Review Questions for Chapter 16:
Subduction-Related Igneous Activity, Part I: Island Arcs.
1. Sketch from memory a cross-section through a typical island arc, showing the major crustal and
plate tectonic features. Include a secondary arc.
2. What is h, and what is a reasonable average value for it? Label it on your sketch for Review
Question 1.
3. What is the major control on the horizontal distance from the trench to the volcanic front and the
width of the volcanic zone? Explain.
4. What is the most likely cause of back-arc spreading?
5. How does the average composition of arc magmas compare to MORBs and OIBs? How does the
range compare? Explain this in three or four concise sentences.
6. In what way is magmatism at a subduction zone a paradox?
7. What is the most likely solution to the paradox of magmatism at a subduction zone? Why is it more
likely in subduction zones that at mid-ocean ridges and intra-oceanic islands? On what basic
principles is the effect based?
8. Describe at least two mechanisms that might be responsible for the common development of calcalkaline andesites in the island-arc environment. In what way is H2O important to each?
9. Describe two systematic spatial variations and one temporal variation in island-arc magma
compositions.
10. What is meant by K2O(57.5) and FeO*/MgO(57.5) in Figure 16.5? Can these values be determined on
the basis of a single rock analysis? Why or why not? Estimate the values of each for the Guatemala
data in Figure 16.7.
11. What are LIL and HFS elements? In what way does the LIL/HFS ratio in arc magmas typically
differ from the ratio in MORBs and OIBs? What is probably responsible and why?
12. What evidence is there to indicate that a depleted MORB-source type of mantle is involved in
island-arc petrogenesis?
13. The EM (enriched mantle) signature in many OIBs may have been resident in the mantle source for
hundreds of millions of years. What isotopic evidence suggests that enriched components were
mixed with the depleted mantle component beneath island arcs relatively recently? Explain.
14. What plate tectonic variables produce the hottest subduction zones, and why?
15. Why were thermal models of subduction zones one of the main arguments against slab melting,
and why is the argument less strong today than it was in the past?
16. What are the two principal criteria for H2O-induced partial melting of an H2O-undersaturated rock
following some P-T-t path in a subduction zone? Explain your reasoning.
17. Recalling the difference between discontinuous and continuous reactions from Chapters 6 and 7,
why would the former type be applied to a dehydration reaction and why would it release H2O
more suddenly from the slab than the latter?
18. Pretend you are an OH– molecule in chlorite in altered subduction oceanic crust. Describe your
route from the beginning of your being subducted to your explosive eruption in an evolved calcalkaline dacite.
19. Describe a similar trip in which you end up in the deep mantle. What is a “choke point,” and under
what circumstances would you most likely escape one?