sept28 ch 9 - soest · parent atoms remaining as a function of time isotopic dating • radioactive...

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Exam 1 GG 101Mean Score 61.4 (SD 13.1)

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Relative Age DatingRelative Age DatingAbsolute Age DatingAbsolute Age DatingAge Dating Using Magnetic RecordsAge Dating Using Magnetic RecordsThe Age of the EarthThe Age of the Earth

Geologic TimeGeologic Time

A major difference between geologists and most other scientists is their attitude about time.

A "long" time may not be important unless it is > 1 million years.

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Earth History Earth History

Two ways to date Two ways to date geologic eventsgeologic events

1) relative dating (fossils,

structure)

2) absolute dating (isotopic, tree

rings, etc.)

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Relative vs Absolute AgeRelative vs Absolute Age• Usually geologists first establish

relative ages then try to get absolute age dates

• Determining relative age relies on a number of geologic principles that were developed during the 17th to early 19th centuries

Steno's LawsSteno's Laws

Nicolaus Steno (1669)• Principle of Superposition

• Principle of Original Horizontality

• Principle of Lateral Continuity

Laws apply to both sedimentary and volcanic rocks.

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Principle of SuperpositionPrinciple of Superposition

In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.

Layer 1Layer 2Layer 3Layer 4

Principle of SuperpositionPrinciple of SuperpositionSedimentary rocks are deposited in a Sedimentary rocks are deposited in a

layerlayer--cake fashion:cake fashion:

Each layer is older than the one Each layer is older than the one above and younger than the one above and younger than the one

belowbelow

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Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Principle of Superposition

Principle of Original HorizontalityPrinciple of Original Horizontality

Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface.

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Principles of original Principles of original horizontality and superpositionhorizontality and superposition

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Principle of Lateral ContinuityPrinciple of Lateral Continuity

Layered rocks are deposited in continuous contact.

Principle of Lateral ContinuityPrinciple of Lateral Continuity

Map view

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Principle of Lateral ContinuityPrinciple of Lateral Continuity

Map view

Principle of Lateral ContinuityPrinciple of Lateral Continuity

Map view

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• Process used to tie separated strata together

• Based on matching physical features such as– Physical continuity - trace of rock unit– Similar rock types - marker beds, coal

seams, rare minerals, odd color

CorrelationCorrelation

CorrelationCorrelation• Within sedimentary layers there are

often the remains of small animals (fossils)

• Fossils are quite useful for correlating between two sections that are not laterally continuous

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Ammonite Fossils Petrified Wood

CorrelationCorrelation• Fossils represent living creatures that

have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age

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Correlating beds using Correlating beds using index fossilsindex fossils

Using Fossils to Correlate RocksUsing Fossils to Correlate Rocks

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UnconformityUnconformity

A buried surface of erosion

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Layers of rock are said to be Layers of rock are said to be conformableconformable when they are when they are

found to have been deposited found to have been deposited essentially without interruptionessentially without interruption

Unconformity represents missing Unconformity represents missing time in the geologic recordtime in the geologic record

Sedimentation of Beds ASedimentation of Beds A--D D Beneath the SeaBeneath the Sea

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Uplift and Exposure of D to Uplift and Exposure of D to ErosionErosion

Continued Erosion Continued Erosion Removes D and Exposes C Removes D and Exposes C

to Erosionto Erosion

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Unconformity:Unconformity:a buried surface of erosiona buried surface of erosion

Subsidence and Subsidence and Sedimentation of E over CSedimentation of E over C

Formation of a Formation of a DisconformityDisconformity

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First type of unconformityFirst type of unconformity

South rim of the Grand Canyon

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South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old1.7 billion years old1.7 billion years old

Paleozoic StrataPaleozoic Strata

PrecambrianPrecambrian

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old

Nonconformity1.7 billion years old1.7 billion years old

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Nonconformity in the Grand Canyon

Nonconformity in the Grand Canyon

Vishnu SchistVishnu Schist(~1700 million years old)(~1700 million years old)

TapeatsTapeats SandstoneSandstone(~550 million years old)(~550 million years old)

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Second type of unconformitySecond type of unconformity

SiccarSiccar Point, ScotlandPoint, Scotland

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SiccarSiccar Point, ScotlandPoint, Scotland

Buried and tilted erosional surface

Sedimentation of Beds ASedimentation of Beds A--D D Beneath the SeaBeneath the Sea

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Deformation and Erosion Deformation and Erosion During Mountain BuildingDuring Mountain Building

ErosionalErosional Surface Cuts Surface Cuts Across Deformed RocksAcross Deformed Rocks

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Subsidence and Subsequent Subsidence and Subsequent Deposition Buries Deposition Buries ErosionalErosional SurfaceSurface

AngularAngularUnconformityUnconformity

Formation of an Formation of an Angular UnconformityAngular Unconformity

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Third type of unconformityThird type of unconformity

Principle of CrossPrinciple of Cross--Cutting Cutting RelationshipsRelationships

• A rock unit must always be older than any feature that cuts or disrupts it– If a rock unit is cut by a fracture

• The rock itself is older than the fracture that cuts across it

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CrossCross--cutting Relationshipscutting Relationships

Relative Geologic DatingRelative Geologic Dating

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The Geologic time scaleThe Geologic time scale

• Divisions in the worldwide stratigraphic column based on variations in preserved fossils

• Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

The Geologic The Geologic Time ScaleTime Scale

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Absolute geochronologyAbsolute geochronology

• Add numbers to the stratigraphic column based on fossils.

• Based on the regular radioactive decay of some chemical elements.

IsotopesIsotopesDifferent forms of the sameelement containing the samenumber of protons, but varyingnumbers of neutrons.

i.e.i.e.:235235U, U, 238238UU 8787Sr, Sr, 8686SrSr 1414C, C, 1212CC

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Naturally Occurring Naturally Occurring Isotopes of CarbonIsotopes of Carbon

HalfHalf--lifelife

The half-life of a radioactive isotope is defined as the time required for half of it to decay.

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Proportion of Proportion of Parent Atoms Parent Atoms

Remaining as a Remaining as a Function of Function of

TimeTime

Isotopic datingIsotopic dating• Radioactive elements (parents) decay to

nonradioactive (stable) elements (daughters).

• The rate at which this decay occurs is constant and known.

• Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

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Radioactivity and Absolute TimeRadioactivity and Absolute Time

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Radioactive Radioactive Decay of Decay of

Rubidium to Rubidium to StrontiumStrontium

by by ββ emissionemission

Production and Decay Production and Decay of Radiocarbonof Radiocarbon

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Major Radioactive Elements Used Major Radioactive Elements Used in Isotopic Datingin Isotopic Dating

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Geologically Useful Decay SchemesGeologically Useful Decay Schemes

Parent Daughter Half-life (years)235U 207Pb 4.5 x 109

238U 206Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47 x 109

14C 14N 5730

Radiometric DatingRadiometric Dating

• We can calculate geologic age if– The half-life of a radioactive isotope is

known– The parent/daughter ratio can be

measured– There is no loss of an isotopes from the

system• e.g., 222Rn is an intermediate daughter product

in the 238U decay series to 208Pb

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Oldest rocks and minerals on EarthOldest rocks and minerals on EarthNarryer Gneiss, Western Australia• Zircons in a metamorphosed sandstone dated

at 4.35 to 4.40 GaAcasta Gneiss, Northwestern Canada and the

Nuvvuagittuq greenstone belt, Northern Quebec

• Rocks dated at 3.80 to 4.28 GaSeveral other regions dated at 3.8 Ga by various

methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.

Age of the EarthAge of the EarthAlthough the oldest rocks found on Earth

are 4.4 Ga, we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.4 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

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Age of the EarthAge of the EarthThis is based on the age of rocks brought

back from the Moon (4.4 Ga), and meteorites (4.6 Ga), that are thought to be good representatives of the early solar system.

These data suggest that the present chemical composition of the crust must have evolved for more than 4.5 Ga.

The geologic The geologic timescale timescale

and absolute agesand absolute agesIsotopic dating of intebeddedvolcanic rocks allows assignment of an absolute age for fossil transitions

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The big assumptionThe big assumption

The half-lives of radioactive isotopes are the same as they

were billions of years ago.

Test of the assumptionTest of the assumption

Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data.

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Bracketing agesBracketing ages

Radiometric Radiometric dates dates

provide provide absolute absolute

ages to the ages to the Geologic Geologic ColumnColumn

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Earth’s Magnetic

Field

MagnetostratigraphyMagnetostratigraphy• Technique that works best in volcanic

rocks• Time scale based on polarity reversal of

Earth's magnetic field• Major problem is that Earth's magnetic

field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks

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Magnetization Magnetization of of

MagnetiteMagnetite

Lavas Lavas record record

magnetic magnetic reversalsreversals

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Magnetic Magnetic reversals reversals over the over the past 20 past 20 million million yearsyears

The Geologic time scaleThe Geologic time scale• Divisions in the worldwide

stratigraphic column based on variations in preserved fossils

• Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

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The The Geologic Geologic Column Column

and and Time ScaleTime Scale