GEOL 100 (Planet Earth) #12 - Earth s Clock Geologic Time - Absolute Age Absolute Age = age of rock (or geologic event) in years. Need constant process as well as record of process; constant process in rocks is radioactive decay Isotope = different form of atom with same number of protons, different number of neutrons 238 U - 92 protons, 146 neutrons 235 U - 92 protons, 143 neutrons Absolute Age - Isotopes Stable isotopes - don't change form over time Unstable (radioactive) isotopes - spontaneously break apart over time Radioactive decay - spontaneous change in nucleus of radioactive isotope Absolute Age - Radioactive Decay Radioactive decay - spontaneous change in nucleus of radioactive isotope Parent (radioactive) isotope Daughter (stable) isotope + energy Absolute Age - Radioactive Decay Radioactive decay - spontaneous change in nucleus of radioactive isotope Parent (radioactive) isotope Daughter (stable) isotope + energy e.g., Uranium Lead Absolute Age - Half-Life Rate of radioactive decay reaction is given by half-life (time needed for half of parent isotopes to change to daughter isotopes). 1
Absolute Age - Half-Life Half-life is measurable and constant quantity for any radioactive isotope (not influenced by chemical bonding, differences in temperature and pressure) Determined by measuring amount of energy released Half-Life: Example Calculation Representative half-life calculation; reaction of parent to daughter isotope assuming half-life = 30 years; assume new mineral forms at time = 0 with all radioactive isotopes and no daughter isotopes Radiometric Age Dating Age of rock is determined by knowing half-life of reaction and amount of parent and daughter isotopes (radiometric age dating) Half-Life: Penny Flipping Simulation Simulate radioactive decay by flipping 160 pennies Need to choose which sides are radioactive, stable Common Radioactive Isotopes Used Commonly used parent isotopes with long half-lives (billions of years) = 40 K, 238 U, 87 Rb. They are used to measure old events (e.g., rocks 1 m.y. to many b.y.) Radiocarbon Dating 14 C half-life = 5,730 years; after ~11 half-lives, no parent isotope remains (can no longer measure age). 14 C used to date organic matter up to 70,000 years old 2
Igneous rocks? Clastic sedimentary rocks? Granite Sandstone Chemical sedimentary rocks? Metamorphic rocks? Rock Salt Gneiss chemical precipitation Igneous rocks yield most reliable radiometric ages of formation Combining Relative and Absolute Ages Use radiometric ages of igneous rock and relative age principles to constrain ages of rocks that you can't date Diagram from last class with radiometric ages of igneous rocks 3
Age of dipping sedimentary rocks? Age of this conglomerate? Lava flow (30 million years) Age of shale? Igneous dike (50 million years) Using fossils and radiometric ages of rocks from all over world resulted in Geologic Time Scalesubdivided and named intervals of geologic 7me that represent Earth's history Geologic Time Scale Age of Earth = 4.6 b.y. (from age of meteorites, moon rocks) Age-dated moon rock Age-dated meteorite Oldest rocks from Earth (continental rock) = 4-4.2 b.y. Why not 4.6 b.y.? PreCambrian = Largest block of time (88% of Earth history); Life evolved ~3-3.5 b.y.a. (bacteria). Only soft-bodied organisms lived, so few fossils are preserved. Extreme volcanism - Early Precambrian Iron Formation - Middle Precambrian 3.5 b.y. old stromatolites (cyanobacteria) 4
PreCambrian/Paleozoic boundary = Appearance of abundant animals with shells, fossils now abundant. Paleozoic ("Old life") Era = Trilobites, crinoids, coral reefs, snails; Appearance of fish, plants, amphibians, insects Abundant ocean life in Paleozoic Abundant new life with hard parts in Early Paleozoic Paleozoic ( Old life ) Era = Carboniferous Period involved extensive forests, swamps (later became coal). Paleozoic/Mesozoic boundary = Mass extinction; half of all families of organisms suddenly disappeared. Carboniferous Forest and Swamp Mesozoic ( Middle life ) Era = Dominance of reptiles (dinosaurs), first birds Mesozoic/Cenozoic boundary = Mass extinction of 1/4 of all families of organisms. Extinction of all dinosaurs. Dinosaurs of Mesozoic Era Mesozoic/Cenozoic (K/T) Mass Extinction 5
Cenozoic ( Recent life ) Era = Dominance of mammals, humans appear, current mass extinction. Mammals of Cenozoic Roll of paper with geologic time scale (1/16 inch = 1 million years) 6