Name ____________________________________________
Plate Tectonics, Convergent Boundaries
Purpose
The purpose of this lab is to have you use real earthquake data to determine the direction and angle of
subduction taking place at convergent boundaries.
Plate Boundaries Background
As you have already learned, there are three different types of plate boundaries.
The first type of plate boundary is the diverging boundary, also known as a spreading center, and is
exemplified by the East Pacific Rise in the Mid-Atlantic Ridge. Impelled by the movement of heated
mantle material upwelling from below the lithosphere, the older lithospheric rock spread apart and newly
generated volcanic material fills the fracture at the spreading center.
As new ocean floor rock is created at the mid ocean ridges, there must be a compensating destruction of
old ocean floor crust because no evidence suggests that the earth is constantly expanding. This
destruction of Earth's old crust appears to happen in the areas where it is subducted into ocean trenches.
A subduction zone is an area in which a cold slab of seafloor is forced back into the mantle beneath
another plate. Plates are moving toward each other at this, the second type of plate boundary, known as
a converging boundary. Geothermal heat and friction increase the temperature of the down-going plate
and create magma, which erupts at the surface as a chain of andesitic volcanic islands or andesitic
volcanoes along a continental margin. An example of this would be the Andes of South America.
Continental collisions produce mountain chains like the Himalayas where the continents are sutured
together. Earthquakes occur at various depths along the subducted plate.
The third common type of plate boundary is formed in areas where lithospheric plates slip horizontally
past each other; it is called a transform boundary, which follows a transform fault. Transform faults are
formed either in the ocean or on a continent by a system of large lithosphere fractures that laterally
connect to spreading ridges, to subduction zones, or a subduction zone and a spreading ridge. The
transform faults are developed as a consequence of differential spreading rates caused by the curvature
of the Earth's spherical surface. The San Andreas Fault system in California is an example of a transform
fault connecting the East Pacific Rise in the Gulf of California to the Juan de Fuca Rise off the coast of
Oregon and Washington.
I hereby declare upon my word of honor that I have neither given nor received any unauthorized aid on
this work.
______________________________________________________
Name ____________________________________________
2013
Converging Boundaries and Subduction Zones
As stated previously, if ocean floors are spreading in areas of mid-ocean ridges, they must be converging
in other areas if the surface area of the earth is to stay constant. Apparently deep ocean trenches represent zones
of subduction where ocean-floor rock sinks into the mantle. One line of evidence for this theory lies in the
distribution of earthquakes of intermediate and deep focal depths. Earthquakes occur only in rigid crustal rock
material, not in plastic or molten mantle or core material.
Most earthquakes are the shallow focus type that occur within the upper 70 km of the earth, within the
lithosphere. Only near trenches do earthquakes of intermediate focal depth occur. Intermediate focal depth can be
defined as 70 to 350 km below the surface. This fact would indicate that something solid and rigid is being forced
deep into the mantle in these areas. Since the earthquakes are occurring within this subducted plate, it can be
profiled by locating earthquakes and their various focal depths relative to the position of the trench.
Data
Assume that you manage a seismic monitoring station set up in the Bonin Island group to the south and east of the
Izu Trench (see Figure 1). Over a week-long period, you record 24 significant earthquakes in the Izu Trench (along
cross section A-A’) and calculate their focal depths and relative distances from your recording station. These data
are compiled in the table below (Table 1).
EVENT
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
TABLE 1: DATA FOR IZU TRENCH EARTHQUAKES
DEPTH (KM)
RELATIVE DISTANCE
(KM)
390
400
32
120
295
350
190
290
540
530
54
200
90
220
215
340
32
150
630
520
230
310
150
240
13
80
680
570
335
390
460
460
70
190
10
50
400
440
230
480
110
260
55
165
58
190
605
520
Name ____________________________________________
2013
Questions
1. Using the graph paper provided, plot the data points from Table 1. When plotting, use the
appropriate symbol for each of the earthquake categories. (See symbols on the graph paper)
2. Group the graph points as shallow, intermediate, and deep-focus earthquakes (kind of already
done for you). Then interpret the data by answering the following questions.
a. The graph plot shows the angle of descent for the downgoing (subducting) plate. What
is the approximate angle for sinking slab in this case?
b. Which plates are involved in this convergent boundary? Which is the upper plate?
Which is the subducting plate? Could the upper plate just as easily have been the
subducting plate? Why or why not?
3. The relationship of the spacing between shallow, intermediate, and deep-focus earthquakes is
similar to the rules for isobars (on a weather map) or contour lines (on a topographic map). The
closer together they are, the steeper the slope of the angle of descent, and vice versa. So, the
further spread apart the deep focus earthquakes are from the intermediate and the shallow
focus earthquakes, the shallower the angle of descent.
a. Using these relationships, analyze the cross sections B-B’ (the Mariana Trench) and C-C’
(the Japan Trench) on the map provided.
i. Compare the angle of descent of the plate at the Izu Trench (where all of your
data is from) with the angle at the Mariana and Japan Trenches. Which is
steepest? Which is shallowest?
3