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Transcript
Name _____________________________________________________ Lab Group Members ________________________________________ , ______________________________ ___________________________________________ , ____________________________________________ Date ______________________ Lab # ________________ The Theory of Plate Tectonics and Sea Floor Spreading Modeling Seafloor Spreading Occurring at Mid Ocean Ridges Introduction: A map of the ocean Seafloor shows a variety of topographic features: flat Abyssal plains, long mountain chains called ridges, and deep ocean trenches. Today we know that Mid-Ocean Ridges are part of chain of mountains some 84,000 km long and the Mid-Atlantic Ridge is the longest mountain chain on Earth but these are relatively new discoveries. Geologist Harry Hess, while serving in the U.S. Navy during World War II began mapping out the oceans floor using a new tool called sonar, which was developed originally to find hidden submarines. His research during and after the war helped Hess to develop a revolutionary theory that later would be called Seafloor Spreading. Through direct sampling, Hess measured the age of the sea floor and realized that new seafloor has continually been forming over millions of years at the mid-ocean ridges. Hess found that on each side of the ridge, sea floor got older as a person moved away from the ridge towards the continents. The older, denser oceanic crust eventually descends into deep trenches along the continents and is recycle, this is called Subduction. The deep sea trenches are long, narrow basins which extend 8-11 km below sea level. Trenches develop at subduction zones, where denser oceanic lithosphere slides back into the mantle under the less dense continental crust. The oldest seafloor rocks are only about 180 million years old compared to the oldest continental rocks, which can be as old as 4,000 million years. Pre Lab: 1. In the diagram above, highlight both the word Mid-ocean ridge and the ridge. Label this plate boundary. highlight both the word Ocean trench and the trench. Label this plate boundary. Using a highlighter, draw in the convection currents in the asthenosphere. 2. Describe what Hess found when he scanned the ocean floor with sonar. __________________________________________________________________________________ __________________________________________________________________________________ 3. Describe what Hess found when he measured the age of the sea floor. __________________________________________________________________________________ _________________________________________________________________________________ Objective: To construct a paper model that will illustrate how the seafloor is created at mid-ocean ridges and is recycled where it descends into trenches. Materials: Colored Pencils Scissors Tape Procedure: Follow the steps in order, check off each step as you complete it. 1. Tear off the past page and color the strips with colored pencils. 2. Cut the strips out after coloring. Tape the end of one strip at blue end to the other strip at the blue end. It helps if you make sure the colored sides are touching/ facing each other, blue to blue, orange touching orange etc. 3. On the lab page Labeled “Sea Floor Diagram”, fold and cut along the short dotted lines. 4. Thread the two ends of your strip of paper through Slit B. 5. Pull one side down through Slit A and the other through Slit C, color side up 6. Pull the strips through the slits so that the same colors on both strips come out from Slit B and go into or disappear into Slits A and C at the same time. Label your Sea Floor. Check off each item below as you label it. Label the Mid-Atlantic Ridge/ Mid Oceanic Ridge Record the relative age of the rock found here. (older vs younger) Name the type of tectonic boundary found at this ocean ridge (use ESRT pg5) Label the Deep Sea Trenches Record the relative age of the rock found here. (older vs younger) Name the type of tectonic boundary found at a zone of Subduction (ESRT pg5) Record the density of the Ocean Crust Analysis: 1. Explains what happens at Slit B? _____________________________________________________ ____________________________________________________________________________________ 2. Name the land feature occurs at this location on the seafloor. __________________________________ ____________________________________________________________________________________ 3. What happens to the tectonic plates at Slit B? (Circle one) Divergence / Subduction 4. Explains what happens at both Slit A and C? _______________________________________________ ____________________________________________________________________________________ 5. What land features occur where sea floor is pulled under continental crust? _______________________ ____________________________________________________________________________________ ____________________________________________________________________________________ 6. What happens to the crustal plates at Slit A and C? (Circle one) Divergence 7. Subduction/ If you were to start sampling and date the rocks along the colored strip starting at Slit B and move outward towards Slit A or Slit C, explain what changes would you see in the age of the rocks. ____________________________________________________________________________________ ____________________________________________________________________________________ 8. Why is it important that your model have an identical color patterns on both sides of the center slit? ____________________________________________________________________________________ ____________________________________________________________________________________ _________________________________________________________________________________ 9. What is something that could possibly cause the plates to be pulled apart at the Mid Ocean ridge? ____________________________________________________________________________________ _________________________________________________________________________________ Conclusion: Briefly stated in your own words: What is Sea Floor Spreading? Make sure you include in your summary data that helped Harry Hess his develop his theory. ____________________________________________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ _______________________________________________________________________________________ SEA FLOOR DIAGRAM Land feature formed at slit A is a ____________________________ Here, ocean crust is formed a ____________ time ago so the rock age is very ___________________ This is a _________________________________ Plate Boundary where subduction occurs. Land feature formed at slit B is a ____________________________ Here, ocean crust is formed a ____________ time ago so the rock age is very ___________________ This is a _________________________________ Plate Boundary where sea floor spreading occurs. . Land feature formed at slit C is a ____________________________ Here, ocean crust is formed a ____________ time ago so the rock age is very ___________________ This is a _________________________________ Plate Boundary where subduction occurs.