GEOL 240Lg 1st Edition Lecture 13 Outline of Last Lecture 1 Continuation of waves 2 Drawings of waves to offer a better explanation Outline Current Lecture I Radiometric Dating II Decay III Relative Dating parent solution IV Decay Rate V Layered Structures VI Composition Current Lecture Radiometric dating based on radioactive decay provides absolute dates 1950 s prior to 1950 s geologists relied solely on relative dating parent daughter 238 U 206 Pb Uranium Lead I when the rock forms no daughter material is present daughter builds through time II Measure ratio of parent daughter III Know decay rate IV age after about 50 000 years you can t really date anything because almost all the matter is long gone for carbon 14 1 2 life is 5 570 years learn the powers of numbers for future tests etc i e 1 x 10 2 5 14 x 10 9 10 1 tens 10 2 hundreds 10 3 thousands 10 6 million 10 9 billion 100 years ago we knew almost nothing of the earth s interior Seismology in 1920 s ongoing Travel time curves of bodywaves 1 p Reflection and refraction 1 s Rock experiments at high temp pressure P Layered structures layered by composition 3 major composition layers crust low density rock mantle high density rock core metal Crust Continental Lower density Quartz feldspar rich rocks granite Rich in silica Variable thickness 15 80 km avg 30 km Thickest beneath collision zones thinnest beneath rifts Oceanic Higher density Rich in iron magnesium Mg Fe Mafic Rich in basalt Thinner 5 km More uniform in thickness Isostacy relative buoyancy Mantle dense rock composed of really Mg Fe rocks more mafic than oceanic crust ultramafic rocks crust MOHO mantle upper mantle is mainly composed of 2 minerals olivine Mg 1Fe 2 Si O 4 pyroxene MgFe SiO
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