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Lecture 3

EAS100 Lecture Notes - Lecture 3: Cenozoic, Acasta Gneiss, Exoskeleton

Earth and Atmospheric Sciences
Course Code
Solweig Balzer

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Foundation of the Earth: Geological time
Isotopic ages
Correlation and cross-cutting relationships
Earth History
Isotopic (radiometric) ages
Absolute ages
Atoms are built from subatomic particles
Subatomic particles
Nucleus (P and N) surrounded by an electron cloud
“Cloud” determines some chemical properties
Neutrons and Isotopes
Neutrons help bind the nucleus together
Number of neutrons can vary
E.g. 12C; 13C; 14C
Radioactive decay
Unstable isotopes break down (decay)
n p + e
Lose 2(n+p)
Emit particles and/or energy: radioactivity
Rate is predictable
Enables measurement of geologic time
Half life (t1/2)
Time for half of the parent atoms of an isotope to decay into its daughter
Measures decay rate
Decay equation
Rate of decay can also be measured by decay constant
λ = ln2/t1/2 = 0.69/t1/2
Number of atoms at time t is related to starting number of atoms N0 by
N = N0e-lt
To get the age we can use:
Requirements for age determination
Need to know
Half life or decay constant of the isotope
Original amount of:
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Parent isotope in the sample
Daughter isotope in the sample
Final amount of parent or daughter isotope in the sample
Useful isotopic systems
Mineral grains must have formed at the same time as the rock
Dating sedimentary rocks?
Use other methods
Few rocks contain materials that can be dated isotopically
For sedimentary strata
Relative ages
Order of strata
Order of strata
Principle of original horizontality
Sedimentary strata are initially horizontal
Strata order (2)
Principle of superposition
In an undisturbed succession, higher rocks are younger
Correlation by lithology
Compare successions of layers
Correlation by fossils
Compare similar fossils or fossil assemblages
Time scale
Correlation gives relative timing of strata
Correlation was used to develop a geological time scale
Eras subdivided into periods
Subsequently numerical ages were added using isotopic methods
Calibrating and extending the timescale
Calibration and extension is based on:
Isotopic (absolute) ages
Fossil (relative) ages
Cross-cutting relationships
Correlation techniques
ka = kilo-annum (thousand years before present)
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