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Transcript
Unit 3: The Dynamic Earth Mr. Ross Brown Brooklyn School for Law and Technology How have the continents drifted? • 26 October 2015 • Do now: How is continental drift like a jigsaw puzzle? Unit highlights • Continental drift and how there was once a single landmass • Seafloor spread as evidence of drift • Plate tectonics • Geologic activity at plate boundaries • Lithospheric plates and convection Coastlines lead to a hypothesis What is Continental Drift? • Mapmakers and explorers noticed similar shorelines on either side of the Atlantic Ocean • Could the continents have once fit together? • 1912- Alfred Wegener proposed the theory of Continental Drift; the continents had moved! • This suggested a single landmass: Pangea • Surrounded by super ocean Panthalassa What is Pangea? • From late Paleozoic to early Mesozoic eras • Assembled from earlier continental units around 300MYA and began to break apart around 175MYA Fossil evidence for Pangea and drift • Similar fossils found on different continents • Mesosaurus, alive 270MYA, found on Africa’s west coast and S. America’s east Who was Mesosaurus? What other evidence supports the theory of Pangea? • Geological evidence – Mountain ranges spanning continents • Climatic evidence – Glacial debris found across S Hemisphere landmasses that are today warmer Was the theory of Pangea fully accepted? • What was the force that made the continents drift apart? What if continents hadn’t drifted? What if continents hadn’t drifted? Homework #6 26 October 2015 1. What observations first led to Wegener’s hypothesis of continental drift? 2. What types of evidence supported Wegener’s hypothesis? What force makes the continents drift? • 27 October 2015 • Do now: Based on what we discussed yesterday, what is the force that moved the continents apart from Pangea, and still moves them today? What is the Mid-Atlantic Ridge? • Undersea mountain range, with a steep valley running down its center. • Part of an 80,000km system of mid-ocean ridges • In 1940s scientists found that none of the rock there was older than from 175MYA • Continental rock is from up to 4BYA! What are Mid-Ocean Ridges What is seafloor spreading? How did the theory of seafloor spreading develop? • What if the valley at the center of the ridge was a rift, or break? • Magma wells up through this break • Possible if ocean floor moved away from ridge • If seafloor moved, then perhaps continents moved, too?! Homework #6, continued 27 October 2015 1. Describe the process of seafloor spreading. How did our understanding of plate movements develop? • 30 October 2015 • Do now: How is ‘plate tectonics’ different from ‘continental drift?’ Plate tectonics • The theory that Earth's outer shell is divided into several plates that glide over a plastic inner layer above the core. The plates act like a hard and rigid shell compared to Earth's interior. How did seafloor spread refine the theory of plate tectonics? • Moved beyond notion of “continental drift” • Plate tectonics: the study of the formation of the earth’s crust • Lithosphere: the rocky stuff. Crust and rigid mantle • Asthenosphere: plastic rock on which lithosphere floats Earth’s plates What happens where plates meet? • Divergent Boundaries: plates move away from each other, forming rifts or mid-ocean ridges • Convergent Boundaries: plates collide – Subduction: Oceanic pushed under continental – Obduction: Continental pushed under oceanic – Orogenic: both push upwards • Transform Boundaries: Plates grind past each other Where plates meet What happens where plates meet? How did the continents form? • 2 November 2015 • Do now: Why are there seashells on the tops of mountains? How does subduction increase continental crust? • As oceanic crust is subducted, some lithosphere is “scraped off” and accumulates on the continent. This is a terrane. What is a terrane? • Pieces of lithosphere with unique geological history How is each terrane unique? • A terrane contains rocks and fossils that are unique from its neighbors • There are major faults at the boundaries of the terrane • The magnetic properties of the terrane do not match its neighbors