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30.5° - North American Plate point 4' AI North \ point 3 African Plate point 1 43 i 0* 42 , 0 point 177 30.0' - 50 km Activity 2.1 Plate Motion from Different Frames of Reference Name: Course/Section: Date . Learning GOAL You will be able to recognize the direction of motion of one plate as observed from another plate across a mid-ocean ridge, given the trends of two axial rifts and the ridge-ridge transform fault between them. You will also use vector arrows to represent plate motion in a reference frame that is external to the plates—a "no-net-rotation" or NNR reference frame—and estimate the position of this ridge-fault-ridge system -2 million years (Myr) in the future. Important Reminder: We can only measure a velocity or a displacement (the movement from an initial location to a different loca- tion) relative to some reference frame. Before we describe a velocity or a displacement, it's important to ask the question "velocity relative to what?" 1111 Motion of Plates Relative to Each Other Across a Shared Boundary. Figure A2.1.1 is a sketch map of the Atlantis Transform Fault and adjacent axial rifts along the northern Mid-Atlantic Ridge. The parallel red lines between points 1 and 2 and points 3 and 4 represent axial rifts along the ridge, and the bold black line is the transform fault along which the two plates move past each other. Figure A2.1.1 A 1. Starting at the yellow star along the axial rift between points 1 and 2, draw a vector arrow on the African Plate that is approxi- mately parallel to the transform fault, pointing toward the interior of the African Plate. (The vector arrow is started for you.) Starting at the same yellow star, draw a vector arrow in the opposite direction on the North American Plate, pointing toward the interior of the North American Plate. These arrows indicate the sense of divergent plate motion across the ridge as the two plates spread apart. Do the same at the yellow star between points 3 and 4 along the ridge axis. (See Fig. 2.1 B-C for guidance.) 2. Using the arrows you just drew as a guide, draw one arrow on each side of the transform fault, parallel to the fault line, to indicate the sense of relative plate motion across the fault. 3. As viewed from point 2, point 3 is located toward the east southeast (azimuth -102°). Viewed in the opposite direction, point 2 is toward the west-northwest (-.282°) as seen from point 3. In what general direction is the African Plate moving relative to the North American Plate? Direction: PI Plate Motion in an External Reference Frame 1. Carefully use scissors to cut along the dashed line in Fig. A2.1.3, making separate maps of the North American and African Plates near the Atlantis Transform Fault. 2. Place the two halves of the cut-out map on Fig. A2.1.2 so that points 1, 2, 3 and 4 are all aligned. The result is what the boundary looks like today. This blank space is the cut-out area for Fig. A2.1.3 on the next page. 47

North American Plate 40 Tz I;,-d 3n /g - 0 / / 0 tz:2nsforrn / / 2 --• --- -._ / 0 30 0' - / 3 ..,/ African 4?/ / Plate ii North 0 1 . 42.5" 42.0' 50 km 43 , 0° F . Seafloorbathymetry i _rom GeoMapA Figure A2.1.2 • 3. Move the two cut-out maps so that points 1 and 4 on the North American Plate coincide with points l n and 4 n on Fig. A2.1.2 and points 1 and 4 on the African Plate coincide with points l a and 4 a . 4. Use a pencil to trace the I n — 2 n — 3 n — 4 n boundary on Fig. A2.1.2 and do the same for the 1, — 2 a — 3 a — 4 a boundary. Those traced lines mark where the oceanic crust along today's plate boundary will likely be after 2 Myr, as observed in a NNR reference frame external to the plates. 5. Using the lines you just drew, assume that the ridge will be located halfway between 1„ and 1, and halfway between 2 n and 2, after 2 Myr of spreading. Carefully draw that ridge axis on Fig. A2.1.2 and do the same halfway between points 3 n — 3 a and 4 n — 4 a . 6. Complete the picture by drawing the transform fault between the ridge ends. 7. You have now made a prediction for where the section of the Mid-Atlantic Ridge around the Atlantis Transform Fault will be located about 2 Myr from today relative to the NNR reference frame. III Plate Motion in Different but Related Reference Frames 1. Draw a vector arrow on Fig. A2.1.2 from point 1 to in. This vector is an estimate of the direction of average motion of that point along the North American Plate boundary in the next 2 Myr, as observed in the NNR reference frame. Now, draw a vector arrow from point 1 to l a , indicating the direction of average motion of the African Plate at that point in the next 2 Myr relative to the NNR frame. Do the same for points 2, 3, and 4. cut-out this area only Figure A2.1.3 • 2. Draw an arrow on Fig. A2.1.2 from point I n to point l a . This vector is an estimate of the average motion of point 1 on the African Plate as observed from the North American Plate. Do the same for points 2, 3, and 4. 3. Use the map scale to estimate the distance the African Plate will move in the next 2 Myr along the Atlantis Transform Fault, as observed from the North American Plate. (Hint: Measure the 2„ — 2 a or 3 n — 3 a distances.) Answer: km 4. Approximately what will be the width of new crust devel- oped along the Mid-Atlantic Ridge between Africa and North America near the Atlantis transform fault during the next 2 Myr? (Hint: Think about your answer to the previous question.) Answer: km 48

San Francisco' Fresno c:> Pinnacles' Volcanics r Neenach Volcanics 1 1 North 0 100 km SF . k•36° Bakersfield Activity 2.2 Plate Motion and the San Andreas Fault Name: Course/Section: Date* Learning GOAL You will examine two points that were originally in the same place along the San Andreas Fault and measure how far they have moved apart. Using GPS velocity data, you will estimate how long it might have taken for this displacement to occur. Ill Displacement Along the San Andreas. The San Andreas Fault in California is an important part of the boundary between the Pacific Plate to the west and the North American Plate to the east. Between about 19.0 and 24.1 million years ago, a volcano erupted near the western margin of the North American Plate, and later that same volcano was split by a fault in the San Andreas Fault system. The displaced halves of that volcano now form the Pinnacles and Neenach Volcanics (Fig. A2.2.1). Pinnacles now moves with the Pacific Plate and Neenach remains on the deforming edge of the North American Plate. Figure A2.2.1 • 1. Measure the distance between Neenach and Pinnacles along the San Andreas Fault. Because the fault is not a straight line on the map, you might want to use a string in the measuring process. Carefully position the string along the curved trace of the fault. Mark the average positions of Neenach and Pinnacles on the string. Straighten the string along a ruler and measure the distance between the two points on the string in millimeters. Each millimeter on the map represents 4 km on Earth's surface. Approximate distance from Neenach to Pinnacles: km 2. The age of the Neenach—Pinnacles Volcanics is between about 19.0 and 24.1 Myr. Based on your answer to the previous question and making the first-order assumption that faulting began soon after volcanism ceased (around 19.0 Myr), estimate the average rate at which Neenach and Pinnacles have moved apart since 19 Myr. Estimated average rate of displacement since 19 Myr: kin/Myr 49

o 41 / MalibL\ \ f s \ \* 0 loo km North Santa Barbara . N N e \•••• • San Diego • Tijuana' MEXICO GPS velocities relative to the stable interior of the North American Plate 50 mm/yr I I I I GPS station 3. There are three permanent GPS sites very near Neenach and two GPS sites near Pinnacles. Based on data processed for those sites by UNAVCO and posted on March 2, 2019, Pinnacles and Neenach are currently moving apart at a rate of about 21.2 trim/yr (-21.2 km/Myr). If we assume that Neenach and Pinnacles have moved apart at a constant rate of 21.2 km/ Myr—a first-order assumption that might not be accurate—estimate when the faulting that split them apart might have begun. (Hint: Use the distance you measured from Neenach to Pinnacles.) Estimated age of faulting: Myr B Motion of the Crust in a Plate Boundary Zone Figure A2.2.2 • 1. Velocities measured at several GPS stations in southern California are shown in Fig. A2.2.2. The length of the arrow provides the speed of the GPS site, measured in a reference frame called NAMO8 that is fixed to the stable interior of the North American Plate—the area east of the Rocky Mountains. Longer arrows indicate faster motion, and an arrow that is 25 mm long on the map represents a speed of 50 nurt/yr. The data for this map were derived from UNAVCO's GPS Velocity Viewer (https://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html) on February 28, 2019. Use the velocity vectors on either side of the San Andreas Fault to estimate how much faster the Pacific Plate is moving relative to the deforming western edge of the North American Plate in southern California. You may give your answer as a range of velocities. Answer: rnm/yr 2. Add half-arrows along the San Andreas Fault to show the sense of motion across the fault based on your best interpretation of the difference in GPS vectors across the fault. Look at Fig. A2.2.1 for an example of the half-arrow symbol used to map faults that have horizontal slip. 4 REFLECT & DISCUSS Many of the GPS velocity vectors that are shown east of Malibu on Fig. A2.2.2 appear to be directed toward a part of the San Andreas Fault near the center of the map, where the fault is labeled. This segment is called the "Big Bend" in the San Andreas Fault. Vectors on the north (upper) and the south (lower) parts of the map seem to be more nearly parallel to the fault. Describe one or more ways that the crust along the Big Bend might be affected by this difference in motion compared with the areas along the fault to the northwest and southeast. 50

7. Go to the Plate Motion Calculator hosted by UNAVCO at https://www.unavco.org/software/geodetic-utilities/plate- motion-calculator/plate-motion-calculator.html . Enter the latitude and longitude of your GPS station, being sure to include the proper sign ( + / — ). Then enter the site name; choose MORVEL 2010 as your model; select your tectonic plate, NNR no-net-rotation for your reference frame, and HTML table w/ local E&N plate velocities as the output format. Ignore the rest of the input boxes. Then press submit. Record your results. N velocity: mm/yr E velocity: mm/yr Speed: mm/yr Azimuth. ° measured clockwise from north The Plate Motion Calculator provides the instantaneous velocity of a given point in a NNR reference frame, assuming that the plate is not deforming. How do the results from the Plate Motion Calculator compare with your earlier results (questions 2-5)? The velocity of your GPS station includes the velocity of the plate it is located on as well as a component related to the deformation of the part of that plate where the GPS station is located. C Return to the JPL-NASA Time Series website and view the map. From the website map, draw a vector arrow from each of the green circles on the map in Fig. A2.3.2 to show the general direction that at least one spot on Africa, Arabia, Australia, China, Europe, India, North America, Russia, and South America are currently moving relative to a NNR reference frame. In a general way, use the length of the arrows to reflect the velocities of the sites, just as the length of the yellow lines on the Time Series website indicates velocity. Figure A2.3.2 A II REFLECT & DISCUSS Do you think that determining the velocity of one point on a plate is sufficient to tell where the entire plate is moving? Why or why not? (Hint: Represent a plate using a piece of paper on a tabletop. Slide the paper a very short distance from its initial position across the tabletop, maybe with a bit of a twist of your wrist. Now consider whether you could reconstruct the motion of the piece of paper from initial to final position if you only have information about the motion of one point on the paper.) 52 They are slightly off but close enough to determine that my calculation based on GPS data was accurate. No, Tectonic plates have complex geometries and may not move as a rigid body. Different regions of a plate might experience varying degrees of strain, and the velocity at one point might not be representative of other regions.

Activity 2.4 Hotspots and Plate Motions Name: Course/Section: Date' Learning GOAL You will learn how the volcanic trail left on a plate by a mantle hotspot can be interpreted to tell us where and how fast the plates are moving relative to Earth's deep mantle. As a lithospheric plate moves over a hotspot in the upper mantle below the plate, a volcano develops directly above the hotspot. As the plate continues to move, the volcano drifts away from the hotspot and eventually becomes dormant. Meanwhile, a new volcano develops over the hotspot next to the older volcano. The result is a trail of volcanoes with one end of the line located over the hotspot and quite active, and the other end distant and inactive. In between is a succession of volcanoes that are progressively older with distance from the hotspot. A Figure 2.13 shows the distribution of the Hawaiian Islands Chain and Emperor Seamount Chain. The numbers indicate the aver- age age of the volcano in millions of years (Myr), obtained from isotopic dating of the basaltic igneous rock of which each island is composed. 1. If both the Emperor and Hawaiian Islands Chains developed as a result of the same mantle hotspot, what is a possible reason that the hotspot trail changes direction at —42 Myr? 2. What was the rate of Pacific Plate motion relative to the Hawaiian hotspot as it was developing the 2,300 km-long Emperor Seamount Chain from 65 Myr to 42 Myr? Express the rate in millimeters per year (mm/yr). In what direction was the plate moving (north-northwest or south-southeast) relative to the hotspot during that time interval? 3. What was the rate of Pacific Plate motion relative to the Hawaiian hotspot from 5.1 to 0.8 Myr, expressed in mm/year? 4. Using Lo'ihi Seamount as the current location of the Hawaiian hotspot, what was the rate of Pacific Plate motion relative to the Hawaiian hotspot from 0.8 Myr to today, expressed in mnVyr? 5. Go to the JPL-NASA GNSS Time Series website at https://sideshow.jpl.nasa.gov/post/series.html . The map locates each GPS station with a green dot and a yellow line that extends outward in the direction that the GPS station is moving relative to the NNR reference frame. GPS station HNLC is located on Oahu. (a) How does the current motion of HNLC on Oahu compare to the direction of Pacific Plate motion relative to the Hawai- ian hotspot over the past —42 million years? (b) GPS station HNLC on Oahu has the following component velocities relative to the NNR reference frame as of March 5, 2019: moving north at 34.607 ± 0.038 mm/yr and moving west at 62.814 ± 0.041 mm/year. Use the Pythagorean Theorem to find the current speed of the Pacific Plate at Oahu relative to the NNR reference frame. Show your work. 6. REFLECT & DISCUSS Based on all of your previous work, explain how the direction and rate of Pacific Plate movement changed over the past —70 million years. 53 the movement of tectonic plates approx. 100 mm/yr in the south west direction 90mm/yr 37.5mm/yr now it's moving northwest The rate has slowed down and slowly changed direction from south west to northwest

A Medicine Lake Volcano MountA Shasta CANADA WASHINGTON 0 Mt. Baker AGlacier Peak —2 L1.1 • Co cr AMt. Rainier Silverthrone Caldera,A Franklin £/ Glacier Volcano 50 100 200 300 400 miles 50 100 500 kilometers Mount Meager Mount Cayley A Mount Garibaldi He A A Mt. Adams _ ci A Mt. Hood czc A Mt. Jefferson ci) c.) OREGON Lassen Peak A CALIFORNIA Colors indicate rocks of normal magnetic polarity (+), like now. The north-seeking end of a compass needle would have pointed in the general direction of the geographic North Pole. White areas have reversed magnetic polarity (—). At those —8 times, the north and south poles exchanged positions. The north-seeking end of a compass would have pointed in the —11 general direction of the geographic South Pole. 12 'MountSt. ' —4 —6 A Three Sisters A Newberry Volcano A Crater Lake Crest of Gorda Ridge Crest of Juan de Fuca Ridge A volcano trench MONTANA Magnetic polarity time scale: ages are in millions of years ago (Myr) Activity 2.6 Paleomagnetic Stripes and Seafloor Spreading Name: Course/Section: Date' Learning GOAL You will learn to interpret marine magnetic anomalies to infer the rate at which plates have diverged and new lithosphere has formed along a mid-ocean ridge. II Analyze the seafloor part of the map in Fig. A2.6.1, which depicts the area just off the Pacific Coast, west of California, Ore- gon, Washington, and southwest Canada. The colored bands are marine magnetic anomalies. Colored bands are rocks with a positive ( + ) magnetic anomaly, so they have normal polarity, like now. The white bands are rocks with a negative (—) mag- netic anomaly, so they have reversed polarity. Different colors indicate the ages of the rocks in millions of years, as shown in the magnetic polarity time scale provided. Magma Figure A2.6.1 • 57

1. Using a pencil, draw a line on the seafloor to show where new ocean crust and lithosphere is forming now (zero millions of years old). Using Figs. 2.1, 2.3, and 2.12 as guides, label the segments of your line that are the Juan de Fuca Ridge and Gorda Ridge (divergent plate boundaries). Then label the segments of your pencil line that are transform fault plate bound- aries. Add half-arrows to the transform fault boundaries to show the motion of the two plates relative to the transform fault. 2. What has been the average rate and direction of seafloor spreading in mm per year (min/yr) west of the Juan de Fuca Ridge, from B to A? Show your work. 3. What has been the average rate and direction of seafloor spreading in mm per year (mm/yr) east of the Juan de Fuca Ridge, from B to C? Show your work. 4. Notice that rocks older than 11 million years are present west of the Juan de Fuca Ridge but not east of the ridge. What could be happening to the seafloor rocks along line segment C-D that would explain why rocks older than 11 million years no longer exist on the seafloor east of the ridge? 5. Notice the black curve with triangular barbs just east of point C: (a) If you could take a submarine to view the sea floor along this line, what feature would you expect to see? (Hint: See Fig. 2.1A.) (b) Based on Fig. 2.1, what lithospheric plate is located east of the black barbed curve? (c) Based on Fig. 2.1, what lithospheric plate is located west of the black barbed curve? 6. REFLECT & DISCUSS Notice the line of volcanoes that form the Cascade Range, extending from northern Califor- nia to southern Canada. These are active volcanoes, meaning that they still erupt from time to time. What sequence of plate tectonic events is causing these volcanoes to form? 58 distance from B to A is = (1.7/3.3) x 500 kilometer = 257.57 kilometers Time taken to move from B to A = 6.8 million years Rate of spreading = (257.57 x 105 centimeters/6.8 x 106 years) = 3.7877 cm/year in north west direction distance from B to A = (1.5/3.3) x 500 kilometer = 227.27 kilometers Time taken to move from B to C = 6.7 million years Rate of spreading = (227.27 x 105 centimeters/6.7 x 106 years) = 3.392 cm/year Sub-duction zone Oceanic trenches, possibly Volcanoes the North American plate. The Juan de Fuca plate. The process of sub duction triggers the liquefaction of the mantle, consequently generating magma that ascends to the surface and gives rise to volcanoes.

2. The area between CNA and BNA in Fig. A2.7.1 consists of oceanic lithosphere added along the Mid-Atlantic Ridge to the North American Plate between 154.3 and 67.7 Myr. What is the average speed at which new lithosphere was added to the North American Plate along that line? Show your work. min/yr Do the same analysis for the lithosphere between points CAF and BAF. What is the average speed at which new lithosphere was added to the African Plate along that line? mm/yr Which plate moved faster relative to the ridge between 154.3 and 67.7 Myr, if either? Ill Given your answers in part A, did the North Atlantic Ocean Basin develop by adding lithosphere symmetrically along the mid- ocean ridge, or was new lithosphere added more rapidly to one side than the other? If spreading was asymmetric, which plate had more lithosphere added, or did the asymmetry vary from plate to plate over time? IIII Use the rates that you calculated previously and the map scale to estimate when the coastlines of North America and Africa might have last touched before they were separated by the opening of the North Atlantic Ocean Basin. ll REFLECT & DISCUSS Based on the rates you calculated previously, estimate the number of meters that Africa and North America have moved apart since the United States was formed in 1776. Discuss what you did to accommodate the uncer- tainty in your estimate. 60 1354 km/86.6 Myr = 15.63 mm/yr 15.63 1279 km/86.6 Myr = 14.76 mm/yr 14.76 The North American Plate The formation of the North Atlantic Ocean Basin is characterized by a more rapid addition of lithosphere on one side of the mid-ocean ridge, leading to an uneven expansion of the ocean basin. This uneven spread is attributed to a faster spreading rate in the direction of the North American plate compared to the rate at which it spreads towards the African plate. distance between CNA to CAF = 5107 km average rate of movement of the Atlantic plate =5107 km/154.3 Myr = 3.3 cm/yr Total time = 6160 km /(3.3 cm/yr) = 186.67 million years distance moved apart between Africa and North America since 1776 = speed * time difference = 2 * ( 3.3 cm/yr * 245 cm ) = 16.17 m I multiplied by 2 because we need the full spreading length, keeping in mind that the gap is created in both sides

• • • • • • 100°W 90°W 80°W • • • . • • 110°W 1.0° • • • Brazil • \'‘' • ' • • • • •• • 6. 4.0 • • • 11 5 • Bolivia • •• • • • ••• • Para- ~ guay • • • • • • • • • • • 0 500 1000 km 120°W • • *I D • • • • • .• •• • • • • • • • • • • •••• • 5%• • • :• *.• 100' W • Argentina Chile • :•••• • • S. *• • ••••%•• ii ** 44 Map of Earthquake Activity in the Eastern Pacific Ocean and South America ;••,* • • • • • • • Ecuador Activity 2.8 Using Earthquakes to Identify Plate Boundaries Name: Course/Section: Date' Learning GOAL You will see how earthquakes help define the boundaries of plates in the eastern Pacific Ocean Basin near South America. Then you will plot actual earthquake focal data on a cross section to explore the subduction of the Nazca Plate beneath the South American Plate and will relate the down-going slab to active volcanoes in the Andes Mountains of western South America. A Use a red colored pencil or pen to outline the location of all plate boundaries on the map in Fig. A2.8.1. Do your work carefully. Then label the East Pacific Rise, Galapagos Ridge, Chile Ridge, and all of the plates. Refer to Fig. 2.1 for help with the tectonic features. • Shallow-focus • Intermediate-focus • Deep-focus Trench earthquakes earthquakes earthquakes 0-69 km deep 70-299 km deep 300-700 km deep (Data from U.S. Geological Survey) Figure A2.8.1 • • Peru • 61