Page 80 - Laboratory Manual in Physical Geology

B. Notice line B–B’ on the map in Part A and the fact that shallow, intermediate, and deep earthquakes occur along it. Volcanoes
also occur at Earth’s surface along this line. Plot the locations of earthquake foci (depth of earthquake vs. its location east or
west of the trench) on the cross section below using data in the accompanying table (provided by the U.S. Geological Survey).
For volcanoes, draw a small triangle on the surface (depth of zero).
Location East Depth of Location East Depth of Location East Depth of
or West of Earthquake (or or West of Earthquake (or or West of Earthquake (or
Trench volcano location) Trench volcano location) Trench volcano location)
200 km West 20 km 220 km East 30 km 410 km East 150 km
160 km West 25 km 250 km East volcano 450 km East 50 km
60 km West 10 km 260 km East 120 km 450 km East 150 km
30 km West 25 km 300 km East volcano 470 km East 180 km
0 (trench) 20 km 300 km East 110 km 500 km East 30 km
10 km East 40 km 330 km East volcano 500 km East 160 km
20 km East 30 km 330 km East 40 km 500 km East 180 km
50 km East 60 km 330 km East 120 km 540 km East 30 km
51 km East 10 km 350 km East volcano 590 km East 20 km
55 km East 30 km 390 km East volcano 640 km East 10 km
60 km East 20 km 390 km East 40 km 710 km East 30 km
80 km East 70 km 390 km East 140 km 780 km East 530 km
100 km East 10 km 410 km East volcano 800 km East 560 km
120 km East 80 km 410 km East 25 km 820 km East 610 km
200 km East 110 km 410 km East 110 km 880 km East 620 km

B West Distance East or West of Trench (km) East B´
200 100 Trench 100 200 300 400 500 600 700 800 900

0

100
Depth of Earthquake (km) 200

300

400

500

600

700

1. What kind of plate boundary is shown in your cross section? _________________________

2. Draw a line in the cross section to show the probable top surface of the subducting plate.

3. Label the part of your cross section that probably represents earthquakes in the lithosphere.
4. At what depth does magma probably originate here just above the subducting plate: __________________________ km

How can you tell?

5. REFLECT & DISCUSS What is the deepest earthquake plotted on your cross section? Do you think there is a lower
limit below which earthquakes are not likely to occur? Explain your answer. (Hint: Think about how pressure and
temperature influence the behavior of rock in the upper mantle).

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