Chapter 16 Ans

Chapter 16 Ans - Answers for Chapter 16: Subduction-Related...

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Answers 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. See Figure 16.2 with the addition of a pod of arc crust behind the back-arc basin. 2. What is h , and what is a reasonable average value for it? Label it on your sketch for Review Question 1. h is the depth to the surface of the subducting slab directly beneath the volcanic arc front. Estimates vary, but a good average is about 110 km. 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. Given that h is relatively constant, the dip of the subducted slab exerts the principal control on the width of the arc-trench gap and of the volcanic zone. The shallower the dip, the greater the horizontal distance to the volcanic front before h = 110 km is attained. 4. What is the most likely cause of back-arc spreading? Traction between the subducting slab and the overlying wedge drags some of the wedge boundary down with the slab. This material loss must be compensated by flow from below and behind the arc, which causes a circulation cell in the wedge and hence back-arc upwelling and spreading. 1
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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. The average is more silicic than MORB-OIB and the range is also greater. This is attributed to far more complex multiple sources, the higher H 2 O content of the many sources, and to the thicker and more sialic crustal welt that can stall rising melts and allow for more fractionation. 6. In what way is magmatism at a subduction zone a paradox? The geothermal gradient is lower at subduction zones where a cool slab of lithosphere is introduced into the mantle, yet melting typically results, a phenomenon typically attributed to higher than average temperatures. 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? Although the temperature is indeed lower, more H 2 O is available, which, as we have seen, can dramatically lower the solidus temperature of rock systems. The melting point lowering is much greater than the geotherm lowering, so melting is indeed possible (inevitable may be more apt). 2
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8. Describe at least two mechanisms that might be responsible for the common development of calc-alkaline andesites in the island-arc environment. In what way is H 2 O important to each? High f
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Chapter 16 Ans - Answers for Chapter 16: Subduction-Related...

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