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Transcript
Earth’s History • Planet Earth is approximately 4.6 billion years old –Rocks of the crust provide clues to Earth’s past • By analyzing these clues we can infer events from the past 1 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 2 Geologic Dating 3 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 4 Tools used to determine Absolute Age • Using radiometric dating Relative Age • Law of superposition • Use of index fossils • Correlation of rock layers 5 Relative Dating 6 Law of Superposition • In a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngest – Lower layers must be in place before younger rocks can be deposited on top of them – Exception: when something occurs to overturn layers 7 Grand Canyon 8 GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM Law of Superposition • Rock layers are older than folds found in them – Layers were there before they were folded 10 Folds/Tilted 11 Law of Superposition • Rock layers are older than faults found in them • This is logical: you can’t break a rock if it does not exist; so rock containing a fault must be older than the fault 12 Law of Superposition • Fossils are generally the same age as the rock layers in which they are found –Animal remains are deposited along with the sediments that will turn into sedimentary rocks 13 Law of Superposition • Intrusions are younger than the rock that they cut through or flow out of 14 Fossils • Generally found in sedimentary rock layers Why don’t fossils exist in most igneous or metamorphic rock? 15 Fossils • 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 16 Index Fossils • Index Fossils--used to date rock layers –Fossils from creatures that existed for a geologically short period of time –Fossils from creatures that had a wide geographic distribution 17 Which fossil would make the best index fossil? Found in only 1 layer (short lived) Found in all samples (widespread)