Transcript PPT
Geologic time Goal to understand how we determine relative and numerical ages of geologic events United States timeline History of the Earth as a cross-country trip ~4600 km United States timeline Oldest mineral crystals (4400 m.y.-old) show up at CA/NV border Oldest rocks (4030 m.y.-old) show up in NV United States timeline Extinction 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 Fossils Faunal 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 Fossils Using fossils we can correlate different sedimentary rocks of the same age over large distances. Sedimentary record Unconformities: Gaps in the rock record formed due to erosion Unconformities Nonconformity: 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 Unconformities Most 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 dating Isotopes: 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 Famous isotope Parent and daughter puppies Radioactive decay Alpha decay: Spontaneous loss of 2 protons and 2 neutrons (helium nucleus)—Atomic number decreases by 2 Radioactive decay Beta decay: Neutron spontaneously changes into an electron and a proton—Atomic number increases by 1 Radioactive decay Electron capture: Proton spontaneously captures an electron to become a neutron—Atomic number decreases by 1 Radioactive decay Half 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 dating Isotopic 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 dating Isotope 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 dating Useful 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 systems Uranium–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 systems Potassium–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 systems Carbon-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 dating Closure 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 Below closure temperature daughter atoms remain Above closure temperature daughter atoms escape Closure temperatures Uranium–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 dating Metamorphism can reset isotopic clock or cause overgrowths on minerals used in isotopic dating. Metamorphic overgrowths on zircons Complications of isotopic dating It 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 dating Fission tracks: Zones of damage left behind when unstable isotopes split and emit high energy particles • Fission track also develop at a known rate