GEOL 101 1nd Edition Lecture 18 Outline of Last Lecture I Principles of Dating II Unconformities III Fossils Outline of Current Lecture I Radioactivity II Geologic Time Scale Current Lecture Radioactivity Types of radioactive decay Alpha emission o Emission of 2 protons and 2 neutrons an alpha particle o Mass number is reduced by 4 and the atomic number is lowered by 2 o Common example U268 Th234 Beta emission o An electron beta particle is ejected from the nucleus creating a proton o Mass number remains unchanged and the atomic number increases by 1 o Common Example Pb214 Bi214 Electron capture o An electron is captured by the nucleus o The electron combines with a proton to form a neutron o Mass number remains unchanged and the atomic number decreases by 1 Half life time regular for of the radioactive nuclei in a sample to decay Principle of Radiometric dating the percent of radioactive atoms that decay during one halflife is always the same 50 o However the actual number of parent atoms that decay continually decreases o Comparing the ratio of parent to daughter isotopes to the number of half life yields the age of the sample Useful radioactive isotopes for providing radiometric ages and their half lives Rubidium 87 Rb87 47 billion years Ga These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Thorium 232 Th232 14 1 Ga Two isotopes of uranium U238 4 5 Ga 235 U 713 million years Ma Potassium 40 K40 1 3 Ga Carbon 14 C14 5 730 years 14 Dating with C radiocarbon dating Very short half life 5730 years which can be used to date very recent events o 70 000 years maximum Is produced in the upper atmosphere by cosmic ray The C gets interrupted in CO2 which is then absorbed by living organisms C14 dating has become an important tool for anthropologists archeologists and geologists who study very recent Earth history Sources of error The mineral containing the radioactive isotope must be in a closed system for the entire duration To avoid potential problems only fresh unweathered rock samples should be used The Geologic Time Scale a calendar of Earth history Subdivides 4 6 billion years of earth s history into units Originally created using relative dates Radiometric dating has allowed us to assign numerical ages to the sequence Large expanses of time are subdivided into smaller blocks of time Those are subdivided even further o Eons Eras Periods o Epochs Stages Eon greatest expanse of time Names of the eons o Phanerozoic visible life the most recent eon began about 542 million years ago o Proterozoic oxygenation of the atmosphere first multicellular organisms o Archean oldest known cyanobacteria fossils photosynthesis begins first evidence of plate tectonics o Hadean the oldest eon very primitive atmosphere and oceans Likely highly acidic environmental conditions Oldest rock on Earth Acasta Gneiss Canada Oldest rock fragments Jack Hills Zircons Australia Era Subdivision of an eon Eras of the Phanerozoic eon o Cenozoic recent life Age of the mammals The Era in which we live o Mesozoic middle life Age of Dinosaurs ends at the Cretaceous Paleogene extinction formerly K T boundary o Paleozoic ancient life Marked by the sudden emergence of a wide variety of life in the fossil record Cambrian Explosion ends at the Permian Extinction Periods subdivision of era Cenozoic periods Paleogene and Neogene formerly Tertiary and Quaternary Mesozoic periods starting with the oldest Triassic Jurassic Cretaceous Paleozoic periods starting with the oldest Cambrian Ordovician Silurian Devonian Mississippian Pennsylvanian Permian Periods are subdivided into Epochs Paleozoic Era o Devonian first tree sized land plants first amphibians fishes flourish o Silurian first land plants first insects fishes flourish o Ordovician first fishes appear invertebrates still prevalent o Cambrian first organisms with shells invertebrates dominate the ocean trilobites cephalopods brachiopods 542 Ma Precambrian Cambrian boundary Marks the beginning of visible life in the fossil record Cambrian Explosion o 251 Ma Permian Triassic boundary Mass Extinction 90 of all marine organisms 70 of terrestrial organisms died Cause is still highly speculative Meteorite Volcanoes Global warming Combination o Permian first mammal like reptiles synapids o Pennsylvanian Extensive swamps produced massive coal deposits amphibians continue to flourish o Mississippian Gymnosperms conifers ginkos palms etc flourish amphibians flourish first reptiles Mesozoic Era o 65 5 Ma Cretaceous Paleogene boundary formerly K T Mass Extinction 75 marine species Causes favored include Chicxulub impact and or massive volcanic eruptions and associated climate change probably cooling o Cretaceous Dinosaurs still dominate first flowering plants angiosperms o Jurassic Dinosaurs flourish first birds o Triassic first dinosaurs first mammals Cenozoic o Quaternary First genus homo in early Pleistocene Pleistocene glaciations and megafauna think Ice Age first homo sapiens appear about 200 000 years ago o Neogene Mammals continue to flourish first hominds family hominidae appear in the Miocene some extraordinarily large species megafaune o Paleogene Mammals birds insects and flowering plants flourish reptiles dwindle First horses primates whales