telling geologic time - centralia college · 1 telling geologic time relative dating –telling if...
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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
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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
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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
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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
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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.
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(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
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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
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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)
5
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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.
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Radioactive Clocks
now One half-life Two half-lives
Radioactive parent Stable daughter
Half-life
(238U) (206Pb)
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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
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A typical lab where this is done
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What can be dated?
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6
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Fig. 08.25
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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
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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
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Comprehending
geologic time