CEE 1030 1nd Edition Lecture 9 Geologic Time 2 Outline of Last Lecture I Review II Geologic Time Scale III Principles a Principle of Uniformitarianism b Principle of Superposition c Principle of Original Horizontality d Principle of Cross Cutting Relationships e Principle of Inclusions IV Unconformities V Geological history of a region Outline of Current Lecture I Structure of an atom II Radioactive decay III Basic principles of isotopic dating IV Methods of radioactive dating V Earth s oldest rocks VI Absolute ages for the Geologic Time Scale VII Volcanic ash layers Current Lecture I II Structure of an atom a Nucleus contains most of an atom s mass i Number of protons atomic number ii Protons neutrons mass number iii Isotopes an element with a different number of neutrons than usual Radioactive decay a Radioactivity spontaneous changes in the structure of a certain unstable atomic nuclei b Parent an unstable radioactive isotope c Daughter products isotopes resulting from decay of parent isotope d Alpha decay beta decay electron capture e How is it useful for dating i Rate at which radioactive decay occurs for a particular unstable isotope is constant 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 III IV V VI ii If you know the rate of decay and number of parent and daughter isotopes present in a sample we can calculate the age iii Half life the time taken for half of atoms in a radioactive isotope to decay Basic principles of isotopic dating a Comparing the ratio of parent to daughter atoms yields the age of the sample b Decay rates are constant and precisely known i There is no evidence to expect decay rates to vary under Earth s physical conditions c Initial concentration of daughter known i Best to choose minerals that do not include daughter element during initial formations d Closed system parent and daughter elements must not diffuse leak in or out of mineral e Diffusion stops below a certain temperature which varies for different elements and different minerals f Minerals in slow cooled metamorphic rocks can give different ages by different techniques g Radiometric dating requires precise measurements h Careful and detailed sample preparation i Sophisticated and expensive equipment j Can use carbon 14 radiocarbon dating to date materials with carbon in them for several thousand years k Uranium lead dating is extremely useful for dating Earth s oldest rocks a Acasta Gneisses near Great Slave Lake in Canada metamorphic Absolute ages for the Geologic Time Scale a Time scale based on sedimentary rock but absolute ages are best determined in igneous or metamorphic rock b Detrital mineral grains will give age of original source rock c Cement minerals give younger ages than sediment deposition age d One must date sedimentary rocks indirectly looking at igneous layers within sediments Volcanic ash layers a Blanket a wide geographical area b Distinctive time marker c Often contain minerals that can be dated by radiometric techniques
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