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EXERCISE 2.9
Estimating the Steepness of Subduction Zones (continued)
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The boundaries of the Wadati-Benioff 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? guns (50
. km
(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
Amchitka
Shumagin Islands
Cook Inlet
Skwentna
Arc-trench gap
Subduction angle
(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?
2.5.4 Transform Faults
We have thus far looked at transform faults only in the oceans-the oceanic frac-
ture zones-but some transform faults also cut continental lithosphere. The most
famous of these faults constitutes the San Andreas fault system of California, which
has caused major damage and loss of life over the past 100 years. The San Andreas
fault system (shown in Exercise 2.10) is not a single fault, but rather a zone several
miles wide containing numerous active transform faults. It extends for more than
1,000 km, connecting segments of the Juan de Fuca Ridge and Cascade Trench at
its northern end to an unnamed mid-ocean ridge segment in the Gulf of California
to the south.
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEATURES
43

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No two subduction zones are identical. Their differences may include the width
of the accretionary prism, the rate of subduction, and the steepness of the sub-
ducted plate as it moves into the mantle. Earthquakes occur in the subducted plate
as it moves, concentrated in what geologists call a Wadati-Benioff zone. We can track
the plate as it is subducted by the depths and locations of earthquake foci (points
where energy is released).
FIGURE 2.12 Anatomy of an island arc-trench system.
Arc trench gap
Volcanic island arc
Accretionary
prism
Trench
Fore-arc
basin
Oceananic lithosphere
Asthenosphere
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEA

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