Title - South Carolina Sea Grant Consortium · Web viewThe downward motion of the cardboard is similar to the motion of a thin slab of oceanic plate that slides, or subducts, beneath

Grade Level: 8th GradeSC Standards: 8th – IIIB3c,g,jNational Standards: Earth and Space Science - Content Standard D

Benioff BoxOriginal Source: Dr. Leslie Sautter, Department of Geology, College of Charleston

Focus Question How are locations of high seismic

activity related to patterns of the location and depth of earthquake origins (foci)?

ObjectivesStudents will:

plot on a map the distribution of earthquake epicenters for shallow, intermediate and deep-focus earthquakes.

demonstrate how the geophysicist, Hugo Benioff, identified locations on the earth where seismic activity (earthquakes) showed distinct patterns with respect to location and depth of earthquake origins.

Key Words bathymetry earthquake epicenter earthquake focus and foci

MaterialsPer student group:

shoe box (at least 9” X 6”) with lid photocopy of map (from attached sheet) 25 wooden (bamboo) skewers markers: red, green, blue Play-Doh: red, green, blue (optional) tape push pin (type with plastic spool on it) exacto knife or good scissors (optional) thin piece of cardboard measuring 8” x

8”

Teacher PreparationReview bathymetry of the seafloor and plate tectonics, including key terms so that you are familiar with these before the activity.

Time Frame50 to 75 minutes

Suggested Learning EnvironmentThe teacher may wish to construct one large model for the class to observe, or students should work in small collaborative groups of 3 or 4.

Relevant pages in Of Sand and SeaChapter I, The Ocean Planet, pp. 10-13.

COASTeam Program, Project Oceanica, College of Charleston1

Benioff BoxTEACHER PAGES (draft)

Procedure

The text below is identical to the text on the Student Pages. Additional information for the teacher is forthcoming!

1. Tape a copy of the attached map (Figure 1) onto the top of the shoe box. Make certain that the latitude and longitude marks are displayed.

2. Your teacher will discuss the bathymetry (the water depth variations) of the ocean floor depicted by the map.a. What seafloor features can be found?

Describe these features.b. Describe any land masses present.

3. Use Table 1 to plot the 25 earthquake epicenters on the map. Label each point using the “epicenter i.d. number” from column 1 of the table. Labeling the epicenters is important, as you’ll need to return to each point later. Use a blue marker to plot the epicenters for “shallow” focus earthquakes. With a green marker, plot the epicenters for “intermediate” focus earthquakes. Use a red marker for deep focus earthquake epicenters.a. Do you observe any pattern to the

distribution of color dots?

4. With a dashed blue line, mark the approximate boundary between the shallow and intermediate focus earthquakes. Use a green marker to mark the boundary between intermediate and deep focus earthquakes.

5. Compare the distribution of epicenters for the shallow, intermediate and deep focus earthquakes to the volcanic islands and seafloor features. Write down your observations. You are now doing what Hugo Benioff did in the early 1950s.

6. Use the push pin to poke a hole at each of the 25 epicenter locations on the map, poking through the shoe-box cardboard.

7. Use the scale bar provided to make a scale model of the foci depths. For each epicenter, measure and mark on a wooden skewer the focus depth, using a blue (shallow), green (intermediate), or red

(deep) pen. Mark all the way around the girth of the skewer to make a color ring. 0 100 200 300 400

Scale bar: (km)

8. Then, use the same marker to color the pointed tip of the skewer. This colored tip is where the earthquake focus is located – at the bottom of the skewer.

9. Carefully break the skewer approximately ½” above the color ring. Then, push the skewer through the appropriate numbered push pin hole on the shoebox map. Push the skewer into the box to the level of the measured depth (the color ring around the skewer).

10. Repeat the procedure in steps 7-9 for each of the 25 skewers. Do NOT lift the top of the shoe box until instructed.

11. When all of the skewers are in place (and before you lift the box top off!!), answer the following question: a. What do you predict will be the

arrangement of earthquake foci (i.e., the colored tips of the skewers) below the earth’s surface (inside the box) when you lift off the shoebox top?

12. Carefully lift the shoebox top off to reveal the third dimension! Compare what you observe to your prediction from step 11. Record these observations.

13. With the exacto knife or scissors, carefully cut along the line marked A – A’ on the map. Slide the cardboard along the skewer tips. This cardboard represents the “plane of earthquake focus depths.”

14. Answer the following thought questions:a. What might the motion of the

cardboard represent? b. Does this motion generate

earthquakes? c. At what seafloor feature does the

cardboard plane (the plane of foci) intersect the seafloor?

d. What role do you think the cardboard plane has in causing the earthquake foci?

The downward motion of the cardboard is similar to the motion of a thin slab of oceanic plate that slides, or subducts,

Benioff BoxTEACHER PAGES (draft)

beneath another plate. This region of subduction is where earthquakes are generated. We now refer to the plane of earthquake foci as a Benioff Zone. The top of the zone is defined on the seafloor by a trench. Earthquakes occur in much greater concentrations on one side of the trench – which tells us the location of the subducting slab of oceanic lithosphere (or plate).

Notice the volcanoes. They are commonly found parallel to deep ocean trenches. Volcanoes typically form on the same side of trenches as earthquakes. Why do volcanoes form? As the subducting slab of oceanic lithosphere descends it heats up, causing many minerals in its composition to melt. The melted, or molten, rock is less dense than surrounding rock, so it rises. Magma

may reach the surface through cracks and fissures in the lithosphere above, resulting in the formation of a volcano. Volcanoes are commonly found parallel to deep ocean trenches. Compare your model to the two figures of convergent plate boundaries (Figures 2 and 3). Determine where the “Benioff Zone” is located on EACH figure.

15. Write a paragraph summarizing the relationships among the Benioff Zone, the trench, and the volcanoes.

AssessmentUse the student’s ability to determine the “Benioff Zone” on the figure of a convergent plate boundary to assess their understanding of seismic activity and locations of earthquakes.

Source: Dr. Leslie Sautter, Project Oceanica, Dept. of Geology and Environmental Geosciences, College of Charleston, SC.website: http://oceanica.cofc.edu email: [email protected] phone: 843-953-5586

http://oceanica.cofc.edu/coasteam/

Figure 1. Contour map of the earthquake zone being studied. The land masses are volcanic islands.

Table 1. Earthquake focus depths for epicenters associated with Figure 1.

Epicenter I.D. Number

Latitude (degrees North)

Longitude (degrees

east)

Focus depth (km)

Shallow (S)Intermediate

(I)Deep (D)

1 28 30’ 5140’ 310 D2 27 50’ 52 20’ 250 I3 28 00’ 54 10’ 50 S4 27 10’ 51 20’ 390 D5 27 00’ 54 00’ 110 I6 27 20’ 55 00’ 45 S7 26 30’ 52 40’ 210 I8 25 40’ 51 00’ 400 D9 25 30’ 52 20’ 260 I

10 25 30’ 54 20’ 100 S11 25 20’ 53 30’ 160 I12 24 50’ 53 00’ 200 I13 24 10’ 51 50’ 320 D14 23 50’ 52 50’ 250 I15 24 30’ 54 40’ 50 S16 22 30’ 51 10’ 380 D17 22 50’ 52 10’ 290 I18 22 40’ 53 20’ 180 I19 23 00’ 54 00’ 40 S20 22 00’ 54 50’ 50 S21 21 20’ 51 50’ 350 D22 21 30’ 52 30’ 220 I23 20 30’ 51 00’ 370 D24 20 50’ 53 20’ 190 I25 20 40’ 54 10’ 100 S

Figure 2. (a) Cross-section diagram of an oceanic-oceanic convergent plate boundary. (b) The Benioff zone is the plane of shallow focus (blue solid dots), intermediate focus (green solid dots) and deep focus

(red solid dots) earthquakes that follows the surface of the subducting plate. The epicenters of these earthquakes (depicted as open circles in corresponding colors) are plotted on the map of the earth’s

surface, directly above the earthquake foci. (Figure from Of Sand and Sea, by P. Keener-Chavis and L. Sautter)

Figure 3. (a) Cross-section diagram of an oceanic-continental convergent plate boundary, and (b) the corresponding distribution of earthquake foci and epicenters. See Figure 2 caption for key to symbols.

(Figure from Of Sand and Sea, by P. Keener-Chavis and L. Sautter)