Laboratory Manual for Introductory Geology 4e

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 seafloor
and a little geologic reasoning based on your knowledge of arc-trench systems.
We will see how this is done in Exercise 2.9.
EXERCISE 2.9
Estimating the Steepness of Subduction Zones
Name:
Course:
Section:
Date:
(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.
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 Wadati-Benioff zone.
0
Volcanic arc
0
Volcanic arc
Trench
100
Trench
100
Depth (km)
200
300
Wadati-Benioff
zone
Amchitka segment
Depth of
melting
Depth (km)
200
300
Shumagin Islands segment
0 200 400
Distance (km)
Volcanic arc
600 800
Trench
0 200 400
Distance (km)
Volcanic arc
600 800
Trench
0
0
100
100
Depth (km)
200
300
Cook Inlet segment
Depth (km)
200
300
Skwentna segment
0 200 400
Distance (km)
600 800
0 200 400
Distance (km)
600 800
(continued )
42 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS