Geologic time

Goalto understand how we determine relative and

numerical ages of geologic events

United States timelineHistory of the Earth as a cross-country trip ~4600 km

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

Relative Dating Principles

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.

Sedimentary recordUnconformities: Gaps in the rock record formed due

to erosion

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

Unconformity Types

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

Radioactive Decay

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

Numerical datingFission tracks: Zones of damage left behind when

unstable isotopes split and emit high energy particles

• Fission track also develop at a known rate