Lab 9 Name Sec PLATE TECTONIC PROCESSES 1. Fill in the blank spaces on the chart with the correct answers. Refer to figures 2.3, 2.4 p.33 (2.2 and 2.3 on p. 23) as needed.
2. With your knowledge of different stresses and resultant strains outlined above, use the map of plate boundaries in Fig. 2.6 on p.36 (2.5 on p. 26)to explain why you think that the earth is expanding, shrinking or staying the same size over time. 3. In looking at both the plate boundary map in Fig. 2.6 on p 36(fig. 2.5 on p.26) and the tomography map in Fig. 2.7 on pg. 37 (Fig. 2.6 on p.27), to what plate areas do the hotter mantle areas correspond? To what plate areas do the cooler mantle areas correspond? Is there a correlation between crustal type and mantle temperature? Between plate boundaries and mantle temperature? Explain your observations. Use Fig. 2.8 on p.40 (2.7, p. 29) to answer the following questions about a mass of peridotite at 80km depth shown at point X. 1. Given the depth and temperature at point X, what is the geothermal gradient in this area? Following the continental geothermal gradient, rocks 80 km beneath a continent would be what temperature? Following the oceanic geothermal gradient, rocks 80 km beneath an ocean basin would be what temperature? 2. Is the peridotite at point X a mass of solid, a mixture of solid and liquid, or a mass of liquid? How do you know?
3. What would happen to the mass of peridotite if it were heated to 1750 C? How do you know? 4. What would happen to the mass of peridotite if it were heated to 2250 C? How do you know? 5. What confining pressure is the mass of peridotite under at point X? 6. At what depth and pressure will the mass of peridotite begin to melt if it moves towards the surface but the temperature remains the same? What is the name of this type of melting? 7. Name and describe a process that could uplift mantle peridotite and start to melt it. At what plate tectonic setting is this occurring right now? PLATE TECTONIC SETTINGS Use Fig. 2.13 p. 46 (2.12 on p. 35) to answer the following questions. 1. The San Andreas Fault is the boundary between the North American plate and the Pacific plate. What kind of plate boundary is it? 2. Given the displacement shown on the fault, what direction is each plate moving? North American: Pacific: 3. Note that the San Andreas Fault cuts across rocks that are Miocene in age (also see geologic time scale on p. xiv). Since the Miocene outcrops originally formed in one area, how much total displacement (in km) has occurred on the fault? 4. If those Miocene rocks are exactly 25 million years old, what is the average annual rate of fault movement in centimeters per year? 5. Do you think the average annual rate of movement you calculated in question 4 is representative of how the San Andreas Fault moves? Explain how you think the fault moves.
6. The San Andreas Fault moved 5m in the devastating 1906 San Francisco earthquake. How often have such destructive events occurred along the San Andreas Fault to account for the total displacement? If earthquakes occur in regular cycles and the number above represents the time between those major events, in what year can San Francisco expect the next earthquake of that magnitude? Use Fig. 2.12 p. 45 (2.11 on p. 33) to answer the following questions. 1. Given the track and ages of volcanic centers over the Yellowstone hot spot, what direction in the North American plate moving? 2. What is the average rate in centimeters per year that the North American plate has moved in the past 13.8 million years? 3. Based on the pattern in ages of the seafloor rocks shown in Figure 2.11, what type of plate boundary crosses line A-D at point B? 4. Again using the seafloor rock ages, what is the average annual rate of plate movement from both A-B and B-D in the past 8 million years? A-B: B-D: 5. On the following page is a vertically exaggerated cross section profile along the line A to D in Fig. 2.12 (2.11). Use the following data to plot earthquake hypocenters on the cross section. A. Medium-Large EQ s B. Small (Micro) EQ s C. Medium-Large EQ s Location [km from A on section] Depth [km below sea level] Location [km from A on section] Depth [km below sea level] Location [km from A on section] Depth [km below sea level] 495 10 1100 50 950 60 500 12 1050 30 780 27 505 11 1025 80 1025 80 497 11 1075 15 800 32 500 12 1120 40 1100 100 1090 75 850 40 1100 60 825 30 1075 70 900 45 1050 85 1000 75 915 65
6. Notice that seafloor rocks older than 8 million years are present west of point B in Fig. 2.12 (2.11), but not east of the red line that passes through point C. Based on this and your hypocenter data, what could be happening to the seafloor rocks from C to D? What kind of plate boundary is represented by the red line? 7. What do the hypocenters of group C represent on your cross section? Draw a best fit line connecting them on the cross section. 8. What types of faults did the earthquakes in group A likely occur on? 9. Note the line of volcanoes in the Cascade Range in Fig. 2.12.(2.11) A. At about what depth is the best fit line for the hypocenters of group C directly below those of group B and the volcanic peaks? B. What do you think the small magnitude earthquakes from group B represent? Why? 10. Let s assume that the melting point of basalt is 1300 C. Using Fig. 2.8 (2.7), if a mass of basalt on the seafloor is subducted to 100 km depth along the oceanic thermal gradient, would it heat up enough to melt? 11. Since the origin of the magma feeding the volcanic arc involves the partial melting of the subducting plate, what else besides the increase in temperature allows the slab to melt at that depth? What is that type of melting called?