Ex_2_Seafloor_Topograph_and_Plate_Tectonics

17 Exercise 2 – Seafloor Topography and Plate Tectonics (Modified from the Exploring Seafloor Topography education module In the Earth Exploration Textbook available at http://serc.carleton.edu/eet/chapters.html) Purpose: This lab will provide: 1) An appreciation of Earth’s structure and tectonic processes. 2) An understanding of how seafloor topography varies in different tectonic settings. 3) An appreciation for the rates at which tectonic plates move. Objectives: In this exercise, you will: 1) Use GeoMapApp to produce, examine, and interpret topographic profiles of various tectonic settings on the seafloor. 2) Use distance/time data and conversions to calculate the rate of tectonic plate movements in km/My and cm/yr. INTRODUCTION For the first part of this exercise, you will produce and examine computer-generated visualizations using GeoMapApp to investigate how continental margins and the seafloor are unique in different tectonic settings. For the second part, you will investigate the rate at which the Earth’s tectonic plates move. Before we begin, let’s review some Plate Tectonics. PLATE TECTONICS AND BOUNDARY TYPES The theory of Plate Tectonics explains how the movement of the lithospheric plates affects the structure of the Earth’s crust including many topographic features of the seafloor. The crust and uppermost mantle make-up a brittle layer called the lithosphere (Fig. 2.1), which is broken up along the Earth’s surface defining at least 13 major lithospheric plates (Fig. 2.2). The base of these lithospheric plates sits atop the asthenosphere , a zone in the mantle that is mechanically weaker and provides a “sliding layer” for the massive lithospheric plates to move across the Earth’s surface (Fig. 2.1). Look at the key on the map of Tectonic Plates in Figure 2.2 to understand how the plates are currently moving relative to each other. Geologists define plate boundaries depending on the relative motion between plates. When neighboring plates move: � toward each other, the plates share a convergent boundary � away from each other, the plates share a divergent boundary � side-by-side, the plates share a transform boundary

18 Juan de Fuca Plate Transform Extensional Compressive Plate Boundary Type Figure 2.1 Tectonic map showing major tectonic plates of the lithosphere. The boundaries are categorized as convergent, divergent or transform depending on the relative motion between adjacent plates.

20 Continental Crust (Granitic composition, ~80 km thick) The average composition of continental crust is similar to that of a rock called granite . Continental crust is rich in lighter elements like silica and aluminum, which makes it more bouyant and sit topographically higher than the dense seafloor. Basalt Oceanic Crust (Basalt, ~7 km thick) Granite Sediment blankets the continental margin (shelf, slope, rise). The sediment comes from weathering continental rocks. Basalt is erupted from volcanic fissures at the mid-ocean ridge to form ocean crust . Basalt is rich in heavy elements like iron and magnesium. Lithospheric Upper Mantle (Very dense rock, defines the base of lithospheric tectonic plates.) Upper Mantle i t h o s p h e r i c m a n t l e – – – – – – – – l i t h o s p h e r i c m a n t l e – – – – – – – – – – – – a n t l e – – – – – a s t h e n o s p h e r i c m a n t l e – – – – – – – – Lithosphere Figure 2.2 Tectonic plates are composed of continental and/or oceanic crust and the uppermost mantle. The plates “slide” along a mechanically weak, ductile layer in the upper mantle called the Asthenosphere. Note that the continental margin (shown in detail in Fig. 2.4) marks the transition from continental to oceanic crust. The difference in composition of continental and oceanic crust is important and plays a role in how the plates interact at plate boundaries, as you will learn in this lab.

21 Figure 2.3 The mid-ocean ridge forms a topographic high, or linear array of volcanic fissures from which basaltic lava is erupted to form the seafloor. Ocean basins “grow” as newly formed crust cools and spreads away from the ridge. This tectonic process is known as seafloor spreading. Image from http://www.divediscover.whoi.edu/ tectonics/movements.html The new seafloor then moves away from the mid-ocean ridge by a process called seafloor spreading . The spreading that occurs at the ridges is driven by (1) mantle convection , (2) So how does ocean crust become so dense that it sinks back into the mantle? Ocean crust becomes denser as it becomes “older & colder” as it spreads away from the mid-ocean ridge. Not only does the crust cool, but the underlying lithospheric mantle (also dense rock) also cools as it spreads further away from the mid-ocean ridge over time. As the lithospheric mantle cools, it hardens against, or underplates , the overlying ocean crust (Fig. 2.1). With the seafloor comprised of such dense rock layers that only thicken over time, its no wonder that the flat, abyssal plain of the seafloor sits so low on our Earth’s surface. Eventually the densest parts of the seafloor become so heavy, that they sink at converging margins. Question 4. Hypothesize where the oldest crust on the seafloor is relative to the mid-ocean ridge (hint: think what happens as a result of seafloor spreading). gravitational collapse of the high peaks along the ridge (called ridge push ), and most dominantly by (3) a process called slab pull . Slab pull occurs when ocean crust becomes so dense, that it begins to sink back into the mantle at subduction zones associated with convergent plate boundaries. Essentially the weight of the down- going slab exerts enough torque to drive seafloor-spreading 1000s of kilometers away!

23 Seafloor topography of active margins: subduction zones Subduction zones occur where two plates converge and one plate goes beneath the other. As you probably noted when you were looking at the plate map (Fig. 2.2), the most common case occurs where ocean crust converges on continental crust . In this situation, the ocean crust (basalt) will become the down-going slab because it is denser than continental crust (granite). Subduction can occur between two oceanic plates. In this case, the down-going slab will be the densest of the two (colder, and likely older). Common features of subduction zones can be seen in Figure 2.5 and are described as follows: Angle of subduction may vary. ocean crust sea level 0 100 200 km 0 km 8 16 Back arc basin Volcanic arc Trench Convergent Plate boundary Relative change in plate velocity due to drag force exerted by convection mantle. This results in extension in the back arc basin. Drag Force Figure 2.5 Schematic cross section illustrating parts of a subduction zone. Subduction zones can involve continental-oceanic convergence, or ocean-ocean convergence. Fluids from the down-going slab cause melting to occur in the overlying mantle wedge, which feed the volcanic arc that forms on the over-riding plate. Although this is a dominantly convergent margin, a back arc basin formed by local extension may occur if the leading edge of the over-riding plate is “pulled” into the subduction zone faster than the rest of the over- riding plate is moving. Volcanic-arcs are chains of volcanoes that form on the overriding plate in a subduction zone. The volcanoes are fed by magma generated by fluid-induced melting in the mantle wedge as the descending slab is dehydrated as it descends deeper into the mantle. Most people recognize subduction zones by their volcanic arc, but the presence of a trench , or narrow topographic low, is also a tell-tell sign that plates are converging and one is being subducted beneath the other. The trench forms as the crust is flexed or “down-warped” by the descending slab. Trenches form the deepest part of the world’s oceans, with the Mariana Trench located in the western Pacific east of the Philippines reaching 11 km (~7 miles) at the Challenger Deep. The world’s tallest peak, Mt. Everest, would still be more than a mile underwater if it were dropped into the Mariana Trench. On occasion, a back-arc basin or topographic low will develop behind a volcanic arc on the overriding plate in a subduction zone. A back-arc basin forms when the subduction angle of the down-going slab increases, creating convection in the mantle that exerts a drag force on the overriding plate (see Fig. 2.5). When the exerted drag force is greater than the compressional forces in the subduction zone, extension can occur behind the volcanic arc to open the back-arc basin.

24 Question 6. Refer back to the plate map in Figure 2.2, A. Locate several active continental margins. Which boundary-type (convergent, divergent, or transform) most often characterizes them? B. What of the following is true about the type of crust found at convergent plate boundaries? option 1. Most convergent boundaries involve continent-continent collisions. option 2. Most convergent boundaries involve continent-ocean collisions. option 3. Most convergent boundaries involve ocean-ocean collisions. Question 7. Do you know what feature forms when two continents collide? (Hint: Look at the India and Eurasian Plates.) Hot Spots: Tracking Tectonic Plate Movement By now you have probably noticed that most volcanoes occur at plate boundaries due to subduction (convergent) or seafloor spreading (divergent). You may have also notice a number of volcanoes that do not coincide with plate boundaries. These interplate volcanic centers are hotspots , or volcanoes that are fed by anomalous hot regions in the mantle (Fig. 2.6A). These regions are occupied by a plastically deforming mass called a mantle plume , which originates at the core-mantle boundary and rises slowly through the mantle by convection. Thin plume tails stretch down to the core and rise to form a bulbous plume head against the base of the lithosphere, where the plume spreads out into a mushroom shape (Fig. 2.6B). Such plume heads are thought to have diameters between ~500 to ~1000 km. The direction of plate motion can be confirmed by the linear array of seamounts , or extinct volcanoes, that formed over the stationary mantle hot spot and are carried away in the direction the plate moves (Fig. 2.6A).

26 PART I: VISUALIZE AND EXPLORE TECTONICS OF THE SEAFLOOR USING GEOMAPAPP You will now use GeoMapApp to digitally produce topographic profiles of four tectonic settings to explore common tectonic features of the continents and seafloor. GeoMapApp is a mapping application that integrates an ever-expanding global topography database with a graphic-user- interface. GeoMapApp is a useful data exploration and visualization tool that was originally developed and currently maintained at NASA’s Lamont-Doherty Earth Observatory as part of the Marine Geoscience Data System. Getting started with GeoMapApp 1) Open GeoMapApp by double clicking the icon on your desktop. You should have version 3.3.9 or later. 2) At the “ Choose a Base Map Projection ” window, make sure the “ Mercator Selected ” map on the left is chosen ( Fig. 2.7 ). Figure 2.7 When opening GeoMapApp you must first chose a map projection. You should select the Mercator option. 3) As you work on this exercise, you may want to refer to Figure 2.8 for a guide to useful tools and functions of GeoMapApp. Question 8. Click the ARROW tool, and simply hover (do not click) your cursor over any point/area on the map. What is the X-Y coordinate system that is displayed in the top toolbar and changes when you move the cursor in 2-D space?

27 coordinates & depth will be listed here GRID DATA DISTANCE/PROFILE TOOL ZOOM +/- TOOLS ARROW TOOL Figure 2.8 Tools you will use in GeoMapApp version 3.3.9.

29 THE ATLANTIC OCEAN BASIN You are now ready to make a topographic profile of the Atlantic Ocean Basin from the east coast of North America to the west coast of Africa. 8) Click the DISTANCE/PROFILE TOOL . Click and drag a line across the area of the map shown (Fig. 2.10) and look at the resultant profile plot. Move your cursor and explore the screen images. Relate what you see on the profile with what you see on the map. Everyone should notice: If you put your cursor on the profile, a dot will appear on the corresponding position on the map and vice versa. Your instructor will provide printed versions of the topographic profile and map-view that you can use to complete the lab. You may want to resize the profile plots on your computer screen to match the printed version. Question 11. Consider the scale of the map and profile… A. Read the Y-axis on the profile to determine the following. Do not forget the units. Maximum elevation: __________________ Minimum elevation: __________________ Total vertical relief: __________________ Figure 2.10 Zoom in on the eastern North American margin to create a digital topographic profile for that region. Be sure to zoom in at a scale similar to the one shown, although this does not have to be exact.

30 B. Read the X-axis on the profile to determine the horizontal distance of the topographic profile? Don’t forget the units. Horizontal distance: ____________________ C. Which is greater, the vertical relief of the profile or the horizontal distance that it crosses? Is it just “ alittle bit ” greater, or by many orders of magnitude ? Topographic profiles of ocean basins usually have a vertical exaggeration of the y-axis, meaning the y-axis has been “stretched” so that “relatively small” vertical features of the seafloor can be shown over such large horizontal distances. Question 12. Calculate the % slope of the continental slope. A. First, determine the rise and run (be sure to convert to the same units) of the continental slope. Rise ______________ Run ______________ B. Now calculate the % slope as (rise/run) x 100. Show your work. C. How steep does the slope actually appear in the profile? D. Why does the slope appear steeper in the profile?

32 Question 17. Based on your previous answers… A. Is the eastern edge of the North American continent on an active or passive margin? B. What feature marks the eastern edge of the North American tectonic plate (not continent)? C. What type of plate boundary occurs on the eastern edge of the North American plate (convergent, divergent, transform)? THE JAPAN ISLAND ARC 9) Use the scroll bars, and/or magnifying glass to navigate and zoom in on Japan in the western Pacific Ocean (Fig. 2.11). Be sure to zoom in at a scale similar to the one shown, although this does not have to be exact. 10) Click the DISTANCE/PROFILE TOOL . Click and drag a line across the area of the map shown (Fig. 2.11) and look at the resultant profile plot. Figure 2.11 Zoom in on the Japan to create a digital topographic profile for that region. Be sure to zoom in at a scale similar to the one shown, although this does not have to be exact.

33 Question 18. Calculate the vertical exaggeration of the profile plot. Recall what you did for Question 8. Show your work. Question 19. On the printed profile, draw brackets &/or arrows and label: A. Volcanic Island Arc B. Trench C. Back-arc basin D. Any seamounts? Question 20. At the top of the profile, draw brackets to show the extent of the following. Do not forget to label the brackets. A. The Pacific plate B. The Eurasian plate C. Continental crust (note it is helpful to look at the colored map). D. Oceanic crust Question 21. Consider the continental margin on the east side of the Volcanic Arc... A. Find, circle and label the continental shelf. B. Is the shelf bigger or smaller on an active margin (compare your Japan profile to the Atlantic Basin profile? C. What feature has “replaced” the slope and rise? Question 22. On the map… A. Trace and label the trench (note this define/coincides with the plate boundaries). B. Label which plate is “over-riding” and which one is “being subducted”. C. Draw arrows (  --  ) to show the convergent direction of movement between both plates. D. Extension does occur locally in the back-arc basin. Based on the reading, explain in your own words why this basin forms? E. Draw two arrows (  --  ) on the map to show where this local extension occurs.

35 Question 25. On the profile… A. Label the islands that extend above sea level. B. Label the seamounts (the islands that do not extend above sea level). C. Label which island/seamount is the youngest and which is the oldest . Question 26. Postulate two reasons that volcanic seamounts do not extend above sea level (Hint: what can happen to make seamounts “not that tall” or worn- away over time?) Question 27. On the map… A. Draw an arrow to show the direction of movement of the Pacific Plate. B. Notice the bend in the Hawaiian-Emperor Chain on the map. Why do you think this bend occurs? C. In what direction was the Pacific Plate moving to form the Emperor seamounts?

36 PART II: HOW QUICKLY DO PLATES MOVE? Use Figure 2.13, which shows the age of the seafloor in million years, to investigate how quickly plates move. Question 28. Find the 2 stripes on Figure 2.13 that are labeled with an 83.5. A. Where was this ocean crust created 83.5 million years ago? B. Draw a line between points A and B with a ruler. C. Using the scale bar, measure the distance along line A-B. This is the distance that the North America and African Plate have moved away from each other in 83.5 million years. Distance = ________km D. You can now calculate the seafloor spreading rate in kilometers per million years (My). This is essentially the velocity that North America and Africa have moved away from each other. Using V=D/T or velocity = distance ÷ time. Show your calculations. Seafloor spreading rate = ____________km/My E. It is difficult to imagine kilometers per million years. Convert the sea floor spreading rate you determined above to units that are “easier to imagine”, such as centimeters per year . Show your calculations. 1km = 1000m, 1m = 100cm, 1 My = 1,000,000 years Seafloor spreading rate= ________________cm/yr