after about 50,000 years, you can’t really date anything because almost all the matter is long goneGEOL 240Lg 1st Edition Lecture 13 Outline of Last Lecture 1) Continuation of waves2) Drawings of waves to offer a better explanationOutline Current Lecture I. Radiometric Dating II. DecayIII. Relative Dating parent solutionIV. Decay Rate V. Layered StructuresVI. CompositionCurrent 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 + daughterIII. Know decay rateIV. age!!!after about 50,000 years, you can’t really date anything because almost all the matter is longgone- 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 crust◦ composed of really Mg+Fe rocks (more mafic than oceanic crust) MOHO▪ ultramafic rocks mantle◦ upper mantle is mainly composed of 2 minerals▪ olivine (Mg∨1Fe)∨2 Si O∨4▪ pyroxene
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