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# 4.9: Lab Exercise (Part D and E)

## Part D – Origin of Magma

Magma is formed from the melting of rock at both convergent and divergent boundaries. However, the processes that occur to melt the rock are quite different. Three different processes are involved in the melting of rocks as we will explore in the following exercise. In Figure 4.7 you can see a graph depicting a variety of temperature and pressure conditions. The increasing temperature with pressure on rocks as you go deeper within the earth through the crust and mantle lithosphere is called the geothermal gradient (shown in black). This gradient shows the actual temperature conditions that exist in the lithosphere. Obviously, the addition or subtraction of heat or pressure can move rocks off that gradient and cause potential change. The orange line represents the temperature and pressure required for a dry mantle rock to start to melt and any point to the right of this line is where melting of lithospheric rock can occur. The blue line represents the temperature and pressure required for a lithospheric rock to melt if water is present.

20. According to the geothermal gradient, rocks buried 75 km beneath the surface would normally be at what temperature? At 75 km depth, rocks will be heated to about _______ degrees Celsius.

a. 1500                         b. 1250                         c. 1000                         d. 750

21. According to the geothermal gradient, rocks at 500 degrees Celsius will be buried how deep? At 500 degrees Celsius, rocks will be buried to about _______ km depth.

a. 8                              b. 12.5                          c. 20                            d. 27

22. What is the physical state of the dry mantle rock at point X?

a. Completely melted     b. Starting to melt          c. Completely solid

23. What happens when the lithosphere at point X is heated to 1500 °C?

a. No change                b. Starts to crystallize      c. Starts to melt

24. At what depth will the dry mantle rock at point X begin to melt if it is uplifted closer to Earth’s surface and its temperature remains the same?

a. 35 km                      b. 25 km                        c. 18 km                       d. 12 km

25. What would happen to the mantle rock at point X if water is added to it?

a. No change                b. Starts to crystallize      c. Starts to melt

## Part E – Boundaries

Earthquakes are great indicators of plate boundaries and are associated with all three boundary types. One type of boundary is unique in having a Wadati-Benioff zone. Answer the following questions using Figure 4.8.

26. Which of the following places represent a Wadati-Benioff zone?

a. 10°S, 110°W                     b. 0°, 0°                 c. 15°S, 180°                 d. 30°N, 75°E

27. The Wadati-Benioff zone is associated with which type of plate boundary?

a. Divergent                         b. Convergent (Continent-Continent)                         c. Convergent (Continent-Ocean or Ocean-Ocean)                         d. Transform

Type 34 46 16.16 N 118 44 58.19 W into the search bar in Google Earth and zoom out to an eye altitude of 10 miles. Quail Lake is a dammed river that is sitting directly over top of the San Andres Fault, which is a well-known transform boundary with the North American Plate on the northern side and the Pacific Plate on the southern side. This boundary is running East-West in this area and you may be able to see the boundary better by zooming out.

28. Examine the path of the river that feeds into and flows out of Quail Lake. What direction is the North American plate moving in comparison to the Pacific Plate at this location?

a. East                                 b. West

29. Given that San Francisco is located on the North American Plate and Los Angles is located on the Pacific Plate, are these two cities getting closer together or farther apart over time?

a. Closer                              b. Farther

## Google Earth: Identifying Plate Boundaries

Reread the section on Plate Boundaries before answering the following questions.

30. Type “15 19 48.78 S 75 12 03.41 W” into the Google Earth Search bar. What type of tectonic plates are present?

a. Ocean-Ocean                         b. Ocean-Continent                         c. Continent-Continent

31. What type of plate tectonic boundary is present?

a. Transform                             b. Convergent                                c. Divergent

32. Type “6 21 49.68 S 29 35 37.87 E” into the Google Earth Search bar. What type of process is going on at this location?

a. Seafloor Spreading                 b. Continental Rifting                      c. Subduction

33. What features would you expect to occur at this type of boundary?

a. Earthquakes and a trench b. Volcanoes and a valley c. Mountains and landslides d. Earthquakes and offset rivers

34. Type “28 04 27.04N 86 55 26.84E” into the Google Earth search bar. What type of tectonic plates are present?

a. Ocean-Ocean                         b. Ocean-Continent                        c. Continent-Continent

35. What type of plate tectonic boundary is present?

a. Transform                             b. Convergent                                c. Divergent

36. Type “46 55 25.66 N 152 01 25.17 E” into the Google Earth Search bar. What type of tectonic plates are present? Make sure to zoom out to get a good view of the relevant features.

a. Ocean-Ocean                         b. Ocean-Continent                        c. Continent-Continent

37. What features would you expect to occur at this type of boundary?

a. Volcanos, Earthquakes, and a Trench                 b. Volcanoes and a Linear Valley                 c. Mountains and Landslides                 d. Earthquakes and Offset Rivers

38. Type “43 41 07.81 N 128 16 56.29 W” into the Google Earth search bar. What type of tectonic plates are present? Hint: Make sure to reread the section on plate boundaries before answering!

a. Ocean-Ocean                         b. Ocean-Continent                        c. Continent-Continent

39. What type of plate tectonic boundary is at this exact location?

a. Transform                             b. Convergent                                c. Divergent

40. This plate boundary isn’t as simple as the previous examples, meaning another nearby plate boundary directly influences it. Zoom out and examine the area, what other type of boundary is nearby?

a. Transform                             b. Convergent                                c. Divergent