Telling Geologic Time - Centralia College · 1 Telling Geologic Time Relative dating –telling if one layer or geologic feature is older or younger than another Absolute dating –calculating

1

Telling Geologic Time

Relative dating – telling if one layer or geologic feature is older or

younger than anotherAbsolute dating – calculating a numerical age using radiometric dating

(with unstable isotopes) or other techniques (TL, amino acids, tree rings)

Trilobite fossils—

these arthropods

went extinct

during the great

extinction at the

end of the

Permian Period

~250 Ma

ka = kilo-annum

Ma = Mega-annum

Ga = Giga-annum

James Hutton“The Father of Geology”

Uniformitarianism

1788 1788 –– James HuttonJames Hutton:: English farmer and intellectual: envisioned

geologic deep time: "no vestige of a beginning, no prospect of an end."

Principles of Relative Dating

Principle of…

• Original Horizontality

• Superposition

• Cross Cutting Relationships

• Inclusions

• Faunal Succession

Copyright © Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a–4

Early Numerical Estimates of Earth’s Age

Cooling of Earth .075 - 1,280Cooling of Sun 4.4 - 500Orbital physics <10 - 3,700,000Salt in ocean 25 - 2,350Sediment accumulation 3 - 15,000Radioactive elements 1,340 - 4,600

million years


Buffon’s “Age of Earth”

(1749)

Surface of Earth consolidated 1

years

Earth consolidated to its center

Earth cool enough to be touched 34,270

2,936

Beginning of life 35,983

Temperature of present reached 74,832

End of life 168,123


History of thought on geotime

1654 1654 –– Archbishop UssherArchbishop Ussher, biblical literalist:

added ages in the Bible: Earth born:

October 26, 4004 B.C 9:00 AM

1749 1749 –– George de BuffonGeorge de Buffon -- HistorieHistorie NaturelleNaturelle: :

Iron ball hypothesis. Iron ball hypothesis. Earth ~ 75,000 yr oldEarth ~ 75,000 yr old

1788 1788 –– James HuttonJames Hutton:: English farmer: deep time:

"no vestige of a beginning,--no prospect of an end."

1775 1775 –– Abraham WernerAbraham Werner, Freiberg Germany. , Freiberg Germany.

Neptunist: all rocks Neptunist: all rocks precipatedprecipated..

Estimating an absolute age for the Earth

2


1899 John Joly1899 John Joly, Irish geologist: used amount of

salt in ocean and rate of salt added annual to

infer age of the Earth at 90 Ma

1867 1867 –– Chas. LyellChas. Lyell: 20 Ma needed for complete : 20 Ma needed for complete

change of change of molluscanmolluscan species. 12 of these events species. 12 of these events

since Ordovician time, hence, since Ordovician time, hence, Earth >240 MaEarth >240 Ma

1812 1812 –– George CuvierGeorge Cuvier, French zoologist , French zoologist

working in Paris basin: recognized fossil life working in Paris basin: recognized fossil life

and breaks in stratigraphic record. Inferred and breaks in stratigraphic record. Inferred

catastrophes caused extinctions. Embraced by catastrophes caused extinctions. Embraced by

biblical literalists (Biblical delugebiblical literalists (Biblical deluge)

1897 Lord Kelvin1897 Lord Kelvin: used thermodynamics to : used thermodynamics to

estimate cooling of Earth, and age from molten estimate cooling of Earth, and age from molten

state: state: 2424--40 Ma40 Ma


the breakthrough… RADIOACTIVITY!

!1895 => Bequerel1895 => Bequerel discovered…uranium activated

his photo plates in total darkness.

1905 => Ernest Rutherford and B.B. 1905 => Ernest Rutherford and B.B.

BoltwoodBoltwood=>radioactive elements disintegrate (“parent”

and “daughter”). Both dated rocks, but early ages were too

high

1903 Marie and Pierre Curie1903 Marie and Pierre Curie => isolated radium

1913 => Frederick Soddy1913 => Frederick Soddy clarified the nature of

isotopes—leading to refined methods and accurate

ages

The age of Geochronology was born!!1947 Libby1947 Libby => radiocarbon dating

(1) Principle of Original Horizontality

Because of gravity, layers form horizontally. Thus tilted layers imply

passage of geologic time as well as structural deformation

2. Principle of Superposition: the rocks

on top are younger (unless ??)

Youngest Strata

Oldest Strata

(3) Principle of Crosscutting Relationships e.g. dikes

and faults must be younger than the rocks they cut

through

(4) Principle of Inclusions

The inclusions have to be

older than the rocks that

contain them.

3

(5) Principle of Faunal Succession

Sketch by

Baron Cuvier

(1769-1832)

Sedimentary strata contain fossils that succeed each other vertically in a

specific, reliable order that can be correlated over wide horizontal

distances

Example of Relative Time 1

Example 2 of Relative Time

For practice, write the order of the units, faults, or intrusions.

Unconformities

Gaps in the Rock = Gaps in Time

Unconformity

an angular

unconformity

to be specific!

Types of Conformity - Angular

UnconformityFormation of an Angular Unconformity

4

Angular Unconformity in the Grand CanyonThe Angular

Unconformity

at Siccar Point,

Scotland—This is

one of the outcrops

that blew Hutton’s

mind in the mid to

late 1780s!

Source: Edward A. Hay, De Anza College, Cupertino, CA

Example of Correlation Colorado Plateau Correlation

Colorado Plateau Correlation


Dating using unstable isotopes

• Isotopes and Radioactive Decay

–– IsotopesIsotopes – Varieties of same element with differing number of neutrons, but same number of protons

–– Radioactive DecayRadioactive Decay - Spontaneous nuclear disintegration of unstable isotopes

• Daughter Product results from decay of parent product

• Half-life—time over which half the original parent isotopes decays.

– Various techniques (e.g. U-Pb, K-Ar, radiocarbon dating)

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Figure 7.17bHalf life:Half life:

The time in which The time in which

half of the half of the parent parent

elements will decay elements will decay

into the into the daughter daughter

elementselements

The time period in The time period in

which each atom has which each atom has

a 50% chance of a 50% chance of

decaying.decaying.


Radioactive Clocks

now One half-life Two half-lives

Radioactive parent Stable daughter

Half-life

(238U) (206Pb)


Radiometric Dating

MethodsMETHOD

Potassium-Argon

Rubidium-Strontium

Samarium-Neodymium

Lutetium-Hafnium

Rhenium-Osmium

Uranium-Lead

Uranium-Lead

Lead-Lead

NUCLIDES

40K - 40Ar

87Rb - 87Sr

147Sm - 143Nd

176Lu - 176Hf

187Re - 187Os

235U - 207Pb

238U - 206Pb

----

HALF-LIFE

(billion years)

1.25

48.8

106

35.9

43.0

0.704

4.47

----Copyright © Houghton Mifflin Company. All rights reserved. Essentials of Geology, 8a–28

A typical lab where this is done


What can be dated?


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Fig. 08.25


Age of the eras and Earth

• Oldest Earth rock 4 b.y.

• Moon rocks dated at 4.2 b.y.

• Dating of meteorites

Between 4.5 & 4.6 billion years old


The Geologic Time Scale

http://pubs.usgs.gov/fs/2007/3015/f

s2007-3015.pdf

Link to updated geologic time scale.

see also pp. x–xi in book


Comprehending

geologic time

Documents

Telling Geologic Time - Centralia College · 1 Telling Geologic Time Relative dating –telling if one layer or geologic feature is older or younger than another Absolute dating –calculating