CEE 1030 1nd Edition Lecture 9: Geologic Time #2Outline of Last Lecture I. ReviewII. Geologic Time ScaleIII. Principlesa. Principle of Uniformitarianismb. Principle of Superpositionc. Principle of Original Horizontalityd. Principle of Cross-Cutting Relationshipse. Principle of InclusionsIV. UnconformitiesV. Geological history of a regionOutline of Current Lecture I. Structure of an atomII. Radioactive decayIII. Basic principles of isotopic datingIV. Methods of radioactive datingV. Earth’s oldest rocksVI. Absolute ages for the Geologic Time ScaleVII. Volcanic ash layersCurrent LectureI. Structure of an atoma. Nucleus contains most of an atom’s massi. Number of protons: atomic numberii. Protons + neutrons = mass numberiii. Isotopes: an element with a different number of neutrons than usualII. Radioactive decaya. Radioactivity: spontaneous changes in the structure of a certain unstable atomic nucleib. Parent: an unstable radioactive isotopec. Daughter products: isotopes resulting from decay of parent isotoped. Alpha decay, beta decay, electron capturee. How is it useful for dating?i. Rate at which radioactive decay occurs for a particular unstable isotope is constantThese 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.ii. If you know the rate of decay and number of parent and daughter isotopes present in a sample, we can calculate the ageiii. Half-life: the time taken for half of atoms in a radioactive isotope to decayIII. Basic principles of isotopic datinga. Comparing the ratio of parent to daughter atoms yields the age of the sampleb. Decay rates are constant and precisely knowni. There is no evidence to expect decay rates to vary under Earth’s physical conditionsc. Initial concentration of daughter knowni. Best to choose minerals that do not include daughter element during initial formationsd. Closed system: parent and daughter elements must not diffuse/leak in or out of minerale. Diffusion stops below a certain temperature, which varies for different elements and different mineralsf. Minerals in slow-cooled metamorphic rocks can give different ages by different techniquesg. Radiometric dating requires precise measurementsh. Careful and detailed sample preparationi. Sophisticated and expensive equipmentj. 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 datingIV. Earth’s oldest rocksa. Acasta Gneisses: near Great Slave Lake in Canada, metamorphicV. Absolute ages for the Geologic Time Scalea. Time scale based on sedimentary rock, but absolute ages are best determined in igneous or metamorphic rockb. Detrital mineral grains will give age of original source rockc. Cement minerals give younger ages than sediment deposition aged. One must date sedimentary rocks indirectly (looking at igneous layers within sediments)VI. Volcanic ash layersa. Blanket a wide geographical areab. Distinctive time markerc. Often contain minerals that can be dated by radiometric
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