ES+131+Virtual+Plate+Tectonics+Lab

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ES 131 Lab: Plate Tectonics INTRODUCTION Plate tectonics is a well-established theory that unifies and provides a framework for all geologic observations. Most geologic phenomenon observed near the Earth’s surface are linked in some way to plate tectonic processes. The theory states that the outer 60-100 km of the Earth is divided into slabs of rigid rock (the lithosphere). These slabs (the plates) rest upon a semi-viscous layer of easily deformable rock (the asthenosphere). Thermal convection within the asthenosphere pushes the plates in horizontal directions at rates ranging from 1 cm to 12 cm/year. This causes the plates to move in relation to one another. Boundaries between the 8 principle plates and several smaller plates are zones of rock deformation, earthquakes and volcanism. This lab utilizes real data that demonstrates and/or validates the theory of Plate Tectonics. Four exercises, modified from Jones and Jones (2003), follow.  Part A examines global maps of tectonic plate boundaries along with maps of earthquake and volcanic activity to identify plate boundary locations and assess relative motion between the plates.  Part B uses maps of the ocean floor to calculate spreading rates across mid ocean ridges in the South Pacific and Atlantic Oceans  Part C interprets maps and utilizes geologic ages for Hawaiian Islands to better understand movement of the underlying Pacific plate over a “hot spot”.  Part D examines a geologic map along a portion of the San Andreas Fault to evaluate the direction and rates of plate movement. OBJECTIVES Upon completion of this exercise, you will be able to understand: 1. the basic differences between major types of plate boundaries 2. what magnetic stripping is and use it to calculate spreading rates 3. the co ncept of “hot spots” and use this under standing to determine the speed and direction of movement of plates 4. how to interpret a geological map of the San Andreas Fault and calculate the rate of movement along the fault PROCEDURE Work through the handouts for this lab. Each exercise consists of a brief explanation, accompanying map(s), and a series of questions that pertain to the map(s). Use this information to interpret the data and see how geologic data supports the theory of Plate Tectonics. PART A. PLATE BOUNDARIES EXPLANATION: This exercise will familiarize you with Earth’s major tecto nic plates, how to identify the type of plate boundary, and how plates move in relation to each other. You will need to refer to Figure 1 , a map of the major plates and boundaries, Figure 2 , a map of historic volcanic activity, and Figure 3 , a map of earthquake distribution and depth.

There are three basic types of plate boundaries: DIVERGENT (constructive) plate boundaries form when two plates are moving away from one another. This occurs along mid-ocean ridge systems (e.g., the Mid Atlantic Ridge, the East Pacific Rise). Magma rises from mantle and erupts onto the ocean floor where it cools and solidifies to create new oceanic crust. The young crust is then pulled apart as additional lava comes up and newer crust is formed along the center of the ridge. As a result of this process, oceanic crust moves outward (horizontally) on both sides of an oceanic ridge, and new crust is continually added to older oceanic crust.  Characteristics: These boundaries commonly occur in the midst of ocean basins and they contain numerous transform fault (fracture zone) offsets. Frequently this results in a zig-zag pattern to the plate boundary. Earthquakes are common although they are usually shallower and of lesser magnitudes than at convergent boundaries. Volcanic activity is sporadic and may not be centered exactly on the plate boundary. The sense of motion is typically perpendicular to and away from the ridge axis. CONVERGENT (destructive) plate boundaries occur in the collision zone between two plates that are moving toward each other. There are three sub-types of convergent boundaries. Ocean- continent boundaries occur where dense oceanic crust collides with lighter continental crust. The oceanic crust sinks, or subducts, beneath the less dense continental crust into the underlying asthenosphere. Ocean-ocean boundaries develop where oceanic crust collides with oceanic crust from another plate. In this case, the older (cooler and denser) plate descends and the younger (warmer and less dense) plate overrides. Finally, continent-continent boundaries form where two continental plates collide with each other. One plate typically indents into the other resulting in the formation of a very large mountain chain (e.g., the Himalayas at the boundary between the Indian Plate and the Eurasian Plate).  Characteristics: These boundaries can be identified by the presence of deep and large earthquakes that are initiated along a subducting plate. Earthquakes are shallow adjacent to the trench and get progressively deeper in the direction of subduction. Hence, epicenters at the surface of earth occur over a broader geographic zone. Plates move perpendicular to and toward subuction zone boundaries. Volcanic activity is prevalent on the margin of the overriding plate and is expressed as linear chains of volcanic islands or continental volcanic arcs TRANSFORM (conservative) plate boundaries occur in the collision zones where two plates slide past each other, and no plate is created or destroyed. These plate boundaries are characterized by long faults between the adjacent plates that are known as transform faults or strike slip faults (e.g. The San Andres Fault that forms the boundary between the American Plate and the Pacific Plate in California is a well-known transform fault. The largest number of transform faults are formed in association with divergent seafloor-spreading regions where they serve as accommodation zones for rates of differential movement along the ridges).  Characteristics : These boundaries are more difficult to identify. They must be deduced by analyzing the overall sense of plate motion based on the locations of divergence and convergence in order to identify regions where plates are forced to slide laterally past one another (rather than toward or away from each other). There is no volcanic activity associated specifically with this type of plate margin and although small shallow earthquakes may be associated with transform faults on the seafloor, large destructive earthquakes are associated with onshore transform faults in southern California, and portions of Asia north of the Mediterranean Sea.

FIGURE 1. PLATE BOUNDARY MAP Plate boundaries are indicated by irregular lines surrounding continental landmasses (light gray shading) or oceanic regions as depicted on map. Names of plates are as indicated. This map is part of “Discovering Plate Boundaries,” a classroom exercise dev eloped by Dale S. Sawyer at Rice University. Additional information about this exercise can be found at http://terra.rice.edu/plateboundary

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Figure 2. VOLCANOLOGY MAP Red dots indicate currently or historically active volcanic features. List obtained from the Smithsonian Institution. This map is part of “Discovering Plate Boundaries,” a classroom exercise developed by Dale S. Sawyer at Rice University. Additional information a bout this exercise can be found at http://terra.rice.edu/plateboundary

SEISMOLOGY MAP Earthquake locations 1980-1996 (magnitudes 4 and greater) Color indicates depth: Red 0-33 km, Green 70-300 km, Blue 300-700 km . This map is part of “Discovering Plate Boundaries,” a classroom exercise developed by Dale S. Sawyer at Rice University. Additional information about this exercise can be found at http://terra.rice.edu/plateboundary

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QUESTIONS: 1. Using Figures 1-3, state whether the following plate boundaries are Divergent, Convergent, or Transform . a) South American Plate and the Nazca Plate _______________ b) Pacific Plate and the North American Plate in California _______________ c) North American Plate and the Eurasian Plate _______________ d) Pacific Plate and the North American Plate in Alaska _______________ e) Australian Plate and the Eurasian Plate _______________ f) Nazca Plate and the Pacific Plate _______________ g) Pacific Plate and the Eurasian Plate _______________ h) African and Eurasian Plate _____________ i) Antarctic Plate and the African Plate _______________ 2. Draw small arrows on Figure 1 showing the sense of motion on either sides on the plate boundaries identified above. Part B. Seafloor Spreading The Earth’s Magnetic Field The Earth is encompassed by a magnetic field. The source of this magnetic field is the Earth’s molten metallic outer core. The field generated is analogous to the lines of force produced by a bar magnet with a north pole at one end and a south pole at the other. Imagine a giant bar magnet passing through the center of the Earth with its north and south poles located near the North and South poles of the Earth’s axis of rota tion (Figure 4). A magnetic compass placed in the field would align itself parallel to the lines of magnetic force. The direction of this force is shown by the arrows in Figure 4 (right), which point from the south magnetic pole toward the north magnetic pole. This condition is known as normal polarity . Contrary to popular belief, a magnetic compass does not point to the north geographic pole (true north) because the magnetic poles are not coincident with Earth’s geographic poles. The geographic poles def ine the Earth’s axis of rotation and are fixed with respect to the equator over geologic time. The magnetic poles, however, shift over time with respect to the geographic poles. Plots of the magnetic poles during historic times indicates that they have stayed in relatively close proximity to the geographic poles (Figure 4 ). Reversals of the Magnetic Poles It has been discovered that lava flows contain minerals with magnetic properties that align themselves with respect to the Earth’s magnetic field whil e the lava is still molten. When the lava solidifies, the magnetic minerals remain aligned parallel to the

lines of force in the Earth’s magnetic field that existed when the lava cooled into rock. One of the amazing features of the Earth’s magnetic field is that its polarity has reversed itself many times over geologic time. That is, the north and south magnetic poles changed places so that a magnetic compass would point to the south magnetic pole during periods of reversed polarity (Figure). The study of magnetism in ancient rocks is called paleomagnetism . The paleomagnetic features of rocks studied all over the world have provided a basis for a detailed chronology of times of normal and reversed polarities over the last 170 million years. The rock layers from which this chronology have been assembled have been dated by radioactive means, thereby providing an absolute time scale that identifies the times when times of reversed and normal polarities occurred. Both normal and reversed polarities are referred to as magnetic anomalies. A chronology of magnetic anomalies is given in Figure 5 . Three elements are noted: a) a time scale in millions of years before the present (Ma age), b) periods of normal (black) and reversed (white) polarities, and c) conventional identification numbers which have been assigned to each anomaly. The numbers are arranged in chronological order with the youngest anomaly designated as 1 and successively older anomalies by numbers 2, 2a, 3, and on up to anomaly 33. Magnetic Anomalies A magnetic anomaly is a departure from the normal scheme of things. With respect to rocks containing magnetic minerals, a magnetic anomaly is a magnetic reading that is greater or less than the normal strength of the magnetic field where the rock occurs. To illustrate, we consider a series of lava beds (basalt) extruded on the seafloor along both sides of an active spreading ridge between two crustal plates. In determining the intensity of the magnetic field in rocks, an instrument called a magnetometer is towed on a long cable behind a ship across the trend of the lava beds. As the magnetometer passes over the seafloor, the strength of the Earth’s magnetic field is continually recorded on board the ship and a satellite navigational system simu ltaneously records the ship’s position. When the seaborne magnetometer passes over basalts that are normally polarized, the strength of the Earth’s magnetic field is slightly intensified, because the current Earth’s normally -polarized magnetic field adds a small component to the preserved ancient magnetism of the normally polarized rocks. This results in a positive anomaly being recorded. If the rocks over which the magnetometer passes were formed during a period of reversed polarity, the strength of the field recorded in the rock is slightly reduced, and a negative anomaly is recorded. Figure 4. Magnetic Time Scale. Magnetic anomaly identification numbers are given above the polarity chart. An absolute time scale in millions of years (Ma) is given below. Normal magnetic anomalies are shown in black, and reversed polarity is shown in white. The absolute age of the anomalies and their polarities are can be read directly from the chart. For example, anomaly 4 is 7 million years old, and it is a positive anomaly.

Figure 5 (below) shows a hypothetical record of magnetic field strengths in relation to basalts on either side of a mid-ocean ridge. High points on the curved line indicate a normal, or positive anomaly, and low points indicate a reversed, or negative anomaly. The pattern of negative and positive anomalies are repeated on either side of the spreading ridge; that is, the magnetic record on one side of the ridge is a mirror image of the record on the opposite side. As a result, the spreading ridge crest is flanked by stripes of alternating positive and negative anomalies that increase in age with distance from each side of the spreading ridge (Figure ). As result, you can deduce from this relationship that the tectonic plates are moving away from the spreading center (divergent place boundary). If the absolute ages of the various anomalies on either side of the ridge are known and the distance between anomalies of the same age is measured, the spreading rate of the two adjacent plates can be determined. Using this information, the previous relative positions of two continents, such as South America and Africa, can be determined for various times in the geologic past. In exercise B1 you will determine the spreading rates on segments of the East pacific Rise and the Mid Atlantic Ridge, and in Exercise B2, you will determine the relative positions of South America and Africa at a specific time in the geologic past. B1: Determination of Spreading Rates on the East Pacific Rise and the Mid- Atlantic Ridge 1. Figure 6 is a map of the magnetic reversals between the Gofar Fracture Zone (F.Z.) and the 23 Degree South Fracture Zone (23 O SFZ) along the East Pacific Rise. This active spreading ridge corresponds to the plate margin between the Pacific Plate and the Nazca Plate. The active spreading ridge is shown as black bands offset by fracture zones, and the numbered lines on either side of the spreading ridge are magnetic reversals ( aka magnetic lineations ) The age of each numbered reversal can be determined from Figure 4. Record the ages of reversals 2, 3, 5 and 5a in Table 1. . 2. Measure the distance between each pair of reversals of the same age on opposite sides of the spreading ridge along the Garrett Fracture Zone, using the bar scale on the map. Record these distances along with the age for the appropriate reversal listed in Table1.

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Figure 6. Magnetic Reversal Map for the East Pacific Rise Table 1. Garrett Fracture Zone Data Reversal Age Distance Between Spreading Rate Number (Ma) Reversals (km) (cm/yr) 2 3 5 5a

Figure 7. Magnetic Reversal Map for the Mid-Atlantic Ridge 3. Now measure the distances between the magnetic lineations on the Mid-Atlantic Ridge shown on Figure 7. Make your measurements along the Ascension Fracture Zone. (Note: You are measuring different reversals in this area. *Reversal 13 is shown only on the east side of the ridge. To make this measurement compatible with the others, measure the distance from reversal 13 to the center of the Mid-Atlantic Ridge and multiply by 2. Record the distances between reversals and their ages in the appropriate columns of Table 2. Table 2. Mid-Atlantic Ridge Fracture Zone Data Reversal Age Distance Between Spreading Rate Number (Ma) Reversals (km) (cm/yr) 5 8 13 21

4. Plot a point on the grid of Figure 8 (right) for each reversal anomaly you listed for the East Pacific Rise in Table 1. Each point will be located where the age (in millions of years) along the vertical axis intersects the distance between reversals (in kilometers) along the horizontal axis. Draw a best-fit line through your points.The line should begin at the intersection of the horizontal and vertical axes of the grid. Label this line “East Pacific Rise.” 5. Now do the same for your data from Figure 7. On figure 8 (right) plot the age (in Ma) versus distance (in km) for each point, draw a best-fit line through the points, and label the line “Mid -Atlantic Ridge. ” 6. The two straight lines on Figure 8 provide a visual comparison between the spreading rates of the East Pacific Rise and the Mid-Atlantic Ridge. Is the spreading rate greater on the East Pacific Rise or the Mid-Atlantic Ridge? If you are not familiar with rate determination, it is sometimes referred to as slope. The steepest line slope in this case would have the greater spreading rate. 7. Calculate the spreading rates in centimeters per year (cm/yr) using information based on reversal number 5 on both the East Pacific Rise and the Mid-Atlantic Ridge. Remember that 1 km = 1,000 meters and 1 meter = 100 cm so this means there are 1,000,000 cm in 1km. Show your work in the space provided. Figure 8. Plot of Magnetic Anomaly Age (Ma) versus Distance (km) for Selected Anomalies from the East pacific Rise and the Mid-Atlantic Ridge

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Restoration of the South Atlantic Coastline 50 Million years Before the Present Given the evidence of spreading along the Mid-Atlantic Ridge, it can be deduced that the Africa and South America plates are moving away from each other, carrying the continents of Africa and South America with them. By using the pattern of magnetic lineations shown on Figure 7, it is possible to reverse the spreading process and restore the positions of the African and South American coastlines to a time when a particular set of magnetic lineations was being formed on the Mid-Atlantic Ridge. For the purposes of this exercise, we will use reversal number 21, which according to the magnetic lineation time scale of Figure 4, was formed 49.6 (or roughly 50) million years ago. Proceed as follows: 1. On Figure 7, draw a red line over each of the magnetic lineations of reversal number 21 on the South American side of the Mid-Atlantic Ridge . Connect the segments of the number 21 reversal with a red line drawn along the fracture zones against which they terminate (if you do not have a red pencil, use another color of your choosing). Start with the point where reversal 21 touches the Ascension F.Z. Follow reversal 21 with your red pencil southward until it reaches the Bode Verde F.Z., then along the Bode Verde F.Z. westward to the northern end of the next fracture zone. Continue until you have reached the southernmost fracture zone on the map (you should have a zig-zag pattern) 2. Attach a piece of tracing paper over Figure 7 with tape or paper clips and repeat the process described previously for reversal 21 on the African side of the Mid-Atlantic Ridge . Draw this line in red pencil on the tracing paper (If you do not have tracing paper, use a regular sheet of paper and use a window or glass patio door) . 3. With the tracing paper still in place, trace with black pencil the coastlines of Africa and South America. Also, trace in black pencil on the tracing paper the boundaries of Figure 7 and the 20 O South latitude line. 4. Detach the tracing paper and slide it toward South America until the red line on the tracing paper matches the red line on Figure 7 (you may have a slight amount of rotation involved here ). When the two lines are matched as closely as possible, hold the tracing paper in place and trace the coastline of South America in red pencil on the tracing paper. Also trace the 20 O South latitude line on the tracing paper. 5. Congratulations! The map you have constructed on the tracing paper shows the Mid-Atlantic Ridge as it existed when magnetic reversal 21 was being formed. Your tracing paper also shows the relative positions of segments of the coastlines of Africa in black pencil and South America in red pencil as they were approximately 50 million years ago. The continents moved with respect to each other because the tectonic plates were to which they were attached moved as spreading continued along the Mid-Atlantic Ridge. What is the evidence that the movement of the two plates was not strictly in an east-west direction? 6. Was the Earth’s magnetic field normal or reversed at the t ime represented by your map on the tracing paper?

Part C: Hot Spots Figure 9 is a map of the islands and seamounts (submerged volcanoes) that form the Hawaiian Islands and the Hawaiian-Emperor Chain. All of the islands and seamounts were formed as volcanoes, and are younger than the crust on which they sit upon. In 1963 John Tuzo Wilson proposed that these volcanoes were formed by a “ hot spot ” - a relatively stationary plume of magma that rises through the Earth’s mantle and melts through the overrid ing plate . Questions: 1. Assuming that the big island of Hawaii is directly over the hot spot, and that the hot spot has been stationary since these islands and seamounts formed, use arrows to show the relative motion of the Pacific Plate over the hot spot. You should use two arrows to account for the bend in the Hawaiian – Emperor Chain. 2. What explanation can you give for the bend in the Hawaiian-Emperor Chain? 3. The table below presents age data for each island or seamount (based on radiometric dating), and its ’ distance from Kilauea (the active volcano in Hawaii that is inferred to be directly over the hot spot). Using the graph (below), plot the age versus distance from Kilauea for each of the volcanoes listed in the table.

4. Draw two (2) best-fit straight-line segments through the data, one from Mauna Kea to Laysan and one from Laysan to Suiko. This is a little tedious. Volcano Name Distance from Kilauea (km) Age in Millions of Years Mona Kea (on Hawaii) 54 0.375 West Maui 221 1.32 Kauai 519 5.1 Nihoa 780 7.2 Necker 1058 10.3 La Perouse Pinnacle 1209 12 Laysan 1818 19.9 Midway 2432 27.7 Abbott 3280 38.7 Daikakuji 3493 42.4 Koko 3758 48.1 Jingu 4175 55.4 Nintoku 4452 56.2 Suiko (southern) 4794 59.6 Suiko (central) 4860 64.7 * Original Source for data USGS Prof Paper 1350

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4. Why do the two lines have different slopes? 5. Calculate the rate of plate movement for each of the line segments you drew by dividing the distance travelled by the time interval over which the travel took place. Give your answer in cm / year. Show your work. Part D: Transform Faults Figure 10 shows a geologic map along part of the San Andreas fault, a transform fault which is the boundary of the North American and Pacific Plates. A fault is a line of fracture along which movement in the horizontal or vertical plane can be measured. At this particlular location, the two plates are sliding past each other. The creek flows to the northwest (see note below) from the Tremblor mountains across the fault. Figure 10 shows the distribution of younger (3800 years to today) and older (10,000 to 3,680 years) sediments deposited by the creek (ages derived by carbon-14 dating). Note: we generally orient maps so that the north direction is aligned toward the top of the page. If you are looking at the map as presented, the creek is flowing from the top of the picture until it reaches the fault. It then flows toward the northwest (your left), and then jogs back to the southwest (toward the bottom of the page). San Andreas Fault Trace

Questions: 1. Why does the present-day stream valley bend sharply at the San Andreas Fault? 2. Is there any evidence for vertical (up and down) movement along the fault? 3. What do you predict will happen to the course of the creek in the future? Draw on the map if you prefer. 4. Draw arrows on the figure to show the relative direction of movement of the plates on either side of the fault. 5. What is the rate of movement along the fault over the past 10,000 years? (this can be determined by dividing the distance of the stream offset across the fault by the age of the oldest sediments in each of the channels) Calculate the rate of offset based on data for each channel. Use 10,000 years for the age of the sediment in the oldest channel, and 3,680 years for the age of the sediment in the younger channel. Give your answers in cm/year. Show your work. 6. The best estimate of the total displacement along the San Andreas fault is about 320 km. If your calculated rate of slippage for problem question 5 above has been the average for the life of this fault, about how old is this fault? Show your work and remember to keep your units consistent.

ES+131+Virtual+Plate+Tectonics+Lab

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