Integrated Science 3
Name:____________________
Subduction Zones and Plate Tectonics
Date: _______ Per: ________
Background
For many years geologists have studied magma and lava chemistry to help understand the role of volcanoes in
forming the Earth’s crust. They have discovered that all volcanoes do not emit the same kind of lava. Some produce lavas
that form only basalt— a mafic rock, high in iron and magnesium. Others produce lavas that form andesite in addition to
basalt. Andesite, a felsic rock, contains more silica and is also less dense than basalt and other mafic rocks. It was also
observed that volcanoes producing only basalt are not found in the same locations as those which emit andesite.
Can plate tectonics theory be used to explain and/or predict this distribution of lavas? Laboratory studies have
shown that a basaltic plate containing a small amount of water can form andesitic magma as it melts. This occurs naturally
at subduction zones, where one plate dives beneath another. In the process, the more dense plate is exposed to increased
heat and pressure which causes it to begin melting. As the plate melts and combines chemically with sea water, it forms an
andesite magma. This magma is thick and high in pressurized gases. It rises to the surface, usually producing an explosive
volcanic eruption as the gases are released.
Other studies have shown that basalt can form from partial melting of mantle material. Such magma can also rise
to the surface to form volcanoes. However, since basaltic magma is less viscous (thick) than andesite, it retains less gas
and produces less explosive eruptions.
Thus, one would expect andesite volcanoes and explosive eruptions to be associated with subduction zones.
Similarly, purely basaltic volcanoes, and their less explosive eruptions, are associated with rift zone eruptions (and also
hot spots).
Focus Questions
● What are some important subduction zone volcanoes?
● What are some important rift zone volcanoes?
● Why are subduction zone and rift zone volcanoes different in lava composition?
● How and why are subduction zone and rift zone eruptions different?
Procedure
1. You have been provided with a map of the Pacific region. Each number on the map represents a volcano
active within recent geological time.
2. Use the data table, below, to find the type of lava for each volcano.
3. Place a light colored dot at each volcano that has andesitic lava. Place a dark colored dot at each volcano
that has only basaltic lava.
4. Draw a line from point ‘A’ to point ‘B’, which separates the basaltic volcanoes from the andesite
volcanoes. The line may curve. It should have all the basaltic volcanoes on one side and all andesite
volcanoes on the other. Geologists call this the “andesite line”.
5. Using information from other maps and handouts, carefully add important subduction zones/ trenches
and spreading centers/rift zones to your ma. Be sure your key clearly indicates which structure is which.
6. Use your knowledge of rift zones and subduction zones to add arrows which indicate the directions of
plate movement on your map.
7. Use your map to answer the appropriate Analysis and Conclusion questions.
Data Table
1. Mauna Loa-Basalt
2. Christmas IslandBasalt
3. Bagana-Andesite
4. Mt. LovingtonAndesite
5. Guam-Andesite
6. Saipan-Andesite
7. Truk-Basalt
8. Krakatoa-Andesite
9. Mayon-Andesite
10. Eniwetok-Basalt
11. Suribachi-Andesite
12. Wake Island-Basalt
13. Fuji-Andesite
14. Asama-Andesite
15. Tyatya-Andesite
16. KlyuchevskayaAndesite
17. Marcus Island-Basalt
18. Tenchi-Basalt
19. Bogosloff-Andesite
20. Kaymai-Andesite
21. Miller SeamountBasalt
22. Mt. Garibaldi-Andesite
23. Mt. Rainier-Andesite
24. Mt. Shasta-Andesite
25. Guadalupe-Basalt
26. Paricutin-Andesite
27. Monte Everman-Basalt
28. Atitlan-Andesite
29. Irazu-Andesite
30. Galapagos-Basalt
31. Chimborozo-Basalt
32. Elmisti-Andesite
33. San Pedro-Andesite
34. Aconcagua-Andesite
35. Juan Fernandez-Basalt
36. Puyehue-Andesite
37. Osorno-Andesite
38. Mt. Burney-Andesite
39. Easter Island-Basalt
40. Orohena-Basalt
41. Samoa-Basalt
42. Espiritu SantoAndesite
43. Mt. Marum-Andesite
44. Fiji-Andesite
45. Ruapehue-Andesite
8. A map of the Cascade Mountain region is provided below for your reference. Use available reference
sources to name the tectonic plate which is being subducted in the Pacific Northwest. Draw and label the
boundaries of this plate on the provided map. Include both the spreading center and subduction zone
associated with this tectonic plate.
9. Use this completed map to answer the appropriate Analysis and Conclusion questions.
20
21
19
16 18
15
22
23
24
14
13
17
1
2
9
11
28
12
6
57
8
25 26
27
3
4
10
00
43
42
29
41
30 31
40
44
32
33
34
39
45
35
36
37
38
A
B
Analysis and Conclusions
Pacific Rim and Basin Map
1. Describe the geological processes which occur at an oceanic trench?
2. Describe the geological processes which occur at a rift zone spreading center?
3. What is the relationship of ocean trenches and rift zones to the “andesite line”?
Cascade Mountains Map
4. Describe and explain a typical eruption in the Cascade Mountains. Be sure to discuss both cause and
effect.
5. Describe and explain a typical eruption along the spreading center of the Juan de Fuca plate. Be sure to
discuss both cause and effect.
6. Why is the Juan de Fuca plate so small? What will eventually happen to the plate? What will eventually
happen to the Cascade volcanoes?