GEOL 101 1nd Edition Lecture 18 Outline of Last Lecture I. Principles of DatingII. UnconformitiesIII. FossilsOutline of Current Lecture I. Radioactivity II. Geologic Time ScaleCurrent LectureRadioactivity -Types of radioactive decay:- Alpha emissiono Emission of 2 protons and 2 neutrons (an alpha particle)o Mass number is reduced by 4 and the atomic number is lowered by 2o Common example: U268  Th234- Beta emissiono An electron (beta particle) is ejected from the nucleus creating a protono Mass number remains unchanged and the atomic number increases by 1o Common Example: Pb214Bi214- Electron captureo An electron is captured by the nucleuso The electron combines with a proton to form a neutrono Mass number remains unchanged and the atomic number decreases by 1-Half-life – time regular for ½ of the radioactive nuclei in a sample to decay-Principle of Radiometric dating—the percent of radioactive atoms that decay during one half-life is always the same (50%)o However, the actual number of parent atoms that decay continually decreaseso 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 GaTwo isotopes of uranium:U2384.5 GaU235713 million years (Ma)Potassium-40 (K40) 1.3 GaCarbon-14 (C14) 5,730 years-Dating with C14 (radiocarbon dating)- Very short half-life (5730 years) which can be used to date very recent eventso 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- Periodso Epochs Stages-Eon – greatest expanse of time- Names of the eons:o Phanerozoic (“visible life”) – the most recent eon, began about 542 million years agoo Proterozoic – oxygenation of the atmosphere, first multicellular organismso Archean – oldest known cyanobacteria fossils, photosynthesis begins, first evidence of plate tectonicso 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 eono 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 flourisho Silurian – first land plants, first insects, fishes flourish o Ordovician – first fishes appear, invertebrates still prevalento 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, amphibianscontinue to flourisho Mississippian – Gymnosperms (conifers, ginkos, palms, etc.) flourish, amphibians flourish, first reptiles- Mesozoic Erao 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 birdso Triassic – first dinosaurs, first mammals- Cenozoico Quaternary – First genus: homo in early Pleistocene, Pleistocene glaciations and megafauna (think “Ice Age”), first homo sapiens appear about 200,000 years agoo 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,