1Geologic Time Terms• Hadean• Archean• Proterozoic• Phanerozoic• Paleozoic• Mesozoic• Cenozoic(Tertiary)• Cambrian• Unconformity• Angular unconformity• Half-life• Alpha particle• Beta particle• Gamma ray• NeutronHow do we determine if layers separated by large distances formed at the same time? UTCOWYHow would we recognize a “gap” in the rock record – if part of the rock record is missing and how much time the “gap” represents?Timing of Geologic Events1) relative-age dating (fossils, stratigraphy, structure)2) absolute-age dating (isotopes, tree rings, etc.)Laws / Principlesof StratigraphyNicolaus Steno (1669)• Law of Superposition• Principle of Original Horizontality• Principle of Lateral Continuity(1638-1686)William Smith (1793)• Principle of Fossil Succession(1769-1839)2Oldest rocksOldest rocksYoungest rocksYoungest rocksLaw of SuperpositionIn a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.Principle of Original HorizontalityLayered strata are deposited horizontally or nearly horizontallyUse of Fossils to Correlate Rock FormationsPrinciple of Fossil SuccessionWilliam “Strata” Smith (1793)• Recognized that different strata contained different fossils• Recognized an order or succession of fossils and strata • Used fossils to correlate formations from different outcropsTypes of UnconformitiesAngular UnconformityNonconformityDisconformityA surfacethat represents a break in the rock record due to erosion or nondeposition.UnconformityAngular Unconformity3DevonianOld Red SandstoneOlder tilted strata(shales and slates)Siccar Point, ScotlandNonconformityDisconformitySeveral unconformities are present in the Grand Canyon1234Several unconformities are present in the Grand Canyon1234Cambrian Tapeats SandstonePrecambrian Wapatai Shale4South rim of the Grand Canyon250 million years old250 million years old550 million years old550 million years old1.7 billion years old1.7 billion years oldPaleozoic StrataPaleozoic StrataPrecambrianPrecambrianSouth rim of the Grand Canyon250 million years old250 million years old550 million years old550 million years oldNonconformity1.7 billion years old1.7 billion years oldVishnu SchistVishnu SchistPrinciple of Cross-cutting RelationshipsHost rocks (red) are older than the intruding rocks (black).What is this surface?Lava Flow(bed H)Your turn Use the geologic principles to place the events in order5How old is the Earth?• By the mid 19th century a relative time scale had been worked out for the sedimentary rocks of Europe (Phanerozoic).• They lacked an absolute time scale.• Kelvin and classical physicists advocated 40 million max. • Darwin and evolutionary biologists advocated billions of years.• Discovery of radioactivity at about 1900 confirmed billions.PhanerozoicGeologic Time ScaleTime divisions (units) of Earth’s history as recorded by rock formations – based originally on relative-dating methods:Fossil groups or assemblagesFossil succession (order of fossils)Stratigraphic relationshipsCross-cutting relationshipsand later…Absolute (isotopic) agesGeologic Time ScaleThe Age of the Earth• Bishop Ussher - 17th Cent. (biblical): 4004BC• Buffon - 18th Cent. (Cooling of spheres): ~50000 Y• Hutton - late 18th Cent. (Geological cycles): Infinite• Darwin - late 19th Cent. (Biological changes): Billions• Kelvin - late 19th C (Sun’s energy): 40 Million Max • Modern - (Radiometric): 4.55 Billion6Absolute-Age DatingAbsolute ages of geologic events and rock formations are based on radioactive elements and the rates at which they decay.Many isotopes of each element occur naturallyIsotope: a variety of an element with the “normal” number of protons, but different number of neutronshttp://ie.lbl.gov/education/isotopes.htmRocks are composed of minerals, Minerals are composed of atoms of different elements1. Proton: positive charge2. Neutron: no charge3. Electron: negative chargeThe number of protonsdetermines the elementÆ “the atomic number”The neutronsof a given element may varyISOTOPE: variations of the same element, with different # of neutrons, and so different atomic mass numberIsotope7Isotopes of Carbon• Types of radioactive decay1. Alpha emission: Emission of 2 protons and 2 neutronsWhich of the following accurately describes alpha emissionA) Atomic number lower by 2; atomic mass unchanged B) Atomic number lower by 2; atomic mass lower by 2 C) Atomic number lower by 2; atomic mass lower by 4 D) Atomic number lower by 4; atomic mass lower by 4Radioactive decay: Spontaneous changes in structure of atomic nucleiWhat is change in 1) Atomic number? 2) Atomic mass?-2 -4Example of alpha emissionU238Æ Th2342. Beta emission–A neutron loses an electron and turns into a proton; the electron is ejected from the nucleusTh234Æ Pa234Which of the following accurately describes beta emissionA) Atomic number unchanged; atomic mass unchanged B) Atomic number increases by 1; atomic mass unchanged C) Atomic number decreases by 1; atomic mass unchanged D) Atomic number increases by 1; atomic mass dec by 13. Electron capture: An electron is captured, combines with a proton to form a neutronWhat is change in 1) Atomic number? 2) Atomic mass?-1 0K40Æ Ar40• Parent: an unstable radioactive isotope• Daughter product: isotopes resulting from decay of parentParents and DaughtersKeep track of the ratio # of daughter (D) to# of parents (N): D/N8Radiometric Dating:Establishing an absolute time scale• Minerals contain naturally radioactive elements– K, U, Th, Rb, Sm• These radioactive parent isotopes decay to stable daughterisotopes• When minerals crystallize from melt, they contain parent only.• If we measure the concentration of daughter isotope in a mineral and we know the decay rate, we can calculate when the mineral crystallized.Types of Radioactive Decay• Particle composed of: Mass# Atomic # Example• alpha 2 neutrons+ 4 2 U, Th, 2 protons• beta- electron 0 -1 40K• beta+ positron 0 +1 40K• gamma photon 0 0 all nuclear reactions• neutron neutron 1 0 235U Common types of radioactive decay An Example: U238to Pb206The half-life of a radioactive isotope is the timerequired for half of the original number of radioactive parent atoms to decay to stable daughter products.Fraction of elements
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