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Seafloor Spreading (B) The History of Planet Earth Seafloor Spreading In the early 20th century, Alfred Wegener developed his theory of continental drift, explaining that the continents on Earth move and historically were in different positions from their current locations today. He used evidence from fossil findings, geologic formations, and coastal outlines of the continents to support his theory. In the 1960s, scientific discoveries about seafloor spreading, combined with the theory of continental drift, led to the theory of plate tectonics. In 1962, Harry Hess and a naval team were mapping the Atlantic ocean floor for the United States Navy when they discovered the Mid-Atlantic Ridge (MAR). His discovery of the newly formed ocean floor was the evidence Wegener was searching for in 1912. Hess explained that the MAR is the result of divergent tectonic plates, which explains how the ocean floor is renewed and why continental plates appear to be younger than oceanic plates. Refer to the map, “Spreading at Mid-Ocean Ridge Between African and North America Plates,” on Page 3 to answer questions 1 and 2. 1. What is the map scale? 2. Using the map scale, calculate the distance between the following locations on the map. Distance in kilometers = Distance in cm x Map scale in cm/km Locations Distance Point A and Point B Point B and Point C Point A and Point C North America and Africa (measure the distance between dots) The map highlights the mid-ocean ridge between the North American Plate and the African Plate. As the North American and African plates move away from each other, magma rises up between the two plates. New oceanic crust forms when the magma cools and the plates continue to move apart. The lines on the map represent time in a unit of millions of years. The newest crust is found at 0, the mid-ocean ridge, and the crust gets older as you move out on either side of the ridge. By selecting a point on the crust, you can use its age and its distance from the mid-ocean ridge to calculate the average speed at which the plate is moving away from the mid-ocean ridge. Use the following formula to calculate this plate speed: Average Plate Speed in cm/yr = distance in cm time in years 1 Seafloor Spreading (B) The History of Planet Earth Use the information from the previous page to answer questions 3–6. 3. Calculate the average speed (cm/yr) of the North American plate for the last 156 million years. For the total distance, measure from Point A to Point B. (Remember: 1 km = 100,000 cm) 4. Calculate the average speed (cm/yr) of the African plate for the last 156 million years. For the total distance, measure from Point B to Point C. 5. Calculate the average spreading rate between the North American and African plates using the formula: Average Spreading Rate (cm/yr) = total distance (cm) / total time (yr) For the total distance, measure from point A to point C. 6. Use the average spreading rate you calculated in question 5 to calculate the amount of spreading that has occurred between North America and Africa during your lifetime. 7. If the average spreading rate stays the same, how much will the distance between Africa and North America increase by the year 3025? 8. Calculate the average spreading rate between the two plates during the last 20 million years. 9. Calculate the average spreading rate between the two plates during the last 35 million years. Does the average spreading rate stay constant over time? 2 North America A 118 84 B 55 35 20 Mid-Ocean Ridge North American Plate 156 55 84 African Plate 0 20 35 C 156 Map Scale 1cm = 500 km 118 Age of Crust in Millions of Years Spreading at Mid-Ocean Ridge between African and North American Plates Afric a Seafloor Spreading (B) The History of Planet Earth 3