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Transcript
Fossils of the Glossopteris Fern, a plant that lived in warm, damp areas was found in Africa, S. America, Australia, and in Antarctica. How does this evidence prove Wegner’s Theory of Continental Drift? A: Antarctica must have been closer to the equator and had a warmer climate. • MYP Unit Question: How does land change? • Area of Interaction: Environment • Learner Profile: Thinker • Standard: Investigate the scientific view of how the Earth’s surface is formed. • Learning Target: I’m learning about Sea Floor Spreading because it will help explain Wegner’s Theory of Continental Drift. • Standard: Investigate the scientific view of how the Earth’s surface is formed. • Learning Target: I’m learning about Sea Floor Spreading because it will help explain Wegner’s Theory of Continental Drift. Opening: What evidence supports the theory of Continental Drift? 1. Shape of Continents (Africa & S. America) 2. Fossil evidence (Glossopteris Fern, Mesosaurus & Lystrosaurus) 3. Rock pattern and formation 4. Climate Change (glaciers & ferns) This was still not enough evidence! Scientists needed a mechanism for how the continents moved. So…Why WERE the continents moving?? …because of sea-floor spreading! The process by which molten material adds new oceanic crust to the ocean floor. • It was not until the 1950s that evidence by different scientists helped to support the theory of continental drift. • Scientists began to use new technology, such as sonar, to map the ocean floor. • Using this technology, scientists made new discoveries that led to the understanding of a mechanism for continental drift: sea-floor spreading. Sonar Animation • http://earthguide.ucsd.edu/eoc/teachers/t_t ectonics/p_sonar.html • Sonar: A device that determines the distance of an object under water by recording echoes of sound waves Mapping the Ocean Floor • Alvin’s crew found strange rocks shaped like pillows or like toothpaste squeezed from a tube • Such rocks can form only when molten material hardens quickly after erupting under water • The presence of these rocks showed that molten material has erupted again and again from cracks along the central valley of the mid-ocean ridge. http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html How does Sea-floor Spreading Work? • a process in which new ocean floor is created as molten material from the earth's mantle rises in margins between plates or ridges and spreads out. Animation – Sea Floor Spreading • http://earthguide.ucsd.edu/eoc/teachers/t_t ectonics/p_seafloorspreading.html • Samples from sea floor drilling showed that the farther away from the ridge go the older the rocks become. Evidence to support Sea-Floor Spreading 1.Age of the ocean floor a. newer material was formed at the Mid Ocean Ridge (a huge, underground mountain system). b. older sea floor was found at the trenches. 2. Magnetic Pole Reversals (poles flipped) #1 Age of the Rock • The age of the ocean rock was different. When scientists sampled the rocks, they found that a. the further away from the ridge the rocks were, the older they were. • b. The younger rocks were always in the center of the ridges #2 Magnetic Stripes • Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes” • 780,000 years ago, magnetic poles reversed themselves • If they reversed today, the needle in a compass would point south instead of north • The rock in the ocean is made of iron, which began as molten material Earth’s magnetic poles have reversed themselves throughout history. The rocks that make up the ocean floor lie in a pattern of magnetized stripes. Sea-Floor Spreading Evidence 1. Brand new rock was found at the midocean ridge. 2. Rock got older and older the farther you got away from the ridge. Oldest rocks were found at the trenches. 3. Magnetic Reversals – Iron in the rocks switched directions every time the Earth’s polarity switched. Volcanoes located along Mid-ocean ridges erupt, creating new ocean floor. Ocean Topography What happens when the Ocean crust meets Continental crust? • http://earthguide.ucsd.edu/eoc/teachers/t_t ectonics/p_subduction.html Subduction at Deep-Ocean Trenches Deep-Ocean Trenches • Deep-Ocean Trenches: A deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle Subduction • Subduction: The process by which oceanic crust sinks through a deepocean trench and back into the mantle; a convergent plate boundary Mid Ocean Ridges and Deep Ocean Trenches Mid-Ocean Ridge • Mid-Ocean Ridge: The undersea mountain chain where new ocean floor is produced; a divergent plate boundary Lithospheric Plates What type of crust (continental or oceanic) does the North American Plate have? Sea Floor Spreading Activity with magnetic reversals Exit Ticket: Where are mid-ocean ridges? Where are trenches?