ME ES 11/12 Q2 1303 SG Earth Science PDF

Earth Science Grade 11/12 Unit 13: Plate Tectonics LESSON 13.3 The Seafloor Spreading Table of Contents Introduction 1 Learning Competenci...

Earth Science Grade 11/12 Unit 13: Plate Tectonics LESSON 13.3 The Seafloor Spreading Table of Contents Introduction 1 Learning Competencies 2 Learning Objectives 2 Warm-Up 2 Learn about It 5 Development of the Seafloor Spreading Hypothesis 5 Parts of the Ocean Floor 9 Key Points 12 Check Your Understanding 12 Photo Credits 15 Bibliography 15  Earth Science Grade 11/12 Unit 13: Plate Tectonics Lesson 13.3 The Seafloor Spreading Volcanic chimneys on the seafloor taken during the Pacific Ring of Fire Expedition Introduction How deep do you think the ocean is? What things can you find in the deep ocean waters? With the aid of modern technology, scientists were able to estimate that the ocean has an average depth of 3.7 kilometers. The deepest zone that they were able to record came from the Mariana trench which was 2,540 kilometers deep. Aside from the bizarre marine life, various geologic structures can be found in the ocean basin. What are these structures? How are these ocean basin structures related to the seafloor spreading hypothesis? 1  Earth Science Grade 11/12 Unit 13: Plate Tectonics Learning Competencies At the end of this lesson, the given DepEd learning competencies should be met by the students. Explain how seafloor spreads (S11/12ES-IIf-32). Describe the structure and evolution of ocean basins (S11/12ES-IIf-33). Learning Objectives In this lesson, you should be able to do the following: Explain how the seafloor spreads. Describe the structure and evolution of ocean basins. Warm-Up Let The Seafloor Spread Materials seafloor spreading sheet template A (with three straight lines), and B (with striped bands or striped bands with arrows) scissors clear tape coloring materials Procedure 1. Get the Seafloor Spreading Sheet Template A. Cut the vertical lines to create three slits. Be careful not to cut further than the vertical lines. 2  Earth Science Grade 11/12 Unit 13: Plate Tectonics Template A guide 2. Secure the slit by placing tape over each slit then recut the slit through the tape. 3. Obtain Seafloor Spreading Sheet Template B. Color every other striped band using yellow, orange, and red. 4. Cut the Seafloor Spreading Sheet Template B into half. Be sure to use the dash line as the guide. Template B guide 5. Insert one end of one of the strips of paper through the middle and left end vertical 3  Earth Science Grade 11/12 Unit 13: Plate Tectonics line of template A. Tape it to make a loop. Do the same to the other sheet but this time in the right end vertical line. Model of seafloor spreading 6. Simulate seafloor spreading by circulating the loops of paper. Guide Questions 1. What does the center slit in the model represent? __________________________________________________________________________________________ __________________________________________________________________________________________ 2. What do the strips of paper represent? __________________________________________________________________________________________ __________________________________________________________________________________________ 3. Looking at your model, how will you describe the “oldest” sections? __________________________________________________________________________________________ __________________________________________________________________________________________ 4. Based on the activity, how will you describe seafloor spreading? __________________________________________________________________________________________ __________________________________________________________________________________________ 4  Earth Science Grade 11/12 Unit 13: Plate Tectonics Learn about It Essential Questions How does the seafloor spread? What evidence can support the seafloor spreading hypothesis? Development of the Seafloor Spreading Hypothesis The seafloor spreading hypothesis was proposed by Harry Hess, a petrologist at Princeton University, and Robert Dietz, an oceanographer in the US Coast and Geodetic Survey. Hess suggested that the oceanic crust together with overlying continental crust are moving. He stated that oceanic crust spreads, starts to cool down, and becomes denser as it moves away from the ridge. This colder and denser oceanic crust is subducted into the oceanic trenches and will be later on melted and recycled in the mantle. Additionally, the movement of the seafloor is driven by convection in the mantle. Harry Hess Robert Dietz (1906–1969) (1914–1995) Fig. 1. Harry Hess and Robert Dietz, proponents of the seafloor spreading hypothesis 5  Earth Science Grade 11/12 Unit 13: Plate Tectonics In line with this, paleomagnetism is used as evidence of seafloor spreading. This is because basaltic lavas contain iron-bearing minerals which record the magnetic orientation of Earth once the lava cooled. The magnetic orientation of the oceanic crust together with the determination of the age of rocks through radiometric dating and data from Deep Sea Drilling Project supports the hypothesis that the ocean floor is spreading. Fig. 2. shows the magnetic orientations of the oceanic crust along the Reykjanes ridge near Iceland which is a portion of the mid-oceanic ridge. The colored stripes represent normal polarity while the white stripes represent reversed polarity. The stripes have an alternating pattern and are arranged symmetrically on both sides of the ridge. Fig. 2. Magnetic orientations of seafloor rocks in Reykjanes ridge This discovery leads to the conclusion of geologists that there is a sequence of events that lead to the alternating bands of normal and reversed polarity of the oceanic crust. In which, new oceanic crust continuously forms as basaltic lava rises along the ridge and as it cools, it spreads outward from the ridge. As the lava cools, it aligns with Earth’s magnetic polarity. The orientation of Earth’s magnetic field changes every half-million years on average. Therefore, the magnetic stripes actually record the magnetic orientation of Earth when the basaltic lava cooled. 6  Earth Science Grade 11/12 Unit 13: Plate Tectonics This mechanism leads the scientists to the concept of seafloor spreading that explains the source of new oceanic crust. The seafloor spreading hypothesis and the plate tectonic theory greatly helped the geologist in explaining the origin and the features of the mid-oceanic ridge which is Earth’s largest mountain chain. Fig. 3. Magnetic reversals in the mid-oceanic ridge The mid-oceanic ridge system is one of the most spectacular features found in the deep ocean floor which covers 20% of Earth’s surface. It is a continuous submarine mountain with a length exceeding 80 000 kilometers, and some parts have a width of more than 1500 kilometers. The slightly elevated area formed by the submarine mountain was caused by the relatively lower density of the fresh hot lava coming through the cracks. As the new 7  Earth Science Grade 11/12 Unit 13: Plate Tectonics lithosphere moved away from the ridge, it started to cool down creating a thicker, denser, and slightly sinking oceanic crust. The heat flow at the ridge is several times greater than the other parts of the ocean basin due to the heat flowing from the asthenosphere beneath the spreading center as well as the heat flow from the rising magma. In the given picture, the red area represents the most recent oceanic crust. Fig. 4. Mid-oceanic ridge system Aside from the discovery of magnetic reversals, oceanographers also studied the thickness of the sediments along the mid-Atlantic ridge. Mid-Atlantic ridge is the segment of mid-oceanic ridge which is 1 to 2 kilometers deep and several kilometers wide. The research was conducted in this area and was found to be made up of fresh basaltic lava that is not yet covered by sediments. They observed that it is at its thinnest near the ridge axis and gradually thickens as it moves farther from the ridge. 8  Earth Science Grade 11/12 Unit 13: Plate Tectonics In addition, shallow earthquakes are common in the mid-oceanic ridge due to the fracturing of the crust as two lithospheric crusts moved away from each other. Transform faults are also extensive in the mid-oceanic ridge dividing it into several short segments that are slightly offset from each other. Offsets from the ridge may range from less than a kilometer to hundreds of kilometers. Fig. 5. Transform fault in a segment of mid-oceanic ridge Essential Questions What are the structures found in the ocean basin? How did the ocean basin evolve? Parts of the Ocean Floor High-temperature geothermal vents called smokers are also found near or along the mid-oceanic ridge. These vents spew dark, mineral-rich fluids once in contact with the hot basaltic lava. Meanwhile, the greatest oceanic depths are found in oceanic trenches. These are long, steep-sided depressions on the seafloor found near convergent plate boundaries and represent 2% of the seafloor area. 9  Earth Science Grade 11/12 Unit 13: Plate Tectonics Fig. 6. Black smokers Fig. 7. Challenger Deep at the southern part of the Mariana trench Seamounts and volcanic islands are also formed in ocean basins. A seamount is a 10  Earth Science Grade 11/12 Unit 13: Plate Tectonics submarine mountain that has a height of 1 kilometer or more above the sea floor. If the structure rises above sea level, it is called a volcanic island. These structures are made up of basalt caused by the hot spot above the mantle plume. They form within a tectonic plate rather than at its boundary. A flat-topped seamount is called a guyot. Lastly, flat areas in the ocean floor are called abyssal plains. These areas consist of basaltic oceanic crust covered by deep sea sediments. Fig. 8. The ocean floor Did You Know? Bruce Heezen and Marie Tharp made the first map of the Atlantic Ocean floor. The data used for their mapping was from the Deep Sea Drilling Project. The chief scientist of the project was Maurice Erwing, the thesis advisor of Bruce Heezen during his doctoral studies. 11  Earth Science Grade 11/12 Unit 13: Plate Tectonics Key Points Seafloor spreading hypothesis states the seafloor moves away from the ridge crest and towards the trenches and is constantly melted and recycled in the subduction zones towards the mantle. Paleomagnetism and data from Deep Sea Drilling Project support the seafloor spreading hypothesis. Mid-oceanic ridge is the longest underwater mountain range which comprises 20% of Earth’s surface. Mid-Atlantic ridge is a segment of mid-oceanic ridge where most research on the ocean floor is conducted. The different features of the ocean floor include abyssal plain, seamount, guyot, trench, smoker, and volcanic island. The Mariana trench is the deepest trench in the world. Check Your Understanding A. Identify the term described in each statement. Write your answer on the provided space before each number. ________________ 1. It is a high-temperature geothermal opening on the ocean seafloor found along the ridge. ________________ 2. It is considered the deepest trench on Earth. ________________ 3. It is an oceanic land formation in the seafloor similar to a plateau. 12  Earth Science Grade 11/12 Unit 13: Plate Tectonics ________________ 4. It is a feature in the ocean seafloor that serves as a passageway for molten materials coming from beneath the surface. ________________ 5. It is a depression in the ocean floor caused by plates moving towards each other. ________________ 6. It is the longest underground mountain chain. ________________ 7. It is a mineral that can imprint magnetic polarity during its formation. ________________ 8. It is a kind of striping pattern found in rocks formed that is aligned at the ridge. ________________ 9. These are the two plate movements found along the mid-oceanic ridge. ________________ 10. It is the kind of earthquake occurring along the mid-oceanic ridge. ________________ 11. It is the study of magnetism and ancient rocks. ________________ 12. It is the kind of lava that is ejected along the ridge. B. On the space provided before each number, write true if the statement is correct. Otherwise, change the underlined word or phrases in order to make the statement correct. ________________ 1. Seafloor spreading is the process that occurs at the ridge, where new oceanic crust is formed. ________________ 2. Alfred Dietz is an oceanographer who helped in mapping 13  Earth Science Grade 11/12 Unit 13: Plate Tectonics the ocean floor. The map further supports the seafloor spreading theory. ________________ 3. Harry Hess is the scientist who proposed the theory of seafloor spreading. ________________ 4. Silicate is the mineral which could imprint magnetic polarity during its formation. ________________ 5. Mid-Atlantic ridge is the longest geographical land formation covering 20% of Earth’s surface. ________________ 6. Scientists observed that the thickness of the oceanic crust changes depending on its location wherein it is thickest away from the ridge. ________________ 7. Paleoclimatology is one of the pieces of evidence that supported seafloor spreading hypothesis. ________________ 8. Abyssal plain is the flat area in the ocean floor. 14  Earth Science Grade 11/12 Unit 13: Plate Tectonics Photo Credits Introduction. expl0036 by NOAA Photo Library is licensed under CC BY 2.0 via Flickr. Fig. 6. expl2366 by NOAA Photo Library is licensed under CC BY 2.0 via Flickr. Bibliography Carlson, Diane H. and Charles C. Plummer. 2009. Physical Geology: Earth Revealed, 8th Edition. New York: McGraw-Hill Companies, Inc. Coenraads, Robert R. 2005. Rocks & Fossils: A Visual Guide. Australia: Weldon Owen Inc. Hamblin, W. Kenneth and Eric H. Christiansen. 2003. Earth’s Dynamic Systems, 10th Edition.New Jersey: Pearson Prentice Hall Monroe, James S. and Reed Wicander.2012. The Changing Earth: Exploring Geology and Evolution, 6th Edition, International Edition. United States: Brooks Cole Publishing Stephen J. Reynolds, et al. 2013. Exploring Geology, 3rd Edition. United States: McGraw-Hill Companies, Inc. Tarbuck, Edward J. and Frederick J. Lutgens. 2015. Earth Science. New Jersey: Pearson Prentice Hall. 15