Lab activity 7 plate tectonics
- 1. Life on an Ocean Planet Lab 7 Chapter 13 – Lab/Activity 3 Riding the Ridge: Studies in Seafloor Spreading Florida Marine Science Standards SC.912.N.1.4: Identify sources of information and assess their reliability according to the strict standards of scientific investigation. SC.912.N.1.5: Describe and provide examples of how similar investigations conducted in many parts of the world result in the same outcome. SC.912.N.2.4: Explain that scientific knowledge is both durable and robust and open to change. Scientific knowledge can change because it is often examined and re-examined by new investigations and scientific argumentation. Because of these frequent examinations, scientific knowledge becomes stronger, leading to its durability. SC.912.E.6.3: Analyze the scientific theory of plate tectonics and identify related major processes and features as a result of moving plates. SC.912.E.6.5: Describe the geologic development of the present day oceans and identify commonly found features. Learning Outcomes: After completing this lab activity and studying Chapter 13, The Theory of Seafloor Spreading, you should be able to: 1. Describe how sediment thickness in relationship to the mid-ocean ridge can be used to support the concept of seafloor spreading. 2. Describe how seafloor depth away from the mid-ocean ridge can be used to support the concept of Seafloor Spreading. 3. Provide an explanation using both verbal and illustrative means of the processes occurring at the mid-ocean ridge. 4. Provide a theory as to how the Earth is able to change its magnetic field every few million years or so. 5. Explain how the magnetic reversals of the Earth leave evidence behind in a telltale signature of surface igneous rocks. 6. Describe how the magnetic reversal signatures or banding of ocean crust is used to support the theory that the seafloor is spread- ing at the ridge and then moving off to the sides. 7. Use the information provided by magnetic banding to estimate spreading rates of the mid-ocean ridges. Vocabulary: Epoch – refers to a time frame in which the earth has a stable magnetic field. Event – refers to the actual magnetic flip. Igneous rock – rock that forms from solidifying lava and has not undergone sediment or metamorphic affects. Igneous rock can occur on land or on the sea floor. Magnetic band – the fixing of the magnetite crystals in igneous rock Magnetic field – the field produced by movement of the inner and outer iron based cores of the Earth. Magnetite – iron composed crystals that are common in basaltic and other igneous rocks. Mid-ocean ridge – the 46,000-mile mountain chain that circumnavigates the globe and is the site of seafloor spreading and production of oceanic crust. Seafloor spreading – the name of the theory in which new seafloor is being produced at the mid-ocean ridge and then moved to either side. Thermal subsidence – the condensation of the oceanic crust as it cools resulting in the sinking into the Earth’s asthenosphere (upper mantle). Transect – a set of data that is acquired along a known path such as a cross station along the ocean floor. Introduction: When James Hutton introduced the world to the concept of a dynamic Earth in the 18th Century, he probably never envi- sioned just how dynamic science would theorize the Earth is. In Hutton’s time, the idea of an ancient Earth was controver- sial enough, much less that never-ending cycles produce and destroy mountains. To propose that continents move or that the crust is actually part of its own production and destruction cycle would have probably been called scientific heresy. The 150 years following James Hutton brought great changes in both thought and technology. This series of activities explores the unveiling of features on the ocean floor, age estimates, magnetism, and ocean depth. It includes an innova- tive way of viewing Earth from models of ocean crust production and destruction. Page 1 of 10
- 2. Life on an Ocean Planet Lab 7 2. Literature Review: It’s ironic in retrospect. At the same time Alfred Wegener was defending his continental drift theory, his own country- men on the German survey vessel Meteor were gathering the first seafloor data that would give the theory merit. From 1925-1927, the Meteor Expedition imaged the seafloor with sonar. These images showed a mountain ridge centered in the middle of Atlantic Ocean. Although this was previously known, sonar provided much better details of the ridge, show- ing it to be larger than previously thought. The new maps revealed a shape that mirrored the coastlines of continents ad- jacent to it. Had Wegener known, the details would have probably spurred him on. Wegener died before the information could be widely published, however. From the end of the World War II until the late 1950s, major advances in marine engineering, sonar, and oceanic drill- ing started to reveal a picture of the ocean floor that Wegener would have appreciated. Due to an ever-growing need for oil after the war, ocean-drilling technology became more common. Oceanic research took place on a wider scale funded by both industries and government. A picture of our ocean planet started to take shape that would revolutionize our con- cepts of how Earth works. During continued deep ocean exploration, scientists found the mid-ocean ridge to be much longer than originally thought. It is the largest geological feature, wrapping all the way around the Earth. In the Atlantic, the mid-ocean ridge mir- rors the adjacent continents. Radiometric data indicate that the ocean floor is much younger than scientists once thought. Scientists initially expected the ridge to be the same age as original crust. Instead, they found no oceanic crust that dates older than 200 million years, which is only about five percent of Earth’s estimated age. Furthermore, based on dating me- thods, scientists determined that the ocean’s crust gets older the farther you get from the ridge. Seismic data accumulated over many decades also indicated that the ridge was very active both seismically and vol- canically. To the scientists, this is another sign that dynamic events occur along the mid-ocean ridge. Magnetometers towed behind ships picked up natural magnetic properties of the ocean floor beneath them. They found banded patterns that border the mid-ocean ridge and mirror image, stripe for stripe, the contours of the mid-ocean ridge. It was as if the Earth’s magnetism flips every million to two million years and then flipped back. In the l960s Harry Hess, a petroleum geologist with Princeton University, and Robert Dietz, an oceanographer with the US Coast and Geodetic Survey of Scripps Institution of Oceanography, independently developed a theory to explain the patterns discovered on the seafloor. They proposed seafloor spreading, which says that the seafloor emerges at the mid-ocean ridges, and, like a giant conveyor belt, moves to the sides and goes back into the Earth at subduction zones. In this way, they theorized that the entire sea bed cycles in 200 million years. Their theory accounted for the patterns and observations made by scientists. At about the same time that Hess and Dietz were developing their theory of seafloor spreading, scientists began to study the unusual magnetic bands on the seafloor volcanic rock. They hypothesized that the Earth’s magnetic field flips every one to two million years. They theorized further that magnetite crystals – a form of iron – in lava align with the mag- netic field and stay aligned when the lava hardens into igneous rock. In 1963, British geologists Frederic Vine and Drummond Matthews combined information from the magnetic banding patterns on the seafloor, the hypothesis that the magnetic field flips and the theory of seafloor spreading. This produced an explanation for the magnetic banding patterns found on the seafloor. New seafloor emerges along the ridge, they said, solidifies with the magnetic field pattern of the Earth at the time, and gradually moves across the ocean bottom with the spreading. Each time the magnetic field flips, a new band forms in the seafloor pattern. This activity provides you with data similar to what scientists had available to them in the 1960s. With these data, you will explore the concepts that support the model of a dynamic Earth – an Earth in which the seafloor emerges and spreads, moving tectonic plates and the continents with them. Hypothesis/Research Question: Based on the the literature review, write your hypothesis/prediction below (what evidence do we have that the plates are moving? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Page 2 of 10
- 3. Life on an Ocean Planet Lab 7 Materials: • Ocean data set of mid-ocean ridge and sediment thickness • Ocean data sets of Atlantic and Pacific mid-ocean ridge and radiometric dating data • Ocean profile of mid-ocean ridge and depth • Magnetic anomaly data for the seafloor • 6 pieces of graph paper per student Procedure Part 1 : Bathymetric and Radiometric Evidence of Seafloor Spreading 1. You will be using the data provided at the end of this lab to produce five interpretative graphs. 2. All graphs will be done in pencil and on the graph paper. 3. Make sure you label all ranges on both the y and x-axis. 4. Graph 1 will require you to plot out the depth versus the distance away from the rift of the mid-Atlantic ridge. 5. Graphs 2A and 2B will require you to plot out distance versus age for both an Atlantic and Pacific transect data set. 6. Graphs 3A and 3B will require you to plot out seafloor age versus ocean depth of an Atlantic and Pacific data set. Once your graphs are completed, complete the observation statements. Data Questions Examine Graph 1 of the distance from the Mid-Ocean ridge and the ocean depth. Provide an interpretation of what the graph is indicating. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Examine Graphs 2A and 2B of the crust age and the distance to the ridge. Provide an interpretation of what the graph is indicating. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Compare Graphs 2A and 2B of the crust age and the distance to the ridge. Are there any differences between the shapes of the curves? If so, provide an explanation as to what is causing the difference. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Examine Graphs 3A and 3B of the crust age and ocean depth. Provide an interpretation of what the graph is showing. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Compare Graphs 3A and 3B of the crust age and ocean depth. Are there any differences between the shapes of the curves? If so, provide an explanation as to what is causing the difference. Page 3 of 10
- 4. Life on an Ocean Planet Lab 7 ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Data Questions Using the equation of velocity, calculate in centimeters per year: 1) Atlantic Ridge. Use data provided to you earlier in Station 1 for the Atlantic and Pacific Ocean. Show all work and do not forget your units. 2) the spreading rate of the Pacific Ridge. 1. Spreading rate of seafloor at Atlantic Ridge ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 2. Spreading rate of seafloor at Pacific Ridge ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Data/Results questions: Using the data obtained from the sea floor depth and age plotted graphs you will need to estimate the AGE of the ocean crust in the Pacific using the line of best fit in Graph 3B. (Approximate years only.) Depth Age 3,000 m/9,840 ft ___________________________________________________________________ 3,300 m/10,827 ft ___________________________________________________________________ 3,600 m/11,811 ft ___________________________________________________________________ 3,900 m/12,795 ft ___________________________________________________________________ Page 4 of 10
- 5. Life on an Ocean Planet Lab 7 Procedure Part 2: Use of Sediment Thickness Patterns as Supporting Evidence of Seafloor Spreading 1. Plot sediment thickness against the distance to the mid-ocean ridge using the data provided to you in Table 3. 2. After completing the plotted graph you will need to draw a line of best fit through the data points to determine the av- erage. Feel free to ask your instructor for assistance if you are not sure how to complete a line of best fit. 3. Next, plot sediment thickness against the age using the data provided to you in Table 3. 4. After completing the plotted graph you will need to draw a line of best fit through the data points to determine the av- erage. Data/Results Questions After completing the plot of sediment thickness and distance from the ridge what pattern do you observe? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ After completing the plot of sediment thickness and sediment age what pattern did you observe? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Procedure Part 3: Paleomagnetic Banding as Supporting Evidence of Sea-floor Spreading 1. Using the data provided in Figure 1 (Observed Marine Magnetic Anomalies along the Mid-Ocean Ridge) you will con- struct an illustration depicting the magnetic field reversals on either side of the ridge. 2. Examine the figure carefully before beginning. a. The top x-axis is displaying the distance of the ocean crust away from the ridge. Zero is referencing to the mid- ocean rift. If you observe to the right and the left of the zero the distance is increasing away from the ridge. Page 5 of 10
- 6. Life on an Ocean Planet Lab 7 b. Directly under the distance axis is raw magnetic data (magnetic anomalies). This data was obtained from a mag- netometer as it passed over the mid-ocean ridge. High peaks represent normal polarity (magnetism directed to- wards the north) and down peaks are representing reversed polarity (magnetism directed towards the south). If you look carefully you should observe similarities on either side of the ridge (point zero distance). c. The x-axis directly under the magnetic anomalies is presenting you with the age of the ocean crust in relationship with the mid-ocean ridge. Age is in millions of years with zero representing current time along the young mid- ocean ridge. You will observe that the age increases the further to the left and right of the ridge you get. NOTE: All points on the x-axis line up. For example if you look at all the data for 400 kilometers to the right or left of the ridge it lines up with the same polarity, and an age of approximately 9 million years. d. Each time frame in which the Earth has a specific magnetism is known as an epoch. Epochs are normally named after scientists who study this phenomenon. An actual flip is known as a reversal or an event. 3. Examine the peaks and valleys of the magnetometer data and draw the magnetic epochs in the open profile. Using a dark colored pencil for the normal polarity and the light colored pencil for the reversed polarity, mark the appropriate bands the figure. You should see similar reversal patterns on either side of the ridge. See your teacher if you are un- sure of what to do or how to mark your polarity band. Page 6 of 10
- 7. Life on an Ocean Planet Lab 7 Data/Results questions Are there any patterns in the magnetic banding profile that you constructed? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Calculate the spreading rate of the ocean crust based on information provided to you in Figure 1. Use the same procedure that you used in Station 1 to calculate the spreading rates. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Analysis of Results: Interpret and analyze your results for each Station by answering the following questions. Part 1: Bathymetric and Radiometric Evidence of Seafloor Spreading 1. Based on the plotted graphs you constructed, provide an explanation as to what is causing the age of the crust to increase the further away from the ridge. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 2. Provide a possible cause that explains the difference in spreading rates between the Atlantic and Pacific mid-ocean ridges. Page 7 of 10
- 8. Life on an Ocean Planet Lab 7 ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 3. Provide an explanation as to why the seafloor increases in depth away from the ridge, and why it tends to level out at great distances away from the ridge. ______________________________________________________________________________________________________ Part 2: Use of Sediment Thickness Patterns as Supporting Evidence of Seafloor Spreading. 1. Provide an explanation of the sediment thickness pattern you observed in your graph from Station 2. ______________________________________________________________________________________________________ 2. Would you expect the sediment thickness away from the ridge to perfectly show the sedimentation rate based on age? For example if the sediment rate is 1mm every 1000 years would you expect that a 100,000-year-old crust would have exactly 100 mm of sediment on top of it? ______________________________________________________________________________________________________ Part 3: Paleomagnetic Banding as Supporting Evidence of Seafloor Spreading 1. Provide an explanation that supports the magnetic banding observed in the profile you drew in Station 3. ______________________________________________________________________________________________________ 2. Why does the magnetic banding profile show such strong evidence for seafloor spreading? ______________________________________________________________________________________________________ 3. Based on the observed data of magnetic reversals what seems to be the average number of years between the ac- tual flips? ______________________________________________________________________________________________________ 4. Based on your magnetic profile do you think that mid-ocean ridges around the world would show magnetic profiles that would be exactly the same? Explain your answer. ______________________________________________________________________________________________________ Conclude and Communicate: 1. Present an explanation that would account for the differences in the plotted data graphs between the Pacific and At- lantic data that was provided to you. ______________________________________________________________________________________________________ 2. If a mid-ocean ridge was to stop its production of oceanic crust, in other words the ridge becomes dormant or dies out, what would you expect to observe with the sediment thickness at the site of the ridge? Explain. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ Page 8 of 10
- 9. Life on an Ocean Planet Lab 7 3. If new ocean floor is constantly being produced at mid-ocean ridge systems, then how is it that our planet’s crust is not getting larger and the magma in the asthenosphere is not being emptied as time progresses? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 4. Recently magnetic banding patterns have been found on the surface of Mars by orbiting probes carrying magnetome- ters. What conclusions can be made by this discovery? ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 5. How important is it that we have found some evidence of dynamic crusts on other worlds? Does this evidence streng- then or weaken the support for seafloor spreading and continental drift on Earth? Explain. ______________________________________________________________________________________________________ ______________________________________________________________________________________________________ 6. Your hypothesis/prediction(s) for this activity: ______________________________________________________________________________________________________ 7. Is your hypothesis-prediction(s) supported by your data? Why or why not? ______________________________________________________________________________________________________ _ Page 9 of 10
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