Year 9 Science: Plate tectonics
What happens in a subduction zone ?
Investigation:
Open the Rapid Earthquake Viewer (REV) to the Earthquake View page
( http://rev.seis.sc.edu/earthquakes.html ).
The circles on the map indicate the location of earthquakes during the
last year. Compare them to the following map that shows the major
plate boundaries of the earth’s crust:
Are there any similarities?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
The small pink dots on the map show many of the historically
recorded earthquakes.
How do these compare to the map of plate boundaries?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
Year 9 Science: Plate tectonics
Click on the earthquakes along the western coast of South America
south of 15° S latitude and north of 35° S latitude.
Using the information in the Earthquake Info box, record the latitude
(first number in Location), longitude (second number), depth, and
magnitude in the data table below. Record data for up to five
earthquakes but don't worry if there are only one or two.
Latitude
Longitude
Depth
Magnitude
The following data table lists information for other earthquakes along
the same portion of the South American and Nazca plate boundary.
Latitude
-23.06
Longitude
-70.59
Depth
Magnitude
16.8
6.4
-29.45
-71.72
27
6.6
-23.17
-70.12
36
6.4
-24.87
-70.89
58.2
6.3
-16.09
-73.99
62
6.6
-27.54
-70.59
62
6.7
-29.67
-71.38
71
6.4
87.2
6.8
-16.66
-72.66
-21.75
-68.78
101.2
6
-15.35
-72.96
110.5
6
-19.99
-69.2
115.5
7.8
Year 9 Science: Plate tectonics
-21.66
-68.33
125.1
6.5
-18.3
-69.53
133.8
6.2
-16.12
-71.4
136.2
6.1
-23.37
-67.69
147.4
6
-22.32
-67.89
162.6
6.9
-24.22
-67
196.2
6.1
-20.93
-67.28
218
6.4
-23.54
-66.45
225
7.2
- 66.3
256.2
6.1
-65.72
276.2
7.1
-22.98
-22
Using both sets of data, construct a plot of the plate boundary to look at
the depths of the earthquakes compared to their distance from the plate
boundary. Since this plate boundary runs approximately north-south,
we are interested in the longitude of each earthquake.
From the above map, you can see that the continental margin of western
South America is approximately parallel to the lines of longitude.
(The plate boundary is located offshore from the continent.)
Year 9 Science: Plate tectonics
Your task is to construct a graph, using paper and pencil or a
computer program, which shows the relationship between the
longitude of the earthquakes and their depth.
Suggest a reason for any pattern that you can see in the data:
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
Some helpful hints for your plot:
Remember you are plotting depth! The numbers you have been given
are the number of kilometers below the surface of the Earth where the
earthquake occurred. Plot depth on the y-axis of your graph and we
want the surface of the Earth up, and the interior of the Earth
down.
The numbers for longitude are all negative, or west of the prime
meridian. This means that the zero line is to the east. Think carefully
about what these values mean. On your plot, you want west to be on the
left side of the x-axis and east to be on the right side of the x-axis.
Pick a scale for your plot that does not have any vertical exaggeration.
This means that you want 1 km on the x-axis to be the same distance as
1 km on the y-axis.
Fortuitously, at 25° one degree of latitude is approx. 100 km.
Year 9 Science: Plate tectonics
Follow-up questions
1.
Make a hypothesis of what you think the graph of the earthquakes occurring
along the Nazca-South America plate boundary will look like in cross-section.
Describe and draw a sketch of the plot below. What pieces of evidence did you
use to make your hypothesis?
2.
How does the graph you sketched in #1 above compare with the graph you created using
the data?
3.
At the western portion of your plot, at what depths do earthquakes occur? What is the
longitude of this region?
4.
As you move to the east, what happens to the depth of the earthquakes?
5.
If you notice any changes occurring in earthquake depth, at what longitude do they
occur?
6.
What does the zero depth line on the graph represent?
7.
Earthquakes usually occur along plate boundaries. What type of plate boundary does the
one on the west edge of South America represent?
8.
If you could follow a point on the Nazca Plate at the surface where it meets the South
American Plate, where would it be in 1 million years? (What is happening to the Nazca
plate over time?)
9.
Label the plates on your graph. Draw a cross section of the area showing the direction the
plates are moving.
10.
Open the Voyager Junior map program (http://jules.unavco.org/VoyagerJr/Earth). Press
the CTRL key on PCs or the Command key on Macs to select multiple items drop-down
box: tectonic plates and earthquakes. Then click on the Make Changes button. This will
produce a map showing earthquake locations. Click on the Legend button to interpret
earthquake depths. Can you find any other plate boundaries similar to the one you
investigated?
11.
Is the plate boundary to the east of Australia the same type? How can you tell?
12.
Your friend states that the plate boundary on the west coast of North America is a
convergent margin with a subduction zone. Do you support this statement? Why or why
not?
13.
What type of plate boundary occurs in the middle of the Atlantic Ocean ? What evidence
can you list for this plate boundary type occurring here?
14.
Use the map found at http://atoc.colorado.edu/~toohey/fig_63.jpg to check your
predictions for plate boundary types.