United States timelineOldest mineral crystals (4400 m.y.-old) show up at
CA/NV border
Oldest rocks (4030 m.y.-old) show up in NV
United States timelineExtinction of the dinosaurs (65 m.y. ago) takes place
at the PA/NJ border
First humans (100 k.y. ago) hanging out on boardwalk
How do we know?Relative dating: Uses basic principles to establish a
sequence of events– Uniformity, original horizontality, superposition,
cross-cutting relationships, inclusion, and faunal succession
FossilsFaunal succession: Fossils of extinct animals
appear in a regular and predictable order.• Once an animal becomes extinct, you will not
find its fossils in younger rocks
FossilsUsing fossils we can correlate different sedimentary
rocks of the same age over large distances.
UnconformitiesNonconformity: Rock layers
deposited on intrusive igneous and/or metamorphic rocks
Angular unconformity: Rock layers deposited on older tilted rock layers
Disconformity: Rocks deposited on older rocks with no angular mismatch—Often requires fossils to recognize
UnconformitiesMost sedimentary rock sequences record 1–5% of
geologic time
Grand canyon record is exceptional: 15–20%
How do we know?Numerical dating or absolute dating: Laboratory
techniques that can tell how long ago in years a particular rock formed or event occurred.
• Based on processes that happen at a known rate– Radioactive decay of atoms– Nuclear fission– Growth of tree rings
Numerical datingIsotopes: Atoms of a certain element with different
numbers of neutrons—Often unstable
Radioactive decay: Spontaneous loss or gain of neutrons in unstable isotopes– Parent atoms: isotopes before decay– Daughter atoms: stable atoms or isotopes
produced during decay
Parent anddaughter puppies
Famous isotope
Radioactive decayAlpha decay: Spontaneous loss of 2 protons and 2
neutrons (helium nucleus)—Atomic number decreases by 2
Radioactive decayBeta decay: Neutron spontaneously changes into an
electron and a proton—Atomic number increases by 1
Radioactive decayElectron capture: Proton spontaneously captures an
electron to become a neutron—Atomic number decreases by 1
Radioactive decayHalf life: Amount of time needed for exactly one-half
of radioactive parent isotopes to decay into daughter products
• Rate is fixed, regardless of number of parent isotopes
• Therefore radioactive decay is exponential
Numerical datingIsotopic dating: Measuring ratios of parent and
daughter atoms to determine numerical age of Earth materials
• Most widely used numerical dating technique
Mineral samples prepared for isotopic
dating
Isotopic datingIsotope ratios are measured using a mass
spectrometer—Machine that can accurately count atoms with slight differences in atomic mass
Sensitive High Resolution Ion MicroProbe (SHRIMP)
Isotopic datingUseful isotopic systems:• Parents must be incorporated into mineral without
daughters• Mineral must retain the daughter products over long
time periods
Zircon(zirconium silicate)
Commonly used isotopic systemsUranium–lead: Two different isotopes of Uranium
decay to two different isotopes of lead, useful for ages >1–10 m.y.
• U-238 decays to Pb-206, half life = 4.5 b.y.• U-235 decays to Pb-207, half life = 713 m.y.• Mineral zircon is commonly used—Found in almost
all felsic and intermediate igneous rocks
Zircons
Commonly used isotopic systemsPotassium–argon: K-40 decays to Ar-39, half life = 1.3
b.y., useful for dates >1 m.y.• Potassium is found in many rock-forming minerals—
amphibole, biotite, muscovite, and potassium feldspar
Commonly used isotopic systemsCarbon-14: C-14 decays to N-14, half life = 5370
years, useful for dates less than ~70,000 years• C-14 forms naturally in the atmosphere and finds its
way into living organisms and calcite shells
Carbon-14 dating puts age of Dead Sea Scrolls at ~2,200–2,00 years
Complications of isotopic datingClosure temperature: Temperature at which minerals
can begin to retain daughter products• Isotopic clock does not start running until minerals
cool below closure temperature• Different for each mineral and isotopic system
Above closure temperature daughter atoms escape
Below closure temperature daughter atoms remain
Closure temperaturesUranium–lead system in zircon: Closure temperature greater
than melting temperature of most rocks• Can date initial formation of igneous rocks
Potassium–argon system: Different closure temperatures for different minerals—(~550ºC for amphibole to ~250ºC for biotite)
• Can date metamorphism or to reconstruct the cooling history of rocks
Complications of isotopic datingMetamorphism can reset isotopic clock or cause
overgrowths on minerals used in isotopic dating.
Metamorphic overgrowths on
zircons
Complications of isotopic datingIt is time consuming and expensive:• Dating a single rock sample can take months of
work• The most advanced mass spectrometers cost more
than $1,000,000
Sensitive High Resolution Ion
MicroProbe (SHRIMP)
Only 10 of these machines in the world
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