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Name Date Class Lesson Outline LESSON 2 Development of a Theory A. Mapping the Ocean Floor 1. Scientists mapped the depth of the ocean floor using a device called a(n) . 2. In the middle of the oceans are large mountain ranges called . a. Existence of these was confirmed through research called echo-sounder research. b. These underwater mountain ranges are much than mountain ranges on land. B. Seafloor Spreading 1. occurs when new oceanic crust forms at a mid-ocean ridge and old crust moves away from the ridge. a. Molten rock, or , rises from the mantle through cracks in the crust. It erupts as from volcanic vents along the mid-ocean ridge. b. The molten rock cools and becomes , the rock that forms the oceanic crust. c. New oceanic crust forms along a mid-ocean ridge, and crust moves away from the ridge. 2. The topography of the includes the abyssal plain and rugged mountains. a. The rugged mountains that make up the mid-ocean ridge can form in different ways. One way is through large amounts of erupting from the center of the ridge, cooling, and building up around the ridge. Another way is through upward-moving pushing on the crust above it, causing it to crack and form jagged, angular mountains on the seafloor. b. Eventually, making smooth seafloor called the forms on top of the oldest oceanic crust, . 3. Seafloor spreading helps explain continental drift because it shows that continents move with the oceanic as it spreads away from mid- ocean ridges. Plate Tectonics 27 Name Date Class Lesson Outline continued C. Development of a Theory 1. Evidence to support seafloor spreading first came from studying the of rocks on the seafloor. 2. Earth’s outer core causes Earth’s . a. The direction of Earth’s magnetic field often. b. When a magnetic field causes a magnet to point north, the magnetic field has . c. A magnetic field reverses direction during a(n) . d. After a magnetic reversal, a magnet points south because Earth’s magnetic field has 3. . form when iron-rich minerals in cooling lava align with the direction of Earth’s magnetic field. a. The direction of a magnetic field in minerals can be determined by using a device called a(n) b. Magnetometers show . magnetic stripes on either side of a mid-ocean ridge. c. These stripes alternate normal polarity and , showing that each stripe was formed at the mid-ocean ridge and then moved away. 4. Sediment collected from the seafloor show that sediment farther away from a mid-ocean ridge is than the sediment that is closer to the ridge. 28 Plate Tecton Name Date Class Content Practice A LESSON 2 Development of a Theory Directions: Complete this concept map by choosing terms from the word bank and writing them in the correct spaces. continental drift mid-ocean ridge rocks continents move new oceanic crust sediment magnetic signatures older oceanic crust temperature Seafloor Spreading is the process by which 1. is a way to explain how 4. forms along a(n) is evidenced by the 6. supporting the 2. of minerals in 7. 5. and and by measurements of seafloor hypothesis. 3. 8. moves away from it. and 9. Name Date Class Content Practice B LESSON 2 Development of a Theory Directions: On each line, write the term that correctly completes each sentence. 1. are mountain ranges in the oceans, where the crust is young and there is almost no sediment. 2. draws dense rock away from mid-ocean ridges. 3. polarity means magnets orient themselves to point north, whereas 4. As crust polarity refers to magnets pointing south. , it records the direction and orientation of Earth’s magnetic field. 5. The movement of the ocean’s crust is confirmed by Earth’s magnetic ____. 6. Magnetic data from the ocean crust support seafloor 7. . move with the ocean crust as the seafloor spreads. 8. What type of boundary is indicated by arrows B? ___________________________ 9. What does letter A represent at this boundary?______________________________ 10. What landform is represented by letter C? _________________________________ 11. What type of boundary does arrows B and E form? __________________________ 12. What land form is present at D? _________________________________________ 13. What process is occurring as indicated by letter F? __________________________ 14. What landforms are likely to occur at these kinds of zones? ___________________ Name Date School to Home Class LESSON 2 Development of a Theory For this activity, you will model Henry Hammond Hess’s theory of seafloor spreading. You will need a sheet of paper, scissors, crayons, and tape. 1. Place the sheet of paper on a flat surface so it is positioned vertically. Cut two strips off the edge of the paper, each 1 inch wide and about 8 inches long. Color the strips so each has four 2-inch × 1-inch sections in the following colors: red, yellow, blue, green. 2. Place the two strips with the matching colors facing each other. Tape the bottom of the green ends together. 3. In the remaining portion of the sheet of paper, cut three 1½-inch slits that are 2 inches apart from one another. 4. Slide the red ends of both strips up through the middle slit. Then slide one red strip down through the slit on the left and the other strip down through the slit on the right. Adjust the strips so the same colors show on both strips. 5. Pull the red ends of both strips. The same color should appear at the same time and disappear at the same time. The strips of paper represent the old ocean crust moving away from the mid-ocean ridge. What is happening at the center slit? 6. What does the change in colors on the strips of paper represent? 32 Plate Tectonics Name Date Key Concept Builder Class LESSON 2 Development of a Theory Key Concept What is seafloor spreading? Directions: Answer each question or respond to each statement on the lines provided. 1. What happens to magma that rises through cracks on the seafloor? 2. What structures are formed by rapidly cooling lava in the ocean? 3. Where is the new rock located? 4. What happens to the rock as the seafloor spreads? 5. Use your answers to help you explain seafloor spreading. Plate Tectonics 33 Name Date Key Concept Builder Class LESSON 2 Development of a Theory Key Concept What is seafloor spreading? Directions: Answer each question on the lines provided. What is known about seafloor spreading 1. What forms basalt? How this supports continental drift 3. What happens to old oceanic crust as new oceanic crust forms? 2. Which type of rock forms oceanic crust? 4. What happens to the density of rock as it cools? 5. Where does the crust move as it becomes denser? 6. Which force draws the cooler, denser crust downward and away from the mid-ocean ridge? 7. What is formed when lava cools and crystallizes on top of the oceanic crust? 9. What shape does the seafloor take where the sediment is the thickest? 8. Where is seafloor sediment thickest? 10. What is this area of the seafloor called? 11. What is the ocean crust always doing? 12. What happens as the ocean crust spreads? 13. What does the crust record as it cools? 15. What forms when Earth’s magnetic field changes direction? 16. What do magnetic stripes confirm? 14. What does basalt contain that makes this possible? 34 Plate Tectonics Name Date Key Concept Builder Class LESSON 2 Development of a Theory Key Concept What evidence is used to support seafloor spreading? Directions: Write the word or phrase that correctly completes each sentence on the lines provided. What has echo-sounder technology shown about the topography of the seafloor? 1. The topography is not . 2. Mountain ranges stretch . 3. Mountain ranges are located . 4. Ocean mountain ranges are longer . What has the magnetometer shown about the magnetic signature of the seafloor? 5. Parallel magnetic stripes are located . 6. Each stripe has . 7. Magnetic stripes represent . What have temperature measurements beneath the seafloor surface revealed? 8. More thermal energy leaves . 9. Less thermal energy leaves . What has dating of sediment that was taken from just above the ocean crust revealed? 10. The sediment that is closest to the mid-ocean ridge is the sediment that is farther away from the ridge. Plate Tectonics than 35 Name Date Key Concept Builder Class LESSON 2 Development of a Theory Key Concept What evidence is used to support seafloor spreading? Directions: Answer each question on the lines provided. Use complete sentences. 1. What does normal polarity mean? 2. What does reversed polarity mean? 3. How can scientists tell when changes occur in Earth’s magnetic field? 4. How often do magnetic reversals occur? 5. What do Earth’s magnetic stripes indicate about ocean crust? 6. How do Earth’s magnetic signatures support the idea that continents move? 36 Plate Tectonics Name Date Enrichment Class LESSON 2 Hot Springs on the Ocean Floor As part of the developing theory of plate tectonics, scientists predicted that hot springs should exist near locations of active seafloor spreading. Using a submersible craft, scientists discovered the first hydrothermal vent on the ocean floor in 1977. Two types of hydrothermal vents have been discovered. Where the seafloor crust is spreading apart due to plate tectonics, seawater sinks deep into cracks in the ocean floor. In one type of vent, the ocean water is superheated by magma and hot rock from Earth’s newly formed crust. The superheated acidic water spews out, carrying minerals from Earth’s crust. Thick deposits of minerals that are rich in copper, iron, manganese, sulfur, and zinc can form. Chimneys When the hot water (380°C or higher) mixes with the cold seawater (2°C), it cools quickly. The minerals in the water settle and gradually form chimney-shaped vent openings. Because the chimneys spew hot, mineral-rich water, they can look like smoke stacks. Another type of hydrothermal vent occurs when seawater reacts chemically with the mantle beneath the ocean crust. This process is not volcanic, and its heat comes from chemical reactions. The water that rises from the seafloor at this type of vent is cool compared to water at magma-heated spots and is chemically alkaline instead of acid. Calcium carbonate and magnesium precipitate out to form large, white towers. Hydrothermal vents are located in all the oceans, but most are along an area of active volcanoes known as mid-ocean ridges. They form where ocean plates have separated, allowing lava to flow and form new crust. Life in the Hot Springs Most scientists once thought that organisms could not survive at such great depths because of extreme pressure, absence of sunlight, and low temperatures. However, because of hot water warming the area around a vent and the rich chemical stew being ejected by the vent, life-forms are plentiful. Bacteria form the base of the food web around hydrothermal vents, and they support other organisms, such as clams and crabs. The base of food webs everywhere else on Earth includes photosynthetic organisms. However, the food web is different for the bacteria found around hydrothermal vents. These special bacteria can convert the toxic sulfur from the vents into energy. Hydrogen sulfide is the main chemical used by microorganisms around the vents to produce food from carbon compounds. This process of using chemicals rather than sunlight for energy is called chemosynthesis. Applying Critical-Thinking Skills Directions: Respond to each statement. 1. Explain why the discovery of the chemosynthetic bacteria was important to scientists’ understanding of this ecosystem. 2. Summarize the significance of hydrothermal vents existing only in geologically active areas on the ocean floor. Plate Tectonics 37 Name Challenge Date Class LESSON 2 Seascape Mapping The abyssal plain might be somewhat “plain,” but most of the ocean’s floor is busy with undersea landforms. Draw your vision of the ocean floor along a mid-ocean ridge. Include the following labels: crust, mid-ocean ridge, seafloor spreading, magma, youngest basalt, older basalt, oldest basalt, sea mount (undersea mountain), hydrothermal vent, abyssal plain, sediment. 38 Plate Tectonics Name Date Skill Practice Model Class LESSON 2: 30 minutes How do rocks on the seafloor vary with age from a mid-ocean ridge? Scientists discovered that ocean crust forms at a mid-ocean ridge and spreads away from the ridge slowly over time. This process is called seafloor spreading. The age of the seafloor is one component that supports this theory. Learn It Scientists use models to represent real-world science. By creating a small three-dimensional model of volcanic activity along the Mid-Atlantic Ridge, scientists can model the seafloor spreading process. They can then compare this process to the actual age of the seafloor. In this skill lab, you will investigate how the age of rocks on the seafloor changes with distance away from the ridge. Try It Do not eat anything used in this lab. 1. Read and complete a lab safety form. 2. Lay the sheet of waxed paper flat on the lab table. Using a plastic spoon, place two spoonfuls of vanilla yogurt in a straight line near the center of the waxed paper, leaving it lumpy and full. 3. Lay the two pieces of foam board over the yogurt, leaving a small opening in the middle. Push the foam boards together and down so the yogurt oozes up and over each of the foam boards. 4. Pull the foam boards apart and add a new row of two spoonfuls of berry yogurt down the middle. Lift the boards and place them partly over the new row. Push them together gently. Observe the outer edges of the new yogurt while you are moving the foam boards together. 5. Repeat step 4 with one more spoonful of vanilla yogurt. Then repeat again with one more spoonful of berry yogurt. Plate Tectonics 39 Name Date Class Skill Practice continued Apply It 6. Compare the map and the model. Where is the Mid-Atlantic Ridge on the map? Where is it represented in your model? 7. Which of your yogurt strips matches today on this map? 60 million years ago? 8. How do scientists determine the ages of different parts of the ocean floor? 9. Conclude What happened to the yogurt when you added more? 10. 40 Key Concept What happens to the material already on the ocean floor when magma erupts along a mid-ocean ridge? Plate Tectonics Name Date Class Lesson Quiz A LESSON 2 Development of a Theory Matching Directions: On the line before each definition, write the letter of the term that matches it correctly. Each term is used only once. 1. Magnets point north when Earth’s magnetic field has this. A. magnetic reversal B. mid-ocean ridge 2. process that forms new oceanic crust and moves it C. normal polarity 3. takes place when Earth’s magnetic field changes D. reversed polarity 4. Magnets point south when Earth’s magnetic field E. seafloor spreading has this. 5. place where new oceanic crust forms True or False Directions: On the line before each statement, write T if the statement is true or F if the statement is false. 6. Seafloor spreading takes place along a mid-ocean ridge. 7. Oceanic crust is older when it is closer to a mid-ocean ridge than it is when it is farther from the ridge. 8. The high thermal-energy flow at a mid-ocean ridge comes from seawater. 9. Magnetic bands on the ocean floor are evidence that Earth’s magnetic field changes. 10. Knowledge of seafloor spreading could have helped Alfred Wegener gain more support for his hypothesis of continental drift. Plate Tectonics 41 Name Date Class Lesson Quiz B LESSON 2 Development of a Theory Completion Directions: On each line, write the term that correctly completes each sentence. 1. A(n) is a mountain range on the ocean floor where new crust forms. 2. When Earth’s magnetic field has normal polarity, a compass needle points 3. . contributes to the high thermal-energy-flow readings near the center of a mid-ocean ridge. are changes in the orientation of Earth’s magnetic field. 4. 5. The abyssal plain is flat due to an accumulation of far from the ridge. 6. When Earth’s magnetic field has reversed polarity, a compass needle points . on the ocean floor are evidence that Earth’s magnetic field 7. changes. Short Answer Directions: Respond to each statement on the lines provided. 8. List the steps involved in seafloor spreading in sequential order. 9. Explain how knowledge of seafloor spreading could have been useful to the acceptance of Alfred Wegener’s continental drift hypothesis. 42 Plate Tectonics