Age Dating of Rocks. Question: How do we know how old rocks are? How do we know how old rocks are?

Age Dating of RocksAge Dating of Rocks

Question: Question:

How do we know how old rocks are?How do we know how old rocks are?

Two types of datingTwo types of dating

Relative Age Dating – approximately Relative Age Dating – approximately how old rocks arehow old rocks areFound by using the age of things around Found by using the age of things around

ititAbsolute Age Dating – exactly how Absolute Age Dating – exactly how

old rocks areold rocks areFound using radiometric datingFound using radiometric dating

Laws of Relative Age DatingLaws of Relative Age Dating

Relative dating – used to determine Relative dating – used to determine the order of events and the relative the order of events and the relative age by examining the position of age by examining the position of rocks in a sequence. rocks in a sequence. Example: if bedrock is broken by a fault, Example: if bedrock is broken by a fault,

then the fault must be younger because then the fault must be younger because it cuts through the rocksit cuts through the rocks

Relative Age DatingRelative Age Dating

This is called the This is called the Law of Cross Law of Cross Cutting Cutting Relationships – If Relationships – If something cuts something cuts across a layer of across a layer of rock it must be rock it must be younger than the younger than the rock.rock.


The Law of Superposition The Law of Superposition – – In an undisturbed layer of In an undisturbed layer of

rock, the oldest rocks are on rock, the oldest rocks are on the bottom and the rocks the bottom and the rocks become younger toward the become younger toward the top.top.

Sediments are deposited in Sediments are deposited in horizontal layers, so the horizontal layers, so the oldest one would be on the oldest one would be on the bottombottom

Sometimes layers flip, so Sometimes layers flip, so geologists must use other geologists must use other clues to determine if this clues to determine if this has happened. has happened.


Principle of Principle of Original Original Horizontality Horizontality – rocks are – rocks are deposited in deposited in horizontal horizontal layerslayers


Unconformities – Unconformities – gaps in the rock gaps in the rock record caused by record caused by missing rock layersmissing rock layersForm when layers Form when layers

are eroded awayare eroded awayAlso when time goes Also when time goes

by without any new by without any new depositiondeposition

Principle of Uniformitarianism – earth Principle of Uniformitarianism – earth processes that are occurring today processes that are occurring today have been occurring since the have been occurring since the beginning of the earth. beginning of the earth.

Three types of Three types of UnconformitiesUnconformities

Angular Unconformity – horizontal Angular Unconformity – horizontal layers of sedimentary rocks are tilted layers of sedimentary rocks are tilted and uplifted, where they are eroded and uplifted, where they are eroded and weathered. Younger sediments and weathered. Younger sediments are then deposited horizontally on are then deposited horizontally on top of the tilted layerstop of the tilted layers


Disconformity – layers missing Disconformity – layers missing between two beds were an erosional between two beds were an erosional surface is presentsurface is present


Nonconformity – sedimentary rock Nonconformity – sedimentary rock layers above igneous or layers above igneous or metamorphic rocksmetamorphic rocks

Correlating layers – finding the same Correlating layers – finding the same rock layer at two separate locations rock layer at two separate locations and matching them up. How?and matching them up. How?Index fossils – similar, rare fossils found Index fossils – similar, rare fossils found

in one layer at multiple locationsin one layer at multiple locationsRock correlation – matching similar rock Rock correlation – matching similar rock

typestypes

Absolute DatingAbsolute Dating

Absolute Dating – process that uses Absolute Dating – process that uses the properties of radioactive decay in the properties of radioactive decay in rocks to determine their exact agerocks to determine their exact age


Radioactive decay – unstable Radioactive decay – unstable isotopes break downisotopes break downA neutron breaks down into a proton A neutron breaks down into a proton

and an electron: the electron is given off and an electron: the electron is given off as a radioactive particleas a radioactive particle

The number of neutrons & protons are The number of neutrons & protons are changed, so a new element is formedchanged, so a new element is formedUranium-238 is the parentUranium-238 is the parentLead-206 is the daughterLead-206 is the daughter


Each parent has a specific rate of Each parent has a specific rate of decay – this is the half-life of the decay – this is the half-life of the elementelementThe time it takes for half of the atoms to The time it takes for half of the atoms to

decaydecayCarbon-14 has a half-life of 5730 yearsCarbon-14 has a half-life of 5730 years


Radiometric dating – measuring the Radiometric dating – measuring the amounts of parent and daughter amounts of parent and daughter materials in a rock and knowing the materials in a rock and knowing the half-life, a geologist calculates the half-life, a geologist calculates the age of a rockage of a rockGeologists must decide what isotope to Geologists must decide what isotope to

useuseMust have an appropriate half-lifeMust have an appropriate half-life


Radiocarbon dating – using carbon-Radiocarbon dating – using carbon-14 to date fossils, bones and wood 14 to date fossils, bones and wood up to 50,000 years oldup to 50,000 years oldMust be a formerly living thing to have Must be a formerly living thing to have

CarbonCarbon

Age Dating of Rocks. Question: How do we know how old rocks are? How do we know how old rocks are?

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