What's THAT Inside our Earth?

The following lesson on inferring the interior structure of the Earth might be best suited in conjunction with (or before) a unit in plate tectonics, or prior to a unit including earthquakes.

This lesson also provides a good example of how models change over time as our understanding of nature improves with more detailed observations.

Students work in teams to investigate two possible models of the Earth, a simple, homogeneous interior and a layered Earth. They will use calculations and create a graph from data gathered from a model of Earth’s interior and using data from the Rapid Earthquake Viewer. Students will then use the graph to generate a model of the interior structure of the Earth. Students gather data directly from seismograms, organize the data into tables, create and interpret a complex graph of the data, and apply that interpretation to inferring a model of the interior structure of the Earth.

Note: This activity is adapted from Earth’s Interior Structure - Seismic Travel Times in a Constant Velocity Sphere. Copyright 2000. L. Braile. Permission granted for reproduction for non-commercial uses.
Additional acknowledgement for assistance in developing this activity to Michael Hubenthal and Sandra Laursen.

Materials / Preparation

Computers for half the students (or pre-printed seismographs as described below).
or, LCD projector if implementing as a whole class demonstration

Materials for Theoreticians:

Large piece of paper: 11” x 17” to draw or print out the semicircle diagram a 1:20 million scale model of a cross section through the Earth. Print onto 11 x 17 inch paper to make a half circle with radius of 20 cm. Be sure that the tic mark identifying the center of the half circle is visible or print out each half and tape together left semicircle, right semicircle.
metric ruler (capable of measuring 40 cm length)
calculator
pencil or pen
compass (drafting instrument) or ~30 cm long piece of string
protractor
scotch tape
print out illustrating a geocentric angle
blank graph paper
Student theoretician worksheet

Materials for Seismologists:

protractor
compass (drafting instrument) or ~30 cm long piece of string metric ruler
calculator
blank graph paper
Student seismologist worksheet

If you will be working offline, you will need to print out seismograms from REV for the seismologist team and choose the earthquakes for them. To print a seismogram in REV, you’ll need to capture an image of the “record section” (the blue and green seismograms). If using a PC right click and choose “Print picture”; on a Mac click on the image, drag and drop it on the desktop and print that. Choose data from earthquakes of magnitude 6 or greater and select 15-20 stations that are distributed between 0 and 180 degrees from the epicenter, with an emphasis on the 80-120 degree interval. Use the default orientation (vertical or up/down).

Student Instructions for Part B (PDF)

Suggested: IRIS poster “Exploring the Earth Using Seismology”. The poster is available free of charge from IRIS. Alternatively a printable pdf is available.

The lesson can be taught without the poster; however, the poster contains diagrams and explanations that help immeasurably.

Additional resources that might be useful to show the class:

Working with the P-Wave and S-Wave Chart
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-170

Examples of P-Wave and S-Wave travel time graphs:
Example 1:
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-167
Example 2:
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-168
Example 3:
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-169

Earthquake Travel Times
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-171

Grouping

Students will work in teams of 4 with 2 assigned to play the role of 18th century theoretician and two assigned to play the role of modern-day seismologist.

Teacher Tips

It is not necessary to use data from just one earthquake. You may use distance/arrival time data from any earthquake and from multiple earthquakes.

To generate a complete data set, students will need to use data across a range of distances, from stations close to the epicenter up to 180 degrees. For both teams it will be helpful to plot additional data for location in the 80 to 120 degree range to create a smoother graph.

If students are not familiar with geocentric angles you might reference the REV glossary entry for “Distance” to provide a visual explanation of this concept.

Caution - graphs constructed using graphing software, such as MS Excel, may show a misrepresentation of the data. You must choose the “XY scatter” otherwise MS Excel will connect the data points together. Blank graph template

Procedures

Preparation

Have students draw a hemisphere on 11 x 17” paper, or print the template onto 11 x 17” paper. Be sure to print at 100% scale and check that the semi-circle radius measures 20 cm.

Introduction/Engage

Begin a discussion with students that reveals how the models of the interior structure of the Earth have changed over time.

Step 1: Begin with the question, “What did people think the shape of the Earth was 1,000 years ago?” Ask questions that direct student responses toward, “The Earth was flat.” Emphasize that this was the generally accepted scientific model and much evidence supported this view. Begin at the far left end of the white/chalk board with a diagram of what people thought the Earth looked like 1,000 years ago. Draw a diagram showing the Earth as flat, surrounded by water. Plan ahead on the usage of white/chalk board space – there will be five diagrams in all. Ask questions such as, “How did they know the Earth was flat and had an edge?” (Sailing ships would disappear over the horizon and often did not return; the horizon appeared flat). Point out to students that in all these cases the observations and evidence/data supported the model.

Step 2: Ask students what the model of the Earth was in the early 1500's. The flat Earth model persisted until the 1500's when Magellan sailed around the world. This led to a revision of the flat Earth model to a model of the Earth being round. As Magellan continued sailing to the west (more or less), his ship eventually returned to its starting point… though Magellan himself did not (he was killed in a conflict with natives in the Philippines). Discuss with students how the new model developed from the old model. Various religions or world views explained what the interior of the Earth contained, or what existed beyond the Earth, i.e., outer space.

Step 3: A new model emerged in the 1700’s with the notion that the Earth was made of rock, and outer space surrounds the Earth. Ask “How did the scientists know the Earth was filled with rock?” (Mines at the time had reached several hundred meters in depth, and in all cases, the mines penetrated into rock, therefore, the Earth must be filled with rock). This was also a time in history when supernatural explanations were set aside in favor of natural, or scientific, explanations. Draw a picture showing the Earth filled with solid rock.

Step 4: A new model emerged in 1906 when seismic waves were analyzed by Richard Oldham, an Irish geologist, and were found to have different characteristics when traveling through the Earth’s interior. What did he discover?

Note: This progression of models of Earth’s structure is an excellent example of how science is a dynamic venture with new information leading to new levels of understanding of the natural world. At each step, the model of the day DID match observations of the time.

Part A

Segue now to a role-playing activity:

A time warp has occurred during the annual seismology conference, transporting 18th century scientists (theoretical seismologists) to modern times. A debate breaks out between the early seismologists (theoretical seismologists or theoreticians) and the modern day seismologists about the structure of the interior of Earth. Students will take on the role of one of these groups and work in teams to explore this debate. How can each group model their assumptions and compare them to observed data?

Frame the activity to the students by asking a few of these questions:

What do you know about the interior of the Earth?

How do you know this? What tools do we have to explore and support this understanding?

Given what you know about seismic waves and earthquakes, how might these phenomena help us answer these questions?
What do you already know about this? What are your ideas?
If necessary, refresh the basics of seismic waves in general; that they radiate out equally in all directions from the epicenter of an earthquake and respond to changes in density with a change in speed or direction depending on the medium.

How can we use this to create a model that will predict the structure of the interior of Earth?

Explain the concept of geocentric angle: Draw a cut-away diagram of the Earth (that does not show the core and mantle) to demonstrate what is meant by “distance in degrees from the epicenter”. The REV glossary entry for “Distance” provides an example of this. Show students examples of recording stations at various degree distances from an epicenter, for example, 45 degrees distant, 90 degrees distant, etc. This introduces the concept of geocentric angles and measuring distance on a sphere using degrees of arc.

Working in teams of 4, two students will act as theoretical seismologists from the 18th century and two students will play the role of current seismologists. Each pair will be making a prediction about the interior of the Earth using different assumptions and will then compare and analyze the data jointly.

Explain to the students:

1A. The theoreticians will develop a simple model of planet Earth that assumes that Earth is made of a single type of material all the way through (a “homogeneous Earth model”). With a homogeneous Earth, all seismic waves will travel at the same speed through this model Earth (11 km/sec). You will construct a geometric diagram of the Earth’s interior and predict earthquake travel times to different seismic stations around the world, based on this model.

1B. The seismologists will use real earthquake data to measure the actual travel times of seismic waves to different points on the globe. You will construct a table of your data and create a diagram showing the earthquake(s) and stations based on geocentric angles.

2. Once both the theoreticians and seismologists have completed their portions of the activity, you’ll get together with your other teammates and explain your findings to the other team members and explain how you arrived at your conclusions.

3. Each team will create a single shared graph that compares earthquake travel times (y axis) as a function of geocentric angle (x axis) for both the modeled view of Earth and the observed data of Earth.

Ask: What predictions can we make about each approach? What data will we need to test the model?

Note: The worksheets (seismologist) (theoretician) include tables for students to fill in and have the necessary headings but not the angles or earthquake stations. This is done deliberately to encourage students to be ‘minds on’ by making them determine which geocentric angles and/or earthquake stations to use; this simulates real science. By not providing the exact angles, students need to think about why they are choosing each geocentric angle and earthquake station.

Note about graphing: Ideally students will create the graphs starting with blank graph paper, choosing the scale for the x (geocentric angle) and y (time) axis as a team. If this is too difficult to do or if time is running short, graph paper that has been set up with the axes is provided.

Analysis

The final graph has two sets of data; data from the theoreticians and data from the seismologists. The line from the modeled homogeneous Earth is a smooth curve (see Figure 1) The lines drawn through the seismologists data should show an anomaly in the data trend at about 105 to 110 degrees. A roughly linear relationship is apparent on stations from 0 degrees to 110 degrees. A separate line should be visible roughly five minutes above the first line, starting at about 110 degrees and continuing to 180 degrees. This second line shows that the P wave is delayed for some reason. It is important to emphasize that something different is happening to seismic waves on either side of the anomaly.

Student data is likely to be less accurate but hopefully will still show a pattern that is different from that of the theoreticians’ depiction of seismic waves constant rate of travel through the Earth.

Figure 1. Travel times of observed and calculated P waves.

Have the teams report to the class what they have discovered. You want them to work towards the idea that there is an anomaly around 110 degrees and this anomaly is caused by something in the interior of the Earth.

4. Lead students into the next section where they draw the mantle / core boundary.

Ask if the modeled data and the observed data agree? Are there places where they significantly differ?

Focus for a moment on the observed data plotted by the seismologists: What shape was the curve? Was it a single smooth curve?

Ask students what might explain the anomaly and what implications or inferences can be drawn from it.

Other questions to ask:

What inferences can you make about structure of Earth’s interior from this comparison? Frame your inferences in terms of the speed of seismic waves.

Now take your inferences a step further: What might cause the seismic wave patterns that you see? Come up with as many ideas as you can – write the ideas on the board.

If the Earth has a homogeneous interior and we selected a different velocity other than 11 km/s, could we obtain a better fit between the observed and modeled travel times? Why or why not?

5. (optional) So far both theoreticians and seismologists have assumed that earthquake travel times depend only on geocentric angle and does not take into account the geology of where the earthquake originates, where it is detected, or what the waves pass through (such as the roots of mountains or ancient subducted slabs).

How do travel times from stations at the same geocentric angle compare? Were these from the same earthquake / different earthquakes?

How could you determine whether travel times vary depending on the specific location of the earthquake, the detecting seismograph stations, and what lies between them?

Part B

In Part B, students create a model to demonstrate shadow zones in order to develop the conclusion that the Earth has a core made of material with a density greater than the mantle. See student instructions Part B

Step 1: Students will need two large pieces of paper for this part. One piece will be used to create a shadow zone template; the other will be used to draw the shadow zone.

Using a compass or on graph paper have students draw a circle 20 cm in diameter to represent a cross-section of the Earth. Mark the center of the circle to represent the center of the Earth (C). Make an arbitrary mark on the circle to represent an earthquake epicenter (E).

Step 2: With the center of the circle (C) as a vertex, use a protractor to measure an angle of 110 degrees with one leg of the angle intersecting the epicenter (E). Mark on the circle the other leg of the 1100 angle, beginning of the shadow zone (S). Draw lines connecting all three points: E, C, and S. See Figure 2 below.

Figure 2 - printable diagram

Step 3: Students draw a mirror image using the same location for the center of Earth and earthquake (E), and using a 110 degree angle in the lower portion of the circle (Figure 3).

Figure 3 - printable diagram

Note:  We observed in Part A of the activity that on the surface of the Earth, recording stations between 0 and 110 degrees receive direct P waves (lower line on graph from Part A).  Recording stations along the arc in blue receive P waves later than expected because they are diffracted by some ‘phenomenon’ in the Earth’s interior (upper line on graph from Part A).

Step 4:  Have students cut out the area contained within blue lines in Figure 3.  This cone represents the raypaths from an earthquake through the Earth at geocentric angle of 110 degrees, where P waves showed some interference during their journey through the Earth. 

Step 5:  Students place the vertex of the wedge shaped cut-out (from Step 4) on a new circle representing the Earth with 20cm radius.  Align the curved arc of the wedge with the opposite side of the circle.  The point on the cone depicts an earthquake epicenter. Students trace the straight edges ES and ES1 cutting through the circle (Earth) Repeat this procedure until an outline of a circle inside the Earth emerges.  Each time the wedge is placed and lines are drawn represents an earthquake.  (Figure 4). 

Figure 4 - printable diagram

Ask the students what they think this new inner circle is?
Note: This emerging circle represents the mantle-core boundary. P waves traveling through the core are diffracted and the seismograms show evidence of some change as they travel through the Earth’s interior.

Note: The circle is known as the “shadow zone”. This name is derived from the observation that seismic waves are deflected away from the Earth’s core due to its high density. S waves do not pass through liquids, such as the outer core and consequently also do not show up on seismograms from that portion of the Earth for a given earthquake.

Step 6: Knowing the radius of the Earth, (6371 km) and the scale of their Earth model students can estimate the radius of the mantle-core boundary.

Step 7: Show students the poster “Exploring the Earth Using Seismology” to review the concepts learned in this activity.

Thought question: Is it possible that the current model may have further refinements?

Additional reading: Earth's Interior

Assessment

At least two modes of formative assessment are available in this activity.

1. Students can be assessed on the correct construction of their data tables.
2. Students can be assessed on the correct construction of their graph.

Quality tables and graphs will have units for numbers and labels of the graph axes and lines

For a summative assessment, teachers can use the answers students generated in Step 6 above: they estimate for the radius of the mantle–core boundary.

Extension

The radius of the mantle/core boundary can also be calculated using trigonometry. See an explanation at Earth's Interior.

The Seismic Waves program by Alan Jones provides a very good overview of the internal structure of the Earth. It may be useful to explore the Seismic Waves program after doing this activity with students.

In this program students select from a list of major earthquakes and then watch a visualization of the progression of each type of seismic wave along with the recording on the seismogram. In order to use this site, you must download and install it. This is a large (4.8MB) file and takes a few moments to download, but it is worth it. Also, this program only works for Windows machines (not Mac).

CAUTION: For security reasons, some schools require that the teacher obtain permission before downloading programs to school computers. For more information and the readme file about the program, visit Alan Jones' website.