Name ________________________ Investigations Into Earth Science Seafloor Magnetism Overview Two areas of the ocean that were first explored were the Mid-Atlantic Ridge in the North Atlantic Ocean near Iceland. Another area was the East Pacific Rise west of Chile. For this activity, you will simulate the discoveries by Vine, Matthews, and Morley by exploring the magnetism of the seafloor and relating your findings to the theory of plate tectonics. Data Preparation You will be simulating the collection of data from two locations. Location 1 – The Reykjanes Ridge off of Iceland. (Part of the Mid-Atlantic Ridge.) Location 2 – The East Pacific Rise west of Chile. (Another mid-ocean ridge system.) Cross sections are provided for both locations along with graphs that you will use to plot the data you collect. Take a moment to observe both cross sections. The heavy line represents the depth to the sea floor for a path across the mid-ocean ridge. The area below the line represents the rocks that make up the sea floor, and the space above the line represents the ocean water. Note that the vertical and horizontal scales are different. Before collecting data, answer the following questions regarding the cross sections. 1) You are probably more familiar with feet and miles than meters and kilometers. Table 1 lists the horizontal and vertical distances shown on the cross sections. Given that 1.0 kilometer = 0.62 miles, convert each distance into the other three units. (That is, convert kilometers into miles, meters, and feet.) Table 1 Kilometers Miles Meters Feet Reykjanes Ridge Horizontal Distance 300 km Vertical Distance 2250 m East Pacific Rise Horizontal Distance 900 km Vertical Distance 4250 m Show your work for the three values for the top row. Investigations Into Earth Science Spring 2013 EARTHSCI 2500 2) Note that the vertical and scale is provided for each cross section. Why do you think that they are different? 3) Note that the vertical exaggeration is approximately the horizontal scale divided by the vertical scale. In general, non-mathematical terms, how would you define vertical exaggeration? What do you think the profile would look like if the vertical and horizontal scales were the same (i.e. no vertical exaggeration)? Now answer some questions for each location. Location 1 – Reykjanes Ridge (North Atlantic Ocean near Iceland) 4) Describe the general shape of the sea floor along this profile? 5) The deepest location on the profile is 2100 m below sea level and the highest peak is 770 m below sea level. If you could walk between these two points, how much elevation would you gain? What would be the approximate slope of your hike in meters per kilometer? Investigations Into Earth Science Spring 2013 EARTHSCI 2500 6) Using the vertical and horizontal scales, complete Table 2 by calculating the real-world or profile distances for the given measurements. (You do not need to calculate any values for the areas shaded gray.) Vertical Scale Horizontal Scale In The Real World Table 2 On Profile 1.0 cm 6 mm mm m 400 (elevation of Latham hall above sea level) km m 140 cm (Distance from UNI to Iowa Legislature in Des Moines) km 7) Show your work for the first or second row. Location 2 – East Pacific Rise 8) Describe in general terms the profile for the East Pacific Rise. 9) Compare and contrast this profile to the Reykjanes Ridge profile. 10) How do the horizontal and vertical scales at this location compare to Location #1? At which location is the topography “stretched” the most? How do you know that? Investigations Into Earth Science Spring 2013 EARTHSCI 2500 Data Collection North = 0° or 360° Working in pairs, use a compass to collect NW = 315° NE = 45° magnetic data along both the Mid-Atlantic Ridge and the East Pacific Rise using the technique demonstrated by your instructor. East = 90° West = 270° For each compass reading, use the meter sticks to measure the distance in centimeters from the beginning of the profile. Later, you SE = 135° SW = 225° will convert these distances into kilometers so you can plot them on the given sea floor South = 180° profile. Magnetism will be recorded as the Figure 1 – Azimuth degrees for different directions azimuth direction in degrees (0° - 360°) indicated by the north arrow on the compass. (See Figure 2 for the azimuth degrees associated with each cardinal Table 3 direction.) Record the distance and magnetism data in your East Pacific Rise Distance notebook, and complete the following follow-up activities and Magnetism cm km questions. Table 3 shows the first five entries for an example data 0 210 table. 11) In an actual survey, data would be continuously recorded. In our simulation we will collect data at discrete intervals. Describe the sampling plan that the class decided to use to collect the data. 10 20 23 30 40 42 50 60 70 80 180 110 90 62 14 0 332 270 242 200 12) Before you begin collecting data, what patterns or trends do you think you will find? Why do you think this will happen? Investigations Into Earth Science Spring 2013 EARTHSCI 2500 Data Processing and Display Now that you have your data from the two ridges, you can use your data to draw some inferences regarding plate tectonics. But first, describe any trends or tentative conclusions that you saw while collecting the data. (Earth scientists often note patterns, trends that they see while collecting their data or hunches on what the data will show. These notes often lead to new insights.) Your data are in tabular form, but the human mind is better at finding patterns in visual data such as graphs, maps, and figures, so you will need to convert your data in a form that is easier for you to analyze. Step 1 – Convert your measured distances (in cm) to scaled distances along the sea floor (km) by using the following conversion factors. Reykjanes Ridge: 1cm = 1 km East Pacific Rise: 1 cm = 3 km Step 2 – For each location, plot your magnetism data on the graph provided. For each graph, the horizontal axis is distance along the survey and the vertical axis is the direction the compass needle was pointing. Figure 2 shows an example graph for the sample data in Table 3. Step 3 – For each location, use your graph to decide which locations have a north (normal) polarity and which areas have a south (reversed) polarity. Use color to denote the two polarities on your graphs. (North) 0° 360° (North) 45 (East) 90°° 315 ° 270° (West) 225 ° 135° (South) 180° 180° (South) 0 km 20 km 40 km 60 km 80 km Step 4 – Once you have decided which parts Figure 2 – Example graph showing sample magnetism data. of your survey had normal and reversed polarities, transfer than information to the sea floor profiles by using color to denote areas of the sea floor that have normal and reversed polarity. Investigations Into Earth Science Spring 2013 EARTHSCI 2500 Data Analysis and Conclusions Now you can use your data to draw some conclusions. 13) In general terms, describe any patterns or trends that you see in your graphs. 14) What inferences can you make regarding the sea floor topography and the sea floor magnetic polarity? Be sure to include the evidence you have to support each claim. 15) For both locations, calculate the distance from the center of the mid-ocean ridge to the first reversal start of the first area with reversed polarity. Radiometric dating of lavas on land show that this reversal happened 780,000 years ago. Over that time period, what was the spreading rate for each ridge? Investigations Into Earth Science Spring 2013 EARTHSCI 2500 Extension Questions 16) Given what you know about the Earth, explain how these data support the claims of plate tectonics. 17) Many people assume that scientists always conduct controlled experiments where they investigate the changes in just one variable at a time and keep all other variables constant, yet we have talked in class about ways in which Earth scientists use other methodologies to gather evidence. Based on your experience from this activity as well as the reading on Vine, Matthews, and Morely, discuss how this investigation was scientific. Also discuss ways in which it deviated from the traditional, linear scientific method. Investigations Into Earth Science Spring 2013 EARTHSCI 2500 Reykjanes Ridge Near Iceland Seafloor Magnetic Polarity (North) 0° 360° (North) 315° 45° 270° (West) (East) 90° 135° 225° 180° (South) (South) 180° 0 km 20 km 40 km 60 km 80 km 100 km 120 km 140 km 160 km 180 km 200 km 220 km 240 km 260 km 280 km 300 km Horizontal Scale = 1:1,275,000 Seafloor Profile NW SE 0m 250 m 500 m 750 m 1000 m 1250 m 1500 m 1750 m 2000 m 2250 m 0 km 20 km Profile provided by Google Earth (www.earth.google.com) 40 km 60 km 80 km 100 km 120 km Horizontal Scale = 1:1,275,000 140 km 160 km 180 km Vertical Scale = 1:31,500 200 km 220 km 240 km Vertical Exaggeration = 40 260 km 280 km 300 km East Pacific Rise Off Chile Seafloor Magnetic Polarity (North) 0° 360° (North) 45° 315° (East) 90° 270° (West) 135° 225° 180° (South) (South) 180° 0 km 100 km 200 km 300 km 400 km Horizontal Scale = 1:3,850,000 500 km Horizontal Scale = 1:31,500 600 km 700 km 800 km Vertical Exaggeration = 123 Seafloor Magnetic Profile NW 900 km SE 2000 m 2250 m 2500 m 2750 m 3000 m 3250 m 3500 m 3750m 4000 m 4250 m 0 km 100 km Profile provided by Google Earth (www.earth.google.com) 200 km 300 km Horizontal Scale = 1:3,850,000 400 km 500 km Vertical Scale = 1:31,500 600 km 700 km Vertical Exaggeration = 123 800 km 900 km