GEO 102 Lecture 12 Outline of Last Lecture I Geologic Time Outline of Current Lecture II Types of unconformities III Correlation IV Absolute Dating V Half life Current Lecture Basic Principles of Rock Formation Cross cutting relationships If one rock formation intrudes or bisects another the rock being cross cut must be older Also inclusions Basic Principles of Rock Formation Unconformities Non deposition Erosion As opposed to a conformable transition Types of Unconformities Angular unconformity Rocks were tiled prior to later sedimentation disconformity Erosional surface between 2 parallel layers of rock nonconformity 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 Sedimentary rocks in contact with non sedimentary rocks Most of these are based on simple fundamental logic it almost seems unnecessary to define them The Really Big Picture How do we take the previous interpretation of a single outcrop and extrapolate global time Correlation Linking outcrops of the same rock over wide areas Even globally Correlation Linking time equal rocks Presently many different depositional environments form simultaneously Later they will be preserved incompletely as a wide range of rocks How do you link the outcrops Rock types Geochemistry Fossils Fossil succession Fossils change in an orderly way through time Index fossils Extinction and evolution Absolute dating Absolute dating Relative dating has been around centuries and but absolute estimates of the age of the Earth varied The goal was a fixed and absolute age Radioactivity can act as a clock Decay and Half life Unstable nuclei eject particles predictably in order to achieve equilibrium alpha particles 2 protons and 2 neutrons beta particles Electron from nucleus Electron capture Forms a neutron after fusing with a proton Half Life For any given atom it can not be predicted when decay will occur However a POPULATION of atoms will decay like clockwork the time it takes for half of the population to decay one half life What do we date Only some elements have radioactive isotopes Only some isotopes have half lives that are useful Only some minerals contain these few isotopes Only some rocks contain pristine minerals There must be no non radioactive alteration of parent or daughter Closed system
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