Earth history

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Earth’s History Earth’s History • Planet Earth is approximately 4.5 X 10 years old –Rocks of the crust provide clues to Earth’s past • By analyzing these clues we can infer events from the past Earth’s History • Principle of Uniformitarianism – Major assumption in geology – Events in the past occurred the same way that they are occurring today Examples Include: • Weathering/erosion • Deposition • Volcanism • Plate tectonics Geologic Time • Plate motions and mountain building events can be placed on the geologic time scale Geologic Time Geologists have divided Earth’s history into time units based on the the fossil record Geologic Time • A study of the fossil record shows – A great variety of plants, animals, and simpler life forms have lived on Earth in the past – That life forms have evolved through time – Most life forms of the geologic past have become extinct Geologic Dating There are two types of Geologic Dating • Absolute –Determines how many years old something is • Relative –Used to determine if one thing is younger or older than another Tools used to determine Absolute Age • Using radiometric dating Relative Age • Law of superposition • Use of index fossils • Correlation of rock layers 10 Location A Location B Location C Rock layer W W W Rock layer W Rock layer W Z X Z Y X Z X Which letter would make a good index fossil? Z Absolute Dating 47 Radioactive Decay • When an unstable radioactive element changes into a stable element 48 Example Unstable Radioactive Isotope Stable Decay Product Carbon 14 (C14) Nitrogen 14 (N14) Uranium 238 (U238) Lead 206 (Pb206) Parent Daughter 49 Radiometric Dating Half life: The time required for 1/2 of a parent material to break down to daughter material 50 • Another look 51 Radiometric Dating • The age of objects can be determined by measuring the relative amounts of radioisotope and decay products 52 Radiometric Dating • The half lives of radioisotopes vary depending upon the isotope –Examples: • C14 = 5,770 years • U238 = 4,500,000,000 years • Rb87 = 47,000,000,000 years 53 • Carbon 14 is used to date biological remains – Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies • Uranium 238 is used to date rocks – Larger half life 54 Half Life Practice Problems 55 • If half the K -40 of a rock has decayed into Ar-40, how old is the rock? • How many years would it take for 75 % of a sample of K-40 to disintegrate? 56 How old are: • A sample that has ½ c-14 and ½ n-14? • A sample that has ¼ K-40 and ¾ Ar-40? • A sample that has ½ U238 ½ Pb-206? 57 How old are: • A sample that has 1/8 Rb-87 and 7/8 Sr-87? • A sample that has ¼ C-14 and ¾ N-14? • A sample that has ¾ C-14 and ¼ N-14? 58 What fraction remains of: • A 50 billion year old sample has Rb-87 and Sr-87 • A 2.6 billion year old sample has K-40 and Ar-40 59 This is so boring Stranz should take a break ;) 60
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