In this lesson, students examine seismic evidence to determine a) Earth must have a layered internal structure and, b) estimate the size of Earth’s core. Using an inquiry approach, students are divided into two teams (theoreticians and seismologists) to test the simplest hypothesis for what is inside of Earth; the Earth is homogeneous throughout. Theoreticians use a scale model of a homogeneous Earth combined with an average seismic wave velocity to make predictions about when seismic waves should arrive at various points around Earth (predicted). Meanwhile, seismologists interpret seismic data from a recent earthquake to determine when seismic waves actually arrive at various points around Earth (observations). After comparing and discussing the fit of the predicted and observed data, students then use a second scale model to further interpret these results and ultimately measure Earth's outer core based on their data.
~ 90 Minutes
By the end of the exercise, students should be able to:
- Demonstrate that Earth can’t be homogenous.
- Explain how the internal structure of Earth (concentric layers of different density and composition) is inferred through the analysis of seismic data.
- Explain the role models play in the scientific process, especially when used in combination with observational data.
- Explain how models are refined through the collection of additional data
- Discuss how working in a team to make data-gathering and procedural decisions provides an efficient means for completing tasks, provides peer support to check work and to develop conceptual understanding.
9th Grade Earth Science
Exploration. Students should have already learned about earthquakes and seismic waves.
Teacher's Guide (.doc - 6mb) (.pdf - 6mb)
Handouts: worksheets, Earth models and seismic data (.zip 4mb)
Electronic supporting resources: powerpoint, and spreadsheet (.zip - 11mb)
Optional: Flow chart useful for reviewing Activity 1 procedure
Optional: Poster - Exploring the Earth Using Seismology
Open (5 Minutes) - Guided questioning plus the image of an egg encourages students to consider how they could “know” what is inside of something without “seeing or experiencing” it. Student attention is captured and the point is emphasized when the teacher shows and then tosses a pin-hole egg to an unsuspecting student with “unexpected” results.
Prior Knowledge (10 Minutes) - Guided questioning plus the image of Earth from space is used to elicit and make explicit students’ prior knowledge about Earth’s interior structure as well has helping students to identify “how they know” this information. The teacher guides the discussion to suggest that students’ own evidence from life experience (excluding lava seen on TV) suggests that there are rocks/dirt underground. By applying Occam’s Razor, which says that the simplest explanation that explains all the data tends to be the best one, a testable hypothesis for students is that Earth is made of solid rock all the way throughout (homogeneous). Given the size of Earth, a model is needed to test this.
Explore/Explain (30 Minutes) – Activity 1 emphasizes the idea of testing a hypothesis by comparing modeled data to observations. Half the students will create a scale, homogeneous Earth model to predict how long it should take seismic waves to reach various distances around Earth. Simultaneously the other half of the students will analyze a set of seismograms from a real earthquake to determine how long it takes for the seismic waves released from a real earthquake to arrive at various points on Earth’s surface. Students will then graph their data and explore how well the model fits reality. Ultimately students will conclude that the observations do not match the predictions so they can reasonably assume that the Earth is not homogenous or made entirely of rock.
Reflect (15 Minutes) – In Activity 2, students reflect further on the data and identify an anomalous feature in the observed data. By creating a second scale Earth model students can map this anomaly back to the real Earth that defines the P-wave shadow zone.
Apply (20 Minutes) – In the continuation of Activity 2 students apply their new understanding of the P wave shadow zone to multiple earthquakes distributed around the globe. As students map out the shadow zone for each event, a pattern defining the boundary of the outer core emerges.
This activity is adapted from;