GEOLOGICAL TIME     I. GEOLOGICAL TIME A. Radiometric isotopic age dating--absolute dates B. Radioactivity--spontaneous decay of the nuclei of unstable atoms (parents) to produce stable new atoms (daughters) 92U238    92U235    92    92 protons 146    143 neutrons 238    235    92U238---decay--- > 82Pb206 parent daughter decay constant= 1.5369x10-10/year U--- > Pb 1. Assumption--rate of decay is not affected by chemical or physical factors C. Half-life--the time required for half of the nuclei of radioactive elements to decay U238 --- > Pb206 = 4.5 billion years zircon (Zr,U)SiO4 < --- common as an accessory mineral in igneous rx uraninite (UO2) < --- uranium ore D. C14/ C12--remarkably constant in the atmosphere and all living things E. Death = start of geological or isotopic clock 6C14 ---- > 7N14 parent         daughter 1. Half-life = 5,730 years, which is short 2. Good only for dating materials < 70,000 years old F. C14/N14 ratio through time 1. Industrial Revolution 2. Nuclear Age II. DECAY SYSTEMS A. Decay system summary parent daughter half-life (m.y.) materials dated C14 N14 0.000573 once living materials K40 Ar40 1,300 anything w/K,orthoclase, musc., biotite, amph., granite, diorite Rb87 Sr87 48,800 anything w/K U235 Pb207 704 uraninite and zircon anything w/U U238 Pb206 4,500 same as U235 - Pb207 B. Relative age dating 1. Superposition--in an undisturbed sedimentary sequence, oldest rxs are at bottom, youngest rxs are at top 2. Principle of cross-cutting relations--cutter is younger than the cuttee 3. Principle of inclusion--rx bodies are younger than inclusions within them a. Example xenolith b. Example pebble in a conglomerate 4. Unconformities--erosion surfaces separating younger strata from older rxs a. James Hutton (late 1700's) Siccar Point, Scotland b. Greatest scientific find of the 18th century c. 3 types of unconformities - Angular unconformity--an unconformity formed above folded and tilted rxs - Nonconformity--developed above igneous and metamorphic rxs - Disconformity--developed in horizontal strata C. Combinations of absolute and relative age dates 1. Dating sediments a. Bentonite--ash fall deposits (volcanic) - Ash fall blanket - K-spar, musc., bt., amph., zircon - K-Ar, Rb-Sr, U-Pb b. Glauconite--an authigenic mineral; crystallizes during sedimentation - K-rich, date K-Ar or Rb-Sr - Small (~2mm) green pellets c. Age bracketing using igneous rocks 2. Correlations--process of relating a rx sequence in one place to another sequence in some distant place on the basis of lithology and fossil assemblage a. Early 1800's, William "Strata" Smith British CE b. Fossil assemblages 3. Principle of floral and faunal succession--observed changes in life forms become more complex and form a definite, recognizable order throughout geological history III. GEOLOGIC ROCK COLUMN--arrangement of rxs according to their ages A. Geologic time scale--time units related to the rx column 1. eon 2. era 3. period 4. epoch Cenozoic Era 66.4 m.y. Quaternary Period humans Tertiary Period Alps and Himalayas Mesozoic Era 245 m.y. Cretaceous Period Rocky Mtns Jurassic Period first birds Triassic Period dinosaurs,first mammals Paleozoic Era 570 m.y. Permian Period Appalachians Pennsylvanian Period coal deposits Mississippian Period first reptiles Devonian Period first amphibians Silurian Period first land plants Ordovician Period first fish Cambrian Period First shells Precambrian Era 4.6 b.y. ago   mainly, single-celled organisms in oceans