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OS 1 The Oceans Fall 2007 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 Divergent Transform N/A Name of Tectonic Process N/A Volcanoes Present? (Y or N) Earthquakes Present? (Y or N) Plate tectonics- 1 Convergent OS 1 The Oceans Fall 2007 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 OS 1 The Oceans Fall 2007 Note: this line should be 1 inch wide, exactly Plate tectonics- 3 OS 1 The Oceans Fall 2007 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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Distance from Rift Valley (km) 5000 to west 3500 to west 2750 to west 1500 to west 750 to west 400 to west 100 to west 100 to east 400 to east 750 to east 1500 to east 2750 to east 3500 to east 5000 to east Water Depth (m) 5250 4800 4500 4000 3500 3000 2800 2800 3000 3500 4000 4500 4800 5250 Rift Valley (located at 0 km) West (km) East (km) 5000 4000 3000 2000 1000 0 1000 2000 3000 4000 5000 0 m ---------------------------------------------------------------------------------------1000 m ----------------------------------------------------------------------------------2000 m ----------------------------------------------------------------------------------3000 m ----------------------------------------------------------------------------------4000 m ----------------------------------------------------------------------------------5000 m ----------------------------------------------------------------------------------6000 m ----------------------------------------------------------------------------------2. 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 OS 1 The Oceans Fall 2007 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: T½ is the same as the square root of T) Station 1 2 3 4 5 6 7 5500 5000 4500 4000 3500 3000 2500 2000 Water Depth (m) 2800 3000 3500 4000 4500 4800 5000 T (my) 2 4 10 20 32 40 42 T1/2 (You calculate missing values) 1.41 2.00 m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m-----------------------------------------------------------------------------------m____________________________________________________________ 0 1 2 3 4 5 6 7 8 9 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 = 2500 + 350 T1/2, 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 OS 1 The Oceans Fall 2007 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 Maui Molokai Oahu Kauai 0 m.y. 1.1 m.y 1.6 m.y. 2.5 m.y. 4.0 m.y. 0 km 140 km 220 km 300 km 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 OS 1 The Oceans Fall 2007 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