FIGURE 2.12 Anatomy of an island arc-trench system. Arc trench gap Volcanic island arc Fore-arc basin...

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FIGURE 2.12 Anatomy of an island arc-trench system. Arc trench gap Volcanic island arc Fore-arc basin Accretionary prism Trench Oceananic lithosphere Asthenosphere EXERCISE 2.9 Estimating the Steepness of Subduction Zones Name: Course: (a) Based on Island arc-trench geometry (see Fig 2.12) and the fact that melting typically occurs at about the same depth in all subduction zones, what is the major factor that controls the width of the arc-trench gap? Explain. (b) Based on your answer to question (a), sketch two Island arc-trench systems, one with a wider arc-trench gap than the other. Depth (km) Narrow arc-trench gap Wide arc-trench gap The Aleutian arc extends westward from mainland Alaska into the Pacific Ocean. Profiles across four segments of the Aleutian arc are given in the graphs that follow, which show the positions of the volcanic arc and trench for each segment. Gray dots in the profile of the Amchitka segment show the earthquakes in the Wadatl-Benloff zone. Volcanic arc Volcanic arc Depth (km) 0 100 200 300 0 100 200 Even without any earthquake information, it is possible to estimate the steepness of a subduction zone from its anatomy. All you need is an accurate map of the sea- floor and a little geologic reasoning based on your knowledge of arc-trench systems. We will see how this is done in Exercise 2.9. 300 200 Volcanic arc 200 Trench Wadat-Benloff zone Amchitka segment 400 Distance (km) Cook Inlet segment 400 Distance (km) 600 600 Depth of melting Trench 800 800 Depth (km) Depth (km) 100 200 300 100 Section: Date: 0 200 300 Shumagin Islands segment 400 Distance (km) 200 Volcanic arc Trench 200 Skwentna segment 400 Distance (km) 600 600 800 Trench 800 (continued) EXERCISE 2.9 Estimating the Steepness of Subduction Zones (continued) Name: Course: Section: Date: The boundaries of the Wadati-Benloff zone track the progress of the subducted slab of lithosphere into the mantle, and the Island arc sits directly above the point at which the mantle above the subducted plate begins to melt. (c) At what depth does melting apparently occur beneath the Amchitka segment of the arc? (d) How does the point at which melting occurs control the distance of the volcanic arc from the trench? (e) Sketch the outlines of the subducted plate for each segment on its profile. Hints: • Assume that melting occurs at the same depth In each segment. • Draw a vertical line from the volcanic arc to the melting depth. • Draw a horizontal line to represent the melting depth. • Draw the subducted slab so that its upper surface passes through the intersection point. •Using your protractor, measure the angle of subduction downward from the horizontal. (f) For each segment, record the following data in the table provided: • The width of the arc-trench gap •The angle of subduction Arc-trench gap Subduction angle Amchitka Shumagin Islands Cook Inlet Skwentna (g) Based on these data, what is the relationship between the steepness of subduction and the width of the gap between the volcanic arc and the trench? km FIGURE 2.12 Anatomy of an island arc-trench system. Arc trench gap Volcanic island arc Fore-arc basin Accretionary prism Trench Oceananic lithosphere Asthenosphere EXERCISE 2.9 Estimating the Steepness of Subduction Zones Name: Course: (a) Based on Island arc-trench geometry (see Fig 2.12) and the fact that melting typically occurs at about the same depth in all subduction zones, what is the major factor that controls the width of the arc-trench gap? Explain. (b) Based on your answer to question (a), sketch two Island arc-trench systems, one with a wider arc-trench gap than the other. Depth (km) Narrow arc-trench gap Wide arc-trench gap The Aleutian arc extends westward from mainland Alaska into the Pacific Ocean. Profiles across four segments of the Aleutian arc are given in the graphs that follow, which show the positions of the volcanic arc and trench for each segment. Gray dots in the profile of the Amchitka segment show the earthquakes in the Wadatl-Benloff zone. Volcanic arc Volcanic arc Depth (km) 0 100 200 300 0 100 200 Even without any earthquake information, it is possible to estimate the steepness of a subduction zone from its anatomy. All you need is an accurate map of the sea- floor and a little geologic reasoning based on your knowledge of arc-trench systems. We will see how this is done in Exercise 2.9. 300 200 Volcanic arc 200 Trench Wadat-Benloff zone Amchitka segment 400 Distance (km) Cook Inlet segment 400 Distance (km) 600 600 Depth of melting Trench 800 800 Depth (km) Depth (km) 100 200 300 100 Section: Date: 0 200 300 Shumagin Islands segment 400 Distance (km) 200 Volcanic arc Trench 200 Skwentna segment 400 Distance (km) 600 600 800 Trench 800 (continued) EXERCISE 2.9 Estimating the Steepness of Subduction Zones (continued) Name: Course: Section: Date: The boundaries of the Wadati-Benloff zone track the progress of the subducted slab of lithosphere into the mantle, and the Island arc sits directly above the point at which the mantle above the subducted plate begins to melt. (c) At what depth does melting apparently occur beneath the Amchitka segment of the arc? (d) How does the point at which melting occurs control the distance of the volcanic arc from the trench? (e) Sketch the outlines of the subducted plate for each segment on its profile. Hints: • Assume that melting occurs at the same depth In each segment. • Draw a vertical line from the volcanic arc to the melting depth. • Draw a horizontal line to represent the melting depth. • Draw the subducted slab so that its upper surface passes through the intersection point. •Using your protractor, measure the angle of subduction downward from the horizontal. (f) For each segment, record the following data in the table provided: • The width of the arc-trench gap •The angle of subduction Arc-trench gap Subduction angle Amchitka Shumagin Islands Cook Inlet Skwentna (g) Based on these data, what is the relationship between the steepness of subduction and the width of the gap between the volcanic arc and the trench? km

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Solutions Step 1 Based on island arctrench geometry and the fact that melting typically occurs at about the same depth in all subduction zones what is the major facts that controls the width of the ar... View the full answer