Ticking away the Ticking away the moments that make up moments that make up

a dull day ….a dull day ….

Geologic TimeGeologic Time

Geologic TimeGeologic Time

►Relative DatingRelative Dating – placing rocks and – placing rocks and events in their proper sequence of events in their proper sequence of formationformation

►Absolute DatingAbsolute Dating – specifying the – specifying the actual number of years that have actual number of years that have passed since an event occurredpassed since an event occurred

Law of superpositionLaw of superposition

Developed by Nicolaus Steno in 1669 while Developed by Nicolaus Steno in 1669 while scanning the Italian country side.scanning the Italian country side.► In an undeformed sequence of sedimentary In an undeformed sequence of sedimentary

rocks (or layered igneous rocks), the oldest rocks (or layered igneous rocks), the oldest rocks are on the bottom. rocks are on the bottom. Brilliant!!!Brilliant!!!

Principle of original Principle of original horizontalityhorizontality

►Layers of sediment are generally Layers of sediment are generally deposited in a horizontal positiondeposited in a horizontal position

►Rock layers that are flat have not been Rock layers that are flat have not been disturbeddisturbed

Cross-cutting & InclusionsCross-cutting & Inclusions

►Principle of Cross-cutting Principle of Cross-cutting relationshipsrelationships Younger features cut across older featuresYounger features cut across older features

►InclusionsInclusions An inclusion is a piece of rock that is An inclusion is a piece of rock that is

enclosed within another rockenclosed within another rock Rock containing the inclusion is youngerRock containing the inclusion is younger

UnconformityUnconformity

An unconformity is a break in the rock An unconformity is a break in the rock record record

produced by erosion and/or nondeposition produced by erosion and/or nondeposition of of

rock units.rock units.► Types of unconformitiesTypes of unconformities

Angular unconformityAngular unconformity – tilted rocks are overlain by – tilted rocks are overlain by flat-lying rocksflat-lying rocks

DisconformityDisconformity – strata on either side of the – strata on either side of the unconformity are parallel unconformity are parallel

NonconformityNonconformity – metamorphic or igneous rocks in – metamorphic or igneous rocks in contact with sedimentary stratacontact with sedimentary strata

The Grand CanyonThe Grand Canyon

So how do they form?So how do they form?

Click here to see animation

Ok Fine…But how do we use Ok Fine…But how do we use this?this?

Correlation of Rock LayersCorrelation of Rock Layers

Matching of rocks of similar ages in different Matching of rocks of similar ages in different

regions is known as regions is known as correlationcorrelation► Correlation often relies upon fossilsCorrelation often relies upon fossils

William Smith (late 1700s) noted that William Smith (late 1700s) noted that sedimentary strata in widely separated area sedimentary strata in widely separated area could be identified and correlated by their could be identified and correlated by their distinctive fossil contentdistinctive fossil content

Principle of fossil successionPrinciple of fossil succession – fossil – fossil organisms succeed one another in a definite organisms succeed one another in a definite and determinable order, and therefore any time and determinable order, and therefore any time period can be recognized by its fossil contentperiod can be recognized by its fossil content

Absolute DatingAbsolute Dating

Reviewing basic atomic structureReviewing basic atomic structureNucleusNucleus

Protons – positively charged particles with massProtons – positively charged particles with mass Neutrons – neutral particles with massNeutrons – neutral particles with mass

Electrons – negatively charged particles that orbit the Electrons – negatively charged particles that orbit the nucleusnucleus

► Atomic numberAtomic number An element’s identifying numberAn element’s identifying number Equal to the number of protons in the atom’s nucleusEqual to the number of protons in the atom’s nucleus

► Mass numberMass number Sum of the number of protons and neutrons in an atom’s Sum of the number of protons and neutrons in an atom’s

nucleusnucleus► IsotopeIsotope

Variant of the same parent atomVariant of the same parent atom Differs in the number of neutronsDiffers in the number of neutrons Results in a different mass number than the parent atomResults in a different mass number than the parent atom

RadioactivityRadioactivitySpontaneous changes (decay) in the structure of atomic Spontaneous changes (decay) in the structure of atomic

nucleinuclei

Types of radioactive decayTypes of radioactive decay Alpha emissionAlpha emission

► Emission of 2 protons and 2 neutrons (an alpha particle)Emission of 2 protons and 2 neutrons (an alpha particle)► Mass number is reduced by 4 and the atomic number is lowered Mass number is reduced by 4 and the atomic number is lowered

by 2by 2

Beta emissionBeta emission► An electron (beta particle) is ejected from the nucleusAn electron (beta particle) is ejected from the nucleus► Mass number remains unchanged and the atomic number Mass number remains unchanged and the atomic number

increases by 1increases by 1

Electron captureElectron capture► An electron is captured by the nucleusAn electron is captured by the nucleus► The electron combines with a proton to form a neutronThe electron combines with a proton to form a neutron► Mass number remains unchanged and the atomic number Mass number remains unchanged and the atomic number

decreases by 1decreases by 1

Radiometric DatingRadiometric Dating

► ParentParent – an unstable – an unstable radioactive isotoperadioactive isotope

► Daughter productDaughter product – – the isotopes resulting the isotopes resulting from the decay of a from the decay of a parentparent

► Half-lifeHalf-life – the time – the time required for one-half of required for one-half of the radioactive nuclei the radioactive nuclei in a sample to decayin a sample to decay

Principle of radioactive Principle of radioactive datingdating

► The percentage of radioactive atoms that The percentage of radioactive atoms that decay during one half-life is always the decay during one half-life is always the same (50 percent)same (50 percent)

►However, the actual number of atoms that However, the actual number of atoms that decay continually decreasesdecay continually decreases

► Comparing the ratio of parent to daughter Comparing the ratio of parent to daughter yields the age of the sampleyields the age of the sample

Useful ElementsUseful Elements

There can be problems…

• A system must be closed• To avoid potential problems, only fresh, unweathered rock samples should be used

Dating with carbon-14 Dating with carbon-14 (radiocarbon dating)(radiocarbon dating)

►Carbon-14 is produced in the upper Carbon-14 is produced in the upper atmosphere atmosphere

►Half-life of only Half-life of only 5730 years5730 years Used to date very recent eventsUsed to date very recent events Useful tool for anthropologists, Useful tool for anthropologists,

archeologists, and geologists who study archeologists, and geologists who study very recent Earth historyvery recent Earth history

The geologic time scale – a The geologic time scale – a “calendar” of Earth history“calendar” of Earth history

►Subdivides geologic history into unitsSubdivides geologic history into units►Originally created using relative datesOriginally created using relative dates

Structure of the geologic Structure of the geologic time scaletime scale

► EonEon – the greatest expanse of time – the greatest expanse of time PhanerozoicPhanerozoic (“visible life”) – the most recent eon, (“visible life”) – the most recent eon,

began about 540 million years agobegan about 540 million years ago ProterozoicProterozoic ArcheanArchean HadeanHadean – the oldest eon – the oldest eon

► EraEra – subdivision of an eon – subdivision of an eon Eras of the Phanerozoic eonEras of the Phanerozoic eon

► Cenozoic (“recent life”)Cenozoic (“recent life”)► Mesozoic (“middle life”)Mesozoic (“middle life”)► Paleozoic (“ancient life”)Paleozoic (“ancient life”)

Eras are subdivided into periodsEras are subdivided into periods► PeriodsPeriods are subdivided into epochs are subdivided into epochs

Difficulties in dating the Difficulties in dating the geologic time scalegeologic time scale

► Not all rocks can be dated by radiometric Not all rocks can be dated by radiometric methodsmethods Grains comprising detrital sedimentary rocks are not Grains comprising detrital sedimentary rocks are not

the same age as the rock in which they formedthe same age as the rock in which they formed The age of a particular mineral in a metamorphic The age of a particular mineral in a metamorphic

rock may not necessarily represent the time when rock may not necessarily represent the time when the rock formedthe rock formed

► Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages