OS 1 The Oceans Fall 2007

Transcription

1 Name: Plate Tectonics (you must turn Activities 1-2 in during Section) Activity 1: Plate Boundaries A plate boundary simply describes where two plates (either oceanic or continental material) meet. There are only three types of plate boundaries Divergent (moving away from each other), Convergent (moving towards each other), and Transform (sliding past each other). 1. Cut out the triangle below, but keep the remaining sheet of paper in one piece. a. Put the triangle back into the hole and move it in the direction of the arrow on plate A. b. On your paper, label the three sides of the triangle with the types of plate boundaries (convergent, divergent, transform) created by the interaction of plate A with plates B, C, and D. Plate C Plate D 2. Complete the following table to describe the characteristics of plate boundaries: Name of Seafloor Feature Name of Tectonic Process Divergent Convergent Transform N/A N/A Volcanoes Present? (Y or N) Earthquakes Present? (Y or N) Plate tectonics- 1

2 Activity 2: Seafloor Spreading Use the map on page 3 for the following questions and be sure to show your work. You will need a ruler and a calculator. If the line (upper left) is not 1 inch, you should reprint your handout with scaling turned off. Given that the distance between Points A and B is 4550 km, you should first calculate the scale of this map. [For example, you could calculate how many miles are represented by one centimeter.] 1. What is the map scale? Select the third stripe to the east of the MAR. Record its age below and carefully measure the distance it has moved from the mid-ocean ridge where it formed. 2. Sea floor age at selected stripe: million years (My) 3. Distance to the Mid-Atlantic Ridge (MAR) at selected stripe (in kilometers): Using the age of the rock you have chosen and its distance from the MAR, calculate the half-rate of sea floor spreading, the velocity at which one strip of this rock has spread away from the MAR. 4. (distance/time) = km per My Now calculate the total rate (velocity) that the two plates are moving away from each other. This is considered, the rate of sea floor spreading. 5. (2 X half-rate = total spreading rate): km per My Given that the total present day distance between North America and Africa (between points A and B) is 4550 km, calculate the age of the North Atlantic Ocean. 6. Age of North Atlantic Ocean: My 7. Convert the total sea floor spreading rate from #5 above to units that are easier to "imagine" such as inches per year. 8. How much has the distance (in inches) between North America and Africa increased since you were born (show your work)? 9. How much closer (in feet) were these two continents when Columbus made his voyages in 1492? Columbus thought the Earth was 25% smaller than it is, because he used the wrong map. In percent, how much smaller was the Atlantic in 1492 compared to today? Plate tectonics- 2

3 Note: this line should be 1 inch wide, exactly Plate tectonics- 3

4 Activity 3: Seafloor Profiles (you can do this at home) In this activity we will think about how water depth of the ocean floor changes as it ages and moves away from the mid-ocean ridge (rift valley). 1. Use the information in the table below to draw a simple profile of the sea floor across a mid-ocean ridge on the graph below the table. First plot the points below and then connect the dots with a continuous line. Station Distance from Rift Valley (km) Water Depth (m) to west to west to west to west to west to west to west to east to east to east to east to east to east to east 5250 Rift Valley (located at 0 km) West (km) East (km) m m m m m m m Explain why the water depth increases with distance from the rift valley of the mid-ocean ridge as shown in your profile above. Plate tectonics- 4

5 Activity 4: Ocean Depth and Age of Crust We have seen that as ocean crust moves away from mid-ocean ridge it gets older and deeper. We now can derive a relationship between water depth and age. 1. Make a plot of the Water Depth (D) vs. T 1/2 (where T = age of oceanic crust) on the graph below. First complete the table below, then plot the data below the table. (Note: T ½ is the same as the square root of T) Station Water Depth (m) T (my) T 1/2 (You calculate missing values) m m m m m m m m T 1/2 (Million Years) Draw a straight line that connects the points on your graph above 2. How does the water depth of the sea floor change with increasing age of the oceanic crust and lithosphere? Based on the above plot, a relationship can be shown between the water depth to the top of the oceanic crust (sea floor in this case) and the age of the crust at that location. This relationship is: D = T 1/2, where D = depth in meters; T = age in million years 3. Predict the water depth of 7 million year-old oceanic crust. 4. Based on the age you calculated for the North Atlantic in Activity 2, how deep is the oldest part of the North Atlantic basin? 5. Most of the Pacific Ocean basin is over 5000 meters deep. Estimate the age of oceanic crust with a water depth of 5000 meters. Plate tectonics- 5

6 Activity 5: Rate of plate movement Another way to calculate plate movement is by looking at island chains that are formed from hot spots. Dating of rocks from the Hawaiian Islands shows that the ages of the volcanoes are progressively older to the northwest (upper left). It appears that the active volcanoes on the "big island" of Hawaii overlie a very hot region in the Earth's mantle. This hot region located deep with the earth, is known as a hot spot. So as the Pacific plate moves slowly over the stationary hot spot in the mantle, molten rock (called magma) rises to pierce a hole in the lithospheric plate. A series of burn marks are left in the plate -- these scars are the volcanic islands and underwater volcanoes that lie on the top of the plate -- thereby creating a chain of seamounts and oceanic islands. Ages of the volcanoes on the Hawaiian Islands. Island Age of Volcanoes m.y.=million years Distance From Center of Hawaii (Big Island) km = kilometers Hawaii 0 m.y. 0 km Maui 1.1 m.y 140 km Molokai 1.6 m.y. 220 km Oahu 2.5 m.y. 300 km Kauai 4.0 m.y. 460 km 1. Draw an arrow on the following map to show the direction of the absolute motion of the Pacific plate over the past 5 million years. Plate tectonics- 6

7 2. Calculate the rate of absolute motion of the Pacific plate based on the data shown above by first determining the rate of motion for each island and then taking the average of these values. Don't forget to specify units. 3. Which ocean basin is experiencing the faster rate of spreading, the Atlantic (Activity 2) or the Pacific (Activity 5)? Plate tectonics- 7