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Educational Activity
Title: Rumbling Down Below!
Description:
This exercise lets students trace the descent of a subducting plate by mapping the depths of surrounding earthquakes.
Prerequisites:
If students have never drawn a cross-section plot you may want to have them first practice on our exercise entitled "A New View". Students should have a cursory understanding of the subduction process.
Objectives:
During this exercise, students will:
·
Read and interpret x, y, & depth coordinate maps.
· Calculate the x and y-axis range and draw a cross sectional graph
· Translate data from a x, y, & depth map into a cross sectional
graph.
· Approximate general trends in the data.
· Make inferences from information and data given.
· Read and interpret maps
Choose one of the following two formats:
a) Adobe PDF (easier to print) [446Kb]
b) JPG Graphic (choose "fit to page" when printing) [149 KB]
Procedure:
1. Give each students a blank copy of the above handout.
Explain that part A is a map the Mariana subduction zone with each coordinate
point being the epicenter (location) of an earthquake that occurred sometime
in the past one hundred years. Explain that the color coding describes
the depth of each earthquake and can be compared with the legend in the
top right corner. (Earthquakes have three dimensional epicenters. Some
are shallow and originate close to the surface, while others can originate
very deep within the earth's mantle, such as 600 meters below the surface).
Part B on the bottom, is a blank cross plot graph that the students will
have to graph from the data in part A.
2. Before plotting, discuss with the students the differences between the two types of plots. (What does the map's x/y axis represent compared to the cross plot's? How can you plot a point from the map onto the cross-plot? What information can not be included in the new plot? .etc)
3. Ask the students to plot all or most of the earthquakes. Explain that the x-axis measures the relative distance from the Subduction Zone (marked by the line). The students will first need to calculate and label the appropriate range for each axis. An example for the x-axis might be +400km to -75km at 25km interval tick marks, with 0km being set at the line of subduction.
HINT: It will be much easier for your students if they keep the same scale for the x-axis in both plots. The y-axis (depth) can be altered for maximum spread.
4. Once the students have finished plotting, have each student, or group of students show their results to the rest of the class.
5. Facilitate a discussion about their results. You may want to include the following topics:
· Is there any pattern to the earthquake plots? Do you see the pattern on Map 1, Map 2, or both?
· List advantages/disadvantages to map 1, and map2. List situations when you might want to use one over the other.
· What could have caused the earth quakes?
· What could the locations of these earthquakes tell us about the subducting plate?
· Why are there "irregularities" to the earthquake plots (occasional earthquakes that are shallower or deeper than their surrounding neighbors)?
6. Ask the students to plot a line that best fits the data (it can be
curvy) in the cross sectional data.
7. Discuss what this line might mean. Why do the earthquakes stop at a certain depth? Create a list of possible reasons why the earthquakes stop. The reasons do not necessarily have to be the correct ones. Instead, encourage your students to consider a wide range of possible explanations. Some reasons might be:
· the subducted plate stops;
· the subducted plate dissolves;
· the subducted plate continues but with less friction due to a change in its own or its surrounding material;
· the earthquakes are not related to the subducting plate at all.
· the earthquake waves do not reach the surface where the seismometers can read them;
· the seismometers themselves are not designed to register earthquakes at deeper depths;
· the chart people ran out of colors so they left out any deeper earthquakes
8. Explain that some of the questions the students are discussing are still under debate in the scientific community. Therefore it might be up to them to one day come up with the final answers!
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