2. Chapter 7: Plate Tectonics Table of Contents Section 2: Seafloor SpreadingSeafloor Spreading

3. A. Mapping the Ocean Floor 1. If you were to lower a rope from a boat until it reached the seafloor, you could record the depth of the ocean at that particular point. 2. This is exactly how it was done until German scientists discovered how to use sound waves to locate submarines during World War I. Seafloor Spreading

4. A. Mapping the Ocean Floor 3. Sound waves echo off the ocean bottom—the longer the sound waves take to return to the ship, the deeper the water is. 4. Using sound waves, researchers discovered an underwater system of ridges, or mountains, and valleys like those found on the continents. Seafloor Spreading

5. A. Mapping the Ocean Floor 5. In some of these ridges are long rift valleys. These rift valleys are like rips in the ocean floor. Volcanic eruptions and earthquakes occur in the rift valleys from time to time. 6. Underwater volcanic eruptions create underwater mountains. When these mountains push out of the water, they create island. 7. In the Atlantic, the Pacific, and in other oceans around the world, a system of ridges, called the mid-ocean ridges, is present. Seafloor Spreading

6. B The Seafloor Moves 1. In the early 1960s, Princeton University scientist Harry Hess suggested an explanation on the ocean floor movement. His now- famous theory is known as seafloor spreading. Seafloor Spreading 2. Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.

7. B. The Seafloor Moves 3. As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created. 4. The older seafloor moves away from the ridge in opposite directions. Seafloor Spreading

8. C. Evidence for Spreading 1. In 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor. 2. Scientists found that the youngest rocks are located at the mid-ocean ridges. The ages of rocks become increasingly older in samples obtained farther from the ridges, adding to the evidence for seafloor spreading. Seafloor Spreading

9. C. Evidence for Spreading Seafloor Spreading 3. As molten material is forced upward along the ridges, it brings heat and chemicals that support exotic life-forms in deep, ocean water. 4. The giant clams, mussels, and tube worms, get heat and chemicals from magma pouring out of rifts in mid-ocean ridges.

10. D. Magnetic Clues 1.Earth’s magnetic field has a north pole and south pole. 2.Invisible lines of magnetic force leave Earth near the south pole and enter the north pole. At this time, Earth’s magnetic field travels from south to north. 3.However, the lines of magnetic force traveled in the opposite direction sometimes. Seafloor Spreading

11. D. Magnetic Clues 4.These direction changes are called magnetic reversals. During a magnetic reversal, the lines of magnetic force run the opposite way. 5. Scientists have determined that Earth’s magnetic field has reversed itself many times in the past. Seafloor Spreading 6.The reversals are recorded in rocks forming along mid-ocean ridges.

12. E. Magnetic Time Scale 1. Whenever Earth’s magnetic field reverses, newly forming iron minerals will record the magnetic reversal. 2. Using a sensing device called a magnetometer (mag nuh TAH muh tur) to detect magnetic fields, scientists found that rocks on the ocean floor show many periods of magnetic reversal. Seafloor Spreading

13. E. Magnetic Time Scale 3. A strong magnetic reading is recorded when the polarity of a rock is the same as the polarity of Earth’s magnetic field. 4. Look at the picture. Normal polarities in rocks show up as large peaks. After the magnetic reversal, the magnetometer records a weak reading. Seafloor Spreading

14. Magnetic Time Scale Seafloor Spreading 5. Over time, the reversals are shown in strips parallel to mid-ocean ridges. Changes in Earth’s magnetic field can be seen on both sides of mid- ocean ridge. 6.This discovery provided strong support that seafloor spreading was indeed occurring. 7. This helped explain how the crust could move—something that the continental drift hypothesis could not do.

15. 2 2 Section Check Question 1 What is seafloor spreading?

16. 2 2 Section Check Question 2 What method of mapping the ocean floor was developed by scientists in the 1940s and 1950s?

17. 2 2 Section Check Question 3 What was the significance of the discovery that the magnetic alignment in rocks on the ocean floor reverses back and forth in strips parallel to mid-ocean ridges?

18. 2 2 Section Check Q1. Answer Seafloor spreading is the process in which hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. This material flows sideways and carries the seafloor away from the ridge in both directions.

19. 2 2 Section Check Q 2. Answer In the 1940s and 1950s, scientists began to use sound waves echoing off the ocean bottom to map large areas of the ocean floor in detail.

20. 2 2 Section Check Q3. Answer The magnetic reversals showed that new rock was being formed at the mid-ocean ridges, and helped explain how the crust could move.

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