as sandstones, limestones, coal, basalt Transition rocksTransition rocks

►chemical and detrital rocks, some fossiliferous rockschemical and detrital rocks, some fossiliferous rocks Primitive rocksPrimitive rocks

►oldest including igneous and metamorphicoldest including igneous and metamorphic

NeptunismNeptunism

► Proposed by Georges Cuvier (1769-1832)Proposed by Georges Cuvier (1769-1832)► Dominated European geologic thinkingDominated European geologic thinking

The physical and biological history of Earth resulted The physical and biological history of Earth resulted from a series of sudden widespread catastrophes from a series of sudden widespread catastrophes which accounted for significant and rapid changes which accounted for significant and rapid changes in Earth and exterminated existing life in the in Earth and exterminated existing life in the affected areaaffected area

► Six major catastrophes occurred, Six major catastrophes occurred, corresponding to the six days of biblical corresponding to the six days of biblical creation.The last one was the biblical floodcreation.The last one was the biblical flood

CatastrophismCatastrophism

► They were not supported by field evidenceThey were not supported by field evidence Basalt was shown to be of igneous originBasalt was shown to be of igneous origin Volcanic rocks interbedded with sedimentary Volcanic rocks interbedded with sedimentary and primitive rocks showed that igneous activity and primitive rocks showed that igneous activity

had occurred throughout geologic timehad occurred throughout geologic time

► More than 6 catastrophes were needed to More than 6 catastrophes were needed to explain field observationsexplain field observations

► The principle of uniformitarianism became The principle of uniformitarianism became the guiding philosophy of geologythe guiding philosophy of geology

Neptunism and Neptunism and Catastrophism Were Catastrophism Were

Eventually abandonedEventually abandoned

► Principle of uniformitarianismPrinciple of uniformitarianism Present-day processes have operated throughout Present-day processes have operated throughout

geologic time.geologic time. Developed by James Hutton, advocated by Charles Developed by James Hutton, advocated by Charles

Lyell (1797-1875)Lyell (1797-1875)► Hutton applied the principle of uniformitarianism when Hutton applied the principle of uniformitarianism when

interpreting rocks at Siccar Point Scotlandinterpreting rocks at Siccar Point Scotland► We now call what he observed an We now call what he observed an unconformity unconformity but he but he

properly interpreted its formationproperly interpreted its formation► Term uniformitarianism was coined by William Whewell in Term uniformitarianism was coined by William Whewell in

18321832

UniformitarianismUniformitarianism

Unconformity at Siccar Unconformity at Siccar PointPoint

►Hutton explained thatHutton explained that the tilted, lower rocks resulted from the tilted, lower rocks resulted from

severe upheavals that formed mountainssevere upheavals that formed mountains

The mountains were then worn away and The mountains were then worn away and covered by younger flat-lying rockscovered by younger flat-lying rocks

the erosional surface represents a gap in the the erosional surface represents a gap in the rock recordrock record

►Hutton viewed Hutton viewed Earth history as Earth history as cyclicalcyclical

UniformitarianismUniformitarianism

deposition uplift

► He also understood that geologic He also understood that geologic processes processes operate over a vast amount of timeoperate over a vast amount of time

► Modern view of uniformitarianismModern view of uniformitarianism Today, geologists assume that the principles or Today, geologists assume that the principles or

laws of nature are constant but the rates and laws of nature are constant but the rates and intensities of change have varied through timeintensities of change have varied through time

erosion

► Lord Kelvin (1824-1907)Lord Kelvin (1824-1907) knew about high temperatures inside of deep knew about high temperatures inside of deep

mines and reasoned that Earth is losing heat mines and reasoned that Earth is losing heat from its interiorfrom its interior

► Assuming Earth was once molten, he used Assuming Earth was once molten, he used the melting temperature of rocksthe melting temperature of rocks the size of Earththe size of Earth and the rate of heat loss to calculate the age of and the rate of heat loss to calculate the age of

Earth as between 400 and 20 million yearsEarth as between 400 and 20 million years

Crisis in GeologyCrisis in Geology

► For the geologic processes envisioned by For the geologic processes envisioned by other geologists at that time, this age was other geologists at that time, this age was too young!too young!

What was the flaw in Kelvin’s calculation?What was the flaw in Kelvin’s calculation?

►Kelvin did not know about radioactivity as a heat Kelvin did not know about radioactivity as a heat source within the Earthsource within the Earth

Crisis in GeologyCrisis in Geology

► The discovery of radioactivity The discovery of radioactivity destroyed Kelvin’s argument for the destroyed Kelvin’s argument for the age of Earth and provided a clock to age of Earth and provided a clock to measure Earth’s agemeasure Earth’s age

► RadioactivityRadioactivity is the spontaneous decay of is the spontaneous decay of an atom’s nucleus to a more stable forman atom’s nucleus to a more stable form The heat from radioactivity helps explain why The heat from radioactivity helps explain why

the Earth is still warm insidethe Earth is still warm inside Radioactivity provides geologists with a powerful Radioactivity provides geologists with a powerful

tool to measure absolute ages of rocks and past tool to measure absolute ages of rocks and past geologic eventsgeologic events

Absolute-Dating MethodsAbsolute-Dating Methods

► Understanding absolute dating requires knowledge of Understanding absolute dating requires knowledge of atoms and isotopesatoms and isotopes

► The The nucleusnucleus of an atom is composed of of an atom is composed of protonsprotons – particles with a positive electrical charge – particles with a positive electrical charge neutronsneutrons – electrically neutral particles – electrically neutral particles electronselectrons – the – the negatively charged particlesnegatively charged particles – encircling the – encircling the

nucleusnucleus► atomic numberatomic number

Equal to the number of protonsEqual to the number of protons helps determine the atom’s chemical properties and the helps determine the atom’s chemical properties and the

element to which it belongselement to which it belongs

AtomsAtoms

► Atomic mass numberAtomic mass number = number of protons + = number of protons + number of neutronsnumber of neutrons The different forms of an element’s atoms with The different forms of an element’s atoms with

varying numbers of neutrons are called varying numbers of neutrons are called isotopesisotopes ► Different isotopes of the same element have different atomic Different isotopes of the same element have different atomic

mass numbers but behave the same chemicallymass numbers but behave the same chemically

► Most isotopes are stable, Most isotopes are stable, but some are unstablebut some are unstable

► Geologists use decay rates of unstable isotopes Geologists use decay rates of unstable isotopes to determine absolute ages of rocksto determine absolute ages of rocks

IsotopesIsotopes

► Radioactive decayRadioactive decay -the process whereby an -the process whereby an unstable atomic nucleus spontaneously changes unstable atomic nucleus spontaneously changes into an atomic nucleus of a different elementinto an atomic nucleus of a different element

► Three types of radioactive decay:Three types of radioactive decay: In In alpha decayalpha decay, two protons and two neutrons , two protons and two neutrons

(alpha particle) are emitted from the nucleus.(alpha particle) are emitted from the nucleus.

Radioactive DecayRadioactive Decay

In In beta decaybeta decay, a neutron emits a fast moving electron , a neutron emits a fast moving electron (beta particle) and becomes a proton.(beta particle) and becomes a proton.

Radioactive DecayRadioactive Decay

In In electron capture decayelectron capture decay, a proton captures an electron , a proton captures an electron and converts to a neutron.and converts to a neutron.

► Some isotopes undergo only one decay step Some isotopes undergo only one decay step before they become stable.before they become stable. Examples:Examples:

► rubidium 87rubidium 87 decays to strontium 87 by a single beta decays to strontium 87 by a single beta emissionemission

► potassium 40potassium 40 decays to argon 40 by a single electron decays to argon 40 by a single electron capturecapture

► But other isotopes undergo several decay stepsBut other isotopes undergo several decay steps Examples:Examples:

► uranium 235uranium 235 decays to lead 207 by 7 alpha steps and 6 decays to lead 207 by 7 alpha steps and 6 beta stepsbeta steps

► uranium 238uranium 238 decays to lead 206 by 8 alpha steps and 6 decays to lead 206 by 8 alpha steps and 6 beta stepsbeta steps

Radioactive DecayRadioactive Decay

Uranium 238 Uranium 238 decaydecay

► The The half-lifehalf-life of a radioactive isotope is the time of a radioactive isotope is the time it takes for one half of the atoms of the original it takes for one half of the atoms of the original unstable unstable parent isotopeparent isotope to decay to atoms of a to decay to atoms of a new more stable new more stable daughter isotopedaughter isotope

► The half-life of a specific radioactive isotope is The half-life of a specific radioactive isotope is constant and can be precisely measuredconstant and can be precisely measured

► Can vary from less than 1/billionth of a second to Can vary from less than 1/billionth of a second to 49 billion years49 billion years

► Is geometric not linear, so has a curved graphIs geometric not linear, so has a curved graph

Half-LivesHalf-Lives

► In this example of In this example of uniform linear change, uniform linear change, water is dripping into a water is dripping into a glass at a constant rateglass at a constant rate

Uniform Linear ChangeUniform Linear Change

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

Geometric Radioactive Geometric Radioactive DecayDecay

► By measuring the parent/daughter ratio and knowing By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined the half-life of the parent which has been determined in the laboratory geologists can calculate the age of a in the laboratory geologists can calculate the age of a sample containing the radioactive elementsample containing the radioactive element

► The parent/daughter ratio is usually determined by a The parent/daughter ratio is usually determined by a mass spectrometer an instrument that measures the mass spectrometer an instrument that measures the proportions of atoms with different massesproportions of atoms with different masses

Determining AgeDetermining Age

► For example: For example: If a rock has a parent/daughter ratio of 1:3 If a rock has a parent/daughter ratio of 1:3

= a parent proportion of 25%, = a parent proportion of 25%, and the half-live is 57 million years, and the half-live is 57 million years,

Determining AgeDetermining Age

25% means it is 2 half-25% means it is 2 half-lives old.lives old.

the rock is 57 x 2 =114 the rock is 57 x 2 =114 million years old.million years old.

► Most radiometric dates are obtained from Most radiometric dates are obtained from igneous rocksigneous rocks

► As magma cools and crystallizes, As magma cools and crystallizes, radioactive parent atoms separate from radioactive parent atoms separate from

previously formed daughter atomspreviously formed daughter atoms Some radioactive parents are included in the Some radioactive parents are included in the

crystal structure of certain mineralscrystal structure of certain minerals

What Materials Can Be What Materials Can Be Dated?Dated?

► The daughter atoms are different elements with The daughter atoms are different elements with different sizes and, therefore, do not generally different sizes and, therefore, do not generally fit into the same minerals as the parentsfit into the same minerals as the parents

► Geologists can use the crystals containing the Geologists can use the crystals containing the parents atoms to date the time of crystallizationparents atoms to date the time of crystallization

What Materials Can Be What Materials Can Be Dated?Dated?

► Crystallization of magma separates parent atoms Crystallization of magma separates parent atoms from previously formed daughtersfrom previously formed daughters

► This resets the radiometric clock to zero.This resets the radiometric clock to zero.► Then the parents gradually decay.Then the parents gradually decay.

Igneous Crystallization Igneous Crystallization

► Generally, sedimentary rocks cannot be radiometrically Generally, sedimentary rocks cannot be radiometrically dated because the date obtained would correspond to dated because the date obtained would correspond to the time of crystallization of the mineral, when it formed the time of crystallization of the mineral, when it formed in an igneous or metamorphic rock,not the time that it in an igneous or metamorphic rock,not the time that it was deposited as a sedimentary particle was deposited as a sedimentary particle

► Exception: dating the mineral glauconite, because it Exception: dating the mineral glauconite, because it forms in certain marine environments as a reaction with forms in certain marine environments as a reaction with clay during the formation of the sedimentary rockclay during the formation of the sedimentary rock

Not Sedimentary RocksNot Sedimentary Rocks

► In glauconite, potassium 40 decays to argon 40In glauconite, potassium 40 decays to argon 40 because argon is a gas, it can easily escape from a because argon is a gas, it can easily escape from a

mineralmineral► A closed system is needed for an accurate dateA closed system is needed for an accurate date

that is, neither parent nor daughter atoms can have been that is, neither parent nor daughter atoms can have been added or removed from the sample since crystallizationadded or removed from the sample since crystallization

► If leakage of daughters has occurred If leakage of daughters has occurred it partially resets the radiometric clock and the age will be it partially resets the radiometric clock and the age will be

too youngtoo young► If parents escape, the date will be too old.If parents escape, the date will be too old.► The most reliable dates use multiple methods.The most reliable dates use multiple methods.

Sources of UncertaintySources of Uncertainty

► During metamorphism, some of the daughter During metamorphism, some of the daughter atoms may escapeatoms may escape leading to a date that is too young.leading to a date that is too young. However, if all of the daughters are forced out during However, if all of the daughters are forced out during

metamorphism, then the date obtained would be the metamorphism, then the date obtained would be the time of metamorphism—a useful piece of information.time of metamorphism—a useful piece of information.

► Dating techniques are always improving.Dating techniques are always improving. Presently measurement error is typically <0.5% of Presently measurement error is typically <0.5% of

the age, and even better than 0.1%the age, and even better than 0.1% A date of 540 million might have an error of ±2.7 A date of 540 million might have an error of ±2.7

million years or as low as ±0.54 millionmillion years or as low as ±0.54 million

Sources of UncertaintySources of Uncertainty

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

Dating MetamorphismDating Metamorphism

b. As time passes, parent b. As time passes, parent atoms decay to daughtersatoms decay to daughters..

c. Metamorphism drives the c. Metamorphism drives the daughters out of the daughters out of the mineral as it mineral as it recrystallizesrecrystallizes..

d. Dating the mineral today d. Dating the mineral today yields a date of 350 million yields a date of 350 million years = time of years = time of metamorphism, provided metamorphism, provided the system remains closed the system remains closed during that time.during that time.

Dating the whole rock Dating the whole rock yields a date of 700 yields a date of 700 million years = time million years = time of crystallization.of crystallization.

► The isotopes used in radiometric dating The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent need to be sufficiently long-lived so the amount of parent

material left is measurablematerial left is measurable

► Such isotopes include:Such isotopes include:ParentsParents DaughtersDaughters Half-Life (years)Half-Life (years)

Long-Lived Radioactive Long-Lived Radioactive Isotope Pairs Used in Isotope Pairs Used in

DatingDating

Uranium 238Uranium 238 Lead 206 Lead 206 4.5 billion4.5 billionUranium 234Uranium 234 Lead 207 Lead 207 704 million704 millionThorium 232Thorium 232 Lead 208 Lead 208 14 billion14 billionRubidium 87Rubidium 87 Strontium 87 Strontium 87 48.8 billion 48.8 billionPotassium 40Potassium 40 Argon 40 Argon 40 1.3 billion1.3 billion

► Uranium in a crystal will damage the crystal Uranium in a crystal will damage the crystal structure as it decaysstructure as it decays

► The damage can be seen as fission tracks The damage can be seen as fission tracks under a microscope after etching the under a microscope after etching the mineralmineral

Fission Track DatingFission Track Dating

► The age of the The age of the sample is related sample is related to to the number of the number of

fission tracks fission tracks the amount of the amount of

uraniumuranium

► Carbon is found in all lifeCarbon is found in all life► It has 3 isotopes It has 3 isotopes

carbon 12 and 13 are stable but carbon 14 is notcarbon 12 and 13 are stable but carbon 14 is not Carbon 14 has a half-life of Carbon 14 has a half-life of 57305730 years years Carbon 14 dating uses the carbon 14/carbon 12 ratio Carbon 14 dating uses the carbon 14/carbon 12 ratio

of material that was once livingof material that was once living

► The short half-life of carbon 14 The short half-life of carbon 14 makes it suitable for dating material < 70,000 years makes it suitable for dating material < 70,000 years

oldold► It is not useful for most rocks, It is not useful for most rocks,

but is useful for archaeology but is useful for archaeology and young geologic materialsand young geologic materials

Radiocarbon Dating MethodRadiocarbon Dating Method

► Carbon 14 is constantly forming Carbon 14 is constantly forming in the in the upper atmosphere upper atmosphere

► When a high-energy neutronWhen a high-energy neutrona type of a type of cosmic ray strikes a nitrogen 14 cosmic ray strikes a nitrogen 14 atomit may be absorbed by the atomit may be absorbed by the nucleus and eject a proton changing nucleus and eject a proton changing it to carbon 14it to carbon 14

► The The 1414C formation rate C formation rate is fairly constant is fairly constant has been calibrated against tree ringshas been calibrated against tree rings

Carbon 14Carbon 14

► The carbon 14 becomes The carbon 14 becomes part of the natural carbon part of the natural carbon cycle and becomes cycle and becomes incorporated into organismsincorporated into organisms

► While the organism lives it While the organism lives it continues to take in carbon continues to take in carbon 14 but when it dies the 14 but when it dies the carbon 14 begins to decaycarbon 14 begins to decay without being replenishedwithout being replenished

► Thus, carbon 14 dating Thus, carbon 14 dating measures the time of deathmeasures the time of death

Carbon 14Carbon 14

► The age of a tree can be determined by The age of a tree can be determined by counting the annual growth rings in lower counting the annual growth rings in lower part of the stem (trunk)part of the stem (trunk)

► The width of the rings are related to climate The width of the rings are related to climate can be correlated from tree to treecan be correlated from tree to tree a procedure called a procedure called cross-datingcross-dating

► The tree-ring time scale now extends back The tree-ring time scale now extends back 14,000 years14,000 years

Tree-Ring Dating MethodTree-Ring Dating Method

► In cross-dating, tree-ring patterns are used from In cross-dating, tree-ring patterns are used from different trees, with overlapping life spansdifferent trees, with overlapping life spans

Tree-Ring Dating MethodTree-Ring Dating Method

SummarySummary

► Early Christian theologians viewed time Early Christian theologians viewed time as linear and as linear and decided that Earth was very young (about 6000 decided that Earth was very young (about 6000 years old)years old)

► A variety of ages for Earth were estimated A variety of ages for Earth were estimated during the during the 1818thth and 19 and 19thth centuries using scientific evidence, centuries using scientific evidence, ages now known to be too youngages now known to be too young

► Neptunism Neptunism and and catastrophismcatastrophism were popular were popular during during the 17the 17thth, 18, 18thth and early 19 and early 19thth centuries because of centuries because of their consistency with scripture, but were not their consistency with scripture, but were not supported by evidencesupported by evidence

SummarySummary

► James Hutton viewed Earth history James Hutton viewed Earth history as cyclical as cyclical and very longand very long His observations were instrumental in establishing His observations were instrumental in establishing

the principle of uniformitarianismthe principle of uniformitarianism► Charles Lyell articulated uniformitarianism Charles Lyell articulated uniformitarianism in a in a

way that soon made it the guiding doctrine of way that soon made it the guiding doctrine of geologygeology

► UniformitarianismUniformitarianism holds that holds that the laws of nature have been constant through time the laws of nature have been constant through time

and that the same processes operating today have and that the same processes operating today have operated in the past, although not necessarily at the operated in the past, although not necessarily at the same ratessame rates

SummarySummary

► The principles of The principles of superpositionsuperposition, , originaloriginal horizontalityhorizontality, , lateral continuitylateral continuity and and cross-cross-cutting relationshipscutting relationships are basic for determining are basic for determining relative geologic ages and for interpreting relative geologic ages and for interpreting Earth historyEarth history

► Radioactivity was discovered during the late Radioactivity was discovered during the late 1919thth century and lead to century and lead to radiometric datingradiometric dating, , which allowed geologists to determine which allowed geologists to determine absolute agesabsolute ages for geologic events for geologic events

SummarySummary

► The most accurate radiometric dates The most accurate radiometric dates are obtained from long-lived radioactive are obtained from long-lived radioactive isotope/daughter pairs in igneous rocksisotope/daughter pairs in igneous rocks Common pairs include:Common pairs include:

►uranium 238 – lead 206uranium 238 – lead 206►uranium 235 – lead 207 uranium 235 – lead 207 ►thorium 232 – lead 208thorium 232 – lead 208►rubidium87 – strontium 87rubidium87 – strontium 87►potassium 40 – argon 40potassium 40 – argon 40

SummarySummary

► The most reliable radiometric ages are The most reliable radiometric ages are obtained using two different pairs in the obtained using two different pairs in the same rocksame rock

► Carbon 14 dating can be used Carbon 14 dating can be used only for organic matter such as wood, only for organic matter such as wood,

bones, and shells bones, and shells and is effective back to about 70,000 yearsand is effective back to about 70,000 years